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8.2: Storing Genetic Information - Biology

8.2: Storing Genetic Information - Biology



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What you’ll learn to do: Explain how DNA stores genetic information

The unique structure of DNA is key to its ability to store and replicated genetic information:

In this outcome, you will learn to describe the double helix structure of DNA: its sugar-phosphate backbone ladder with nitrogenous base “rungs” of ladder.

Learning Objectives

  • Diagram the structure of DNA
  • Relate the structure of DNA to the storage of genetic information

Structure of DNA

The building blocks of DNA are nucleotides. The important components of each nucleotide are a nitrogenous base, deoxyribose (5-carbon sugar), and a phosphate group (see Figure 2). Each nucleotide is named depending on its nitrogenous base. The nitrogenous base can be a purine, such as adenine (A) and guanine (G), or a pyrimidine, such as cytosine (C) and thymine (T). Uracil (U) is also a pyrimidine (as seen in Figure 2), but it only occurs in RNA, which we will talk more about later.

The nucleotides combine with each other by covalent bonds known as phosphodiester bonds or linkages. The phosphate residue is attached to the hydroxyl group of the 5′ carbon of one sugar of one nucleotide and the hydroxyl group of the 3′ carbon of the sugar of the next nucleotide, thereby forming a 5′-3′ phosphodiester bond.

In the 1950s, Francis Crick and James Watson worked together to determine the structure of DNA at the University of Cambridge, England. Other scientists like Linus Pauling and Maurice Wilkins were also actively exploring this field. Pauling had discovered the secondary structure of proteins using X-ray crystallography. In Wilkins’ lab, researcher Rosalind Franklin was using X-ray diffraction methods to understand the structure of DNA. Watson and Crick were able to piece together the puzzle of the DNA molecule on the basis of Franklin’s data because Crick had also studied X-ray diffraction (Figure 3). In 1962, James Watson, Francis Crick, and Maurice Wilkins were awarded the Nobel Prize in Medicine. Unfortunately, by then Franklin had died, and Nobel prizes are not awarded posthumously.

Watson and Crick proposed that DNA is made up of two strands that are twisted around each other to form a right-handed helix. Base pairing takes place between a purine and pyrimidine; namely, A pairs with T and G pairs with C. Adenine and thymine are complementary base pairs, and cytosine and guanine are also complementary base pairs. The base pairs are stabilized by hydrogen bonds; adenine and thymine form two hydrogen bonds and cytosine and guanine form three hydrogen bonds. The two strands are anti-parallel in nature; that is, the 3′ end of one strand faces the 5′ end of the other strand. The sugar and phosphate of the nucleotides form the backbone of the structure, whereas the nitrogenous bases are stacked inside. Each base pair is separated from the other base pair by a distance of 0.34 nm, and each turn of the helix measures 3.4 nm. Therefore, ten base pairs are present per turn of the helix. The diameter of the DNA double helix is 2 nm, and it is uniform throughout. Only the pairing between a purine and pyrimidine can explain the uniform diameter. The twisting of the two strands around each other results in the formation of uniformly spaced major and minor grooves (Figure 4).

Genetic Information

The genetic information of an organism is stored in DNA molecules. How can one kind of molecule contain all the instructions for making complicated living beings like ourselves? What component or feature of DNA can contain this information? It has to come from the nitrogen bases, because, as you already know, the backbone of all DNA molecules is the same. But there are only four bases found in DNA: G, A, C, and T. The sequence of these four bases can provide all the instructions needed to build any living organism. It might be hard to imagine that 4 different “letters” can communicate so much information. But think about the English language, which can represent a huge amount of information using just 26 letters. Even more profound is the binary code used to write computer programs. This code contains only ones and zeros, and think of all the things your computer can do. The DNA alphabet can encode very complex instructions using just four letters, though the messages end up being really long. For example, the E. coli bacterium carries its genetic instructions in a DNA molecule that contains more than five million nucleotides. The human genome (all the DNA of an organism) consists of around three billion nucleotides divided up between 23 paired DNA molecules, or chromosomes.

The information stored in the order of bases is organized into genes: each gene contains information for making a functional product. The genetic information is first copied to another nucleic acid polymer, RNA (ribonucleic acid), preserving the order of the nucleotide bases. Genes that contain instructions for making proteins are converted to messenger RNA (mRNA). Some specialized genes contain instructions for making functional RNA molecules that don’t make proteins. These RNA molecules function by affecting cellular processes directly; for example some of these RNA molecules regulate the expression of mRNA. Other genes produce RNA molecules that are required for protein synthesis, transfer RNA (tRNA), and ribosomal RNA (rRNA).

In order for DNA to function effectively at storing information, two key processes are required. First, information stored in the DNA molecule must be copied, with minimal errors, every time a cell divides. This ensures that both daughter cells inherit the complete set of genetic information from the parent cell. Second, the information stored in the DNA molecule must be translated, or expressed. In order for the stored information to be useful, cells must be able to access the instructions for making specific proteins, so the correct proteins are made in the right place at the right time.

Both copying and reading the information stored in DNA relies on base pairing between two nucleic acid polymer strands. Recall that DNA structure is a double helix (see Figure 5).

The sugar deoxyribose with the phosphate group forms the scaffold or backbone of the molecule (highlighted in yellow in Figure 5). Bases point inward. Complementary bases form hydrogen bonds with each other within the double helix. See how the bigger bases (purines) pair with the smaller ones (pyrimidines). This keeps the width of the double helix constant. More specifically, A pairs with T and C pairs with G. As we discuss the function of DNA in subsequent sections, keep in mind that there is a chemical reason for specific pairing of bases.

To illustrate the connection between information in DNA and an observable characteristic of an organism, let’s consider a gene that provides the instructions for building the hormone insulin. Insulin is responsible for regulating blood sugar levels. The insulin gene contains instructions for assembling the protein insulin from individual amino acids. Changing the sequence of nucleotides in the DNA molecule can change the amino acids in the final protein, leading to protein malfunction. If insulin does not function correctly, it might be unable to bind to another protein (insulin receptor). On the organismal level of organization, this molecular event (change of DNA sequence) can lead to a disease state—in this case, diabetes.

Practice Questions

The order of nucleotides in a gene (in DNA) is the key to how information is stored. For example, consider these two words: stable and tables. Both words are built from the same letters (subunits), but the different order of these subunits results in very different meanings. In DNA, the information is stored in units of 3 letters. Use the following key to decode the encrypted message. This should help you to see how information can be stored in the linear order of nucleotides in DNA.

ABC = aDEF = dGHI = eJKL = f
MNO = hPQR = iSTU = mVWX = n
YZA = oBCD = rEFG = sHIJ = t
KLM = wNOP = jQRS = pTUV = y

Encrypted Message: HIJMNOPQREFG – PQREFG – MNOYZAKLM – DEFVWXABC – EFGHIJYZABCDGHIEFG – PQRVWXJKLYZABCDSTUABCHIJPQRYZAVWX

[practice-area rows=”2″][/practice-area]
[reveal-answer q=”236947″]Show Answer[/reveal-answer]
[hidden-answer a=”236947″]This is how DNA stores information.

[/hidden-answer]

Where in the DNA is information stored?

  1. The shape of the DNA
  2. The sugar-phosphate backbone
  3. The sequence of bases
  4. The presence of two strands.

[reveal-answer q=”767717″]Show Answer[/reveal-answer]
[hidden-answer a=”767717″]Answer c. The sequence of the bases codes for the instructions for protein synthesis. The shape is DNA is not related to information storage. The sugar-phosphate backbone only acts as a scaffold. The presence of two strands is important for replication, but their information content is equivalent, as they are complementary to each other.

[/hidden-answer]

Which statement is correct?

  1. The sequence of DNA bases is arranged into chromosomes, most of which contain the instructions to build an amino acid.
  2. The sequence of DNA strands is arranged into chromosomes, most of which contain the instructions to build a protein.
  3. The sequence of DNA bases is arranged into genes, most of which contain the instructions to build a protein.
  4. The sequence of DNA phosphates is arranged into genes, most of which contain the instructions to build a cell.

[reveal-answer q=”363484″]Show Answer[/reveal-answer]
[hidden-answer a=”363484″]Answer c. The sequence of DNA bases is arranged into genes, most of which contain the instructions to build a protein. DNA stores information in the sequence of its bases. The information is grouped into genes. Protein is what is mainly coded.[/hidden-answer]

Check Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.

Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.


8.2: Storing Genetic Information - Biology

The unique structure of DNA is key to its ability to store and replicated genetic information:

In this outcome, you will learn to describe the double helix structure of DNA: its sugar-phosphate backbone ladder with nitrogenous base “rungs” of ladder.

Learning Outcomes

  • Diagram the structure of DNA
  • Relate the structure of DNA to the storage of genetic information

8.2: Storing Genetic Information - Biology

The genetic information of an organism is stored in DNA molecules. How can one kind of molecule contain all the instructions for making complicated living beings like ourselves? What component or feature of DNA can contain this information? It has to come from the nitrogen bases, because, as you already know, the backbone of all DNA molecules is the same. But there are only four bases found in DNA: G, A, C, and T. The sequence of these four bases can provide all the instructions needed to build any living organism. It might be hard to imagine that 4 different “letters” can communicate so much information. But think about the English language, which can represent a huge amount of information using just 26 letters. Even more profound is the binary code used to write computer programs. This code contains only ones and zeros, and think of all the things your computer can do. The DNA alphabet can encode very complex instructions using just four letters, though the messages end up being really long. For example, the E. coli bacterium carries its genetic instructions in a DNA molecule that contains more than five million nucleotides. The human genome (all the DNA of an organism) consists of around three billion nucleotides divided up between 23 paired DNA molecules, or chromosomes.

The information stored in the order of bases is organized into genes: each gene contains information for making a functional product. The genetic information is first copied to another nucleic acid polymer, RNA (ribonucleic acid), preserving the order of the nucleotide bases. Genes that contain instructions for making proteins are converted to messenger RNA (mRNA). Some specialized genes contain instructions for making functional RNA molecules that don’t make proteins. These RNA molecules function by affecting cellular processes directly for example some of these RNA molecules regulate the expression of mRNA. Other genes produce RNA molecules that are required for protein synthesis, transfer RNA (tRNA), and ribosomal RNA (rRNA).

In order for DNA to function effectively at storing information, two key processes are required. First, information stored in the DNA molecule must be copied, with minimal errors, every time a cell divides. This ensures that both daughter cells inherit the complete set of genetic information from the parent cell. Second, the information stored in the DNA molecule must be translated, or expressed. In order for the stored information to be useful, cells must be able to access the instructions for making specific proteins, so the correct proteins are made in the right place at the right time.

Figure 1. DNA’s double helix. Graphic modified from “DNA chemical structure,” by Madeleine Price Ball, CC-BY-SA-2.0

Both copying and reading the information stored in DNA relies on base pairing between two nucleic acid polymer strands. Recall that DNA structure is a double helix (see Figure 1).

The sugar deoxyribose with the phosphate group forms the scaffold or backbone of the molecule (highlighted in yellow in Figure 1). Bases point inward. Complementary bases form hydrogen bonds with each other within the double helix. See how the bigger bases (purines) pair with the smaller ones (pyrimidines). This keeps the width of the double helix constant. More specifically, A pairs with T and C pairs with G. As we discuss the function of DNA in subsequent sections, keep in mind that there is a chemical reason for specific pairing of bases.

To illustrate the connection between information in DNA and an observable characteristic of an organism, let’s consider a gene that provides the instructions for building the hormone insulin. Insulin is responsible for regulating blood sugar levels. The insulin gene contains instructions for assembling the protein insulin from individual amino acids. Changing the sequence of nucleotides in the DNA molecule can change the amino acids in the final protein, leading to protein malfunction. If insulin does not function correctly, it might be unable to bind to another protein (insulin receptor). On the organismal level of organization, this molecular event (change of DNA sequence) can lead to a disease state—in this case, diabetes.

Practice Questions

The order of nucleotides in a gene (in DNA) is the key to how information is stored. For example, consider these two words: stable and tables. Both words are built from the same letters (subunits), but the different order of these subunits results in very different meanings. In DNA, the information is stored in units of 3 letters. Use the following key to decode the encrypted message. This should help you to see how information can be stored in the linear order of nucleotides in DNA.

ABC = a DEF = d GHI = e JKL = f
MNO = h PQR = i STU = m VWX = n
YZA = o BCD = r EFG = s HIJ = t
KLM = w NOP = j QRS = p TUV = y

Encrypted Message: HIJMNOPQREFG – PQREFG – MNOYZAKLM – DEFVWXABC – EFGHIJYZABCDGHIEFG – PQRVWXJKLYZABCDSTUABCHIJPQRYZAVWX


The structural diversity of artificial genetic polymers

Synthetic genetics is a subdiscipline of synthetic biology that aims to develop artificial genetic polymers (also referred to as xeno-nucleic acids or XNAs) that can replicate in vitro and eventually in model cellular organisms. This field of science combines organic chemistry with polymerase engineering to create alternative forms of DNA that can store genetic information and evolve in response to external stimuli. Practitioners of synthetic genetics postulate that XNA could be used to safeguard synthetic biology organisms by storing genetic information in orthogonal chromosomes. XNA polymers are also under active investigation as a source of nuclease resistant affinity reagents (aptamers) and catalysts (xenozymes) with practical applications in disease diagnosis and treatment. In this review, we provide a structural perspective on known antiparallel duplex structures in which at least one strand of the Watson-Crick duplex is composed entirely of XNA. Currently, only a handful of XNA structures have been archived in the Protein Data Bank as compared to the more than 100 000 structures that are now available. Given the growing interest in xenobiology projects, we chose to compare the structural features of XNA polymers and discuss their potential to access new regions of nucleic acid fold space.

© The Author(s) 2015. Published by Oxford University Press on behalf of Nucleic Acids Research.

Figures

Representative structures illustrate the structural…

Representative structures illustrate the structural diversity and plasticity of natural and artificial nucleic…

XNA backbone repeating units. Chemical…

XNA backbone repeating units. Chemical structures of the natural and artificial (XNA) nucleic…

Geometric parameters of natural A-…

Geometric parameters of natural A- and B-form helices. ( A ) Natural genetic…

Pseudorotation phase angles P for…

Pseudorotation phase angles P for XNA duplexes. Angles were only calculated for oligonucleotides…

(χ, δ ) angle covariance…

(χ, δ ) angle covariance matrices reflect the structural diversity and plasticity of…


The Significance of DNA and Its Importance in Modern Biology

Deoxyribonucleic acid, otherwise referred to as ‘DNA’, is more than just a bunch of nucleotides perfectly organised and orchestrated to form the infamous ‘double helix’. Commonly studied at a molecular level, DNA and its structure have been fundamental in understanding evolution. The importance of DNA cannot be overstated, with it having several applications in many fields. An understanding and appreciation of the structure and function of DNA has opened up many areas of research, such as genetic engineering, which is an area of study of increasing interest. Forensic science and genealogy also rely heavily on DNA fingerprinting and sequencing for information.

DNA is thought of as a blueprint for the body to make proteins. Structurally, a DNA molecule has a sugar-phosphate backbone with base pairs linked by hydrogen bonds, these hydrogen bonds help stabilise DNA’s helical structure. Stability of the DNA structure is achieved through base stacking (hydrophobic interactions). Consider a sentence: the sentence itself is the polymer, and the letters which compose it, the individual units of the sentence, are its monomers. DNA is composed in a similar way: it is a polymer made up of monomers called nucleotides. There are four different nucleotides: A (adenine), C (cytosine), G (guanine), and T (thymine). Nucleotides are the building blocks of DNA - like the letters of a sentence, they can be arranged in various ways, changing the overall function of the DNA strand [1]. DNA molecules are packaged in chromosomes, storing genetic information individuals have a different sequence and arrangement of chromosomes, genes and alleles (variants to genes). This unique patterning of genetic material is the basis by which extraction of information about an organism is formed.

Diseases and medical conditions can be interpreted and understood using DNA: gene mutations that damage DNA have adverse effects on the health and well-being of an individual. A study investigating the significance of DNA concluded that inherited disorders are all passed on through defective DNA, for example, oxidative stress damages DNA to such an extent that germ cells, involved in conception, become damaged. The result, among others, of such damage to DNA is infertility, which is passed down the germ line childhood cancer can also arise [2].

The battle against cancer also relies on manipulating DNA replication. With no cure for this condition at present, scientists worldwide are now, more than ever, focused on researching DNA. It is said that cancer patients have “tumour-derived DNA fragments [circulating tumour DNA (ctDNA)]” and the discovery of these fragments has proven to be of value in assessing the spread and recurrence of cancer [3]. Epidemiology of COVID-19 can also be understood using DNA, particularly in the analysis of susceptible individuals. Successful viral entry and replication depend on the interaction of SARS-CoV-2 and the angiotensin-converting enzyme 2 (ACE2). Studies show that ACE2 gene polymorphism has an effect on the severity of COVID-19. It also has implications in individual susceptibility to the disease [4]. Genetic variation of ACE2, a direct result of DNA mutation, means different varieties of this enzyme will have different interactions with the virus, explaining why and how contracting the virus has different effects on people.

While DNA studies are mainly seen in clinical settings, they are of use elsewhere: DNA has a longevity of several years, making it useful in historical tracing and archaeology. Such practical applications have helped history scholars learn where Richard III was buried, and also learn that Czar Nicholas II's children were killed during the Russian Revolution, disregarding speculations about their disappearance. Simple tracing of the unique DNA sequences allowed for such truths to be uncovered [5].

DNA can also be used in agriculture to identify crops as well as in “cloning of important agronomic trait genes, and molecular marker-assisted breeding” [6]. Applications of this can also be seen in the colours of biotechnology. Green biotechnology (for agriculture) sees its application in molecular engineering in plant selection to genetically modify plants with desirable traits [7, 8]. The concept behind the manufacturing of ‘golden rice’ uses DNA as the basis for such modifications, with the rice being genetically modified to increase vitamin A content. Although many controversies exist regarding consumption of ‘golden rice’, it can be especially useful in helping reduce a widespread deficiency of vitamin A (retinol) in places such as Bangladesh, where 3,000 children die daily due to vitamin A deficiency [9].

DNA is versatile, having a wide spectrum of uses across various disciplines. The list of examples of its uses and applications is exhaustive, illustrating and demonstrating just how important it is, especially in modern biology. It is an undeniable fact that the discovery of DNA has positively impacted the world of science, assisting scientists on their quest to learn more about the genetics of various organisms. Such information has been beneficial in understanding epidemiology, ultimately improving our quality of life.

References

[1] Khan Academy , "DNA structure and function," Khan Academy, 2020. [Online]. Available: https://www.khanacademy.org/test-prep/mcat/biomolecules/dna/a/dna-structure-and-function. [Accessed 5 July 2020].

[2] R. J. Aitken , G. N. De Iuliis and R. I. McLachlan, "Biological and clinical significance of DNA damage in the male germ line," International Journal of Andrology, no. 32, p. 48, 2008.

[3] E. Heitzer, "Point: Circulating Tumor DNA for Modern Cancer Management," Clinical Chemistry , vol. 66, no. 1, p. 143, 2020.

[4] C. A. Devaux, J.-M. Rolain and D. Raoult , "ACE2 receptor polymorphism: Susceptibility to SARS-CoV-2, hypertension, multi-organ failure, and COVID-19 disease outcome," Journal of Microbiology, Immunology and Infection, vol. 53, pp. 425-435, 2020.

[5] J. Wilson, "CNN Health," Turner Broadcasting System , 25 April 2013. [Online]. Available: https://edition.cnn.com/2013/04/25/health/national-dna-day-tests/index.html. [Accessed 3 July 2020].

[6] C. Wang, X. Zhu, L. Shangguan and J. Fang, "Applications of DNA Technologies in Agriculture," Current Genomics, vol. 17, no. 4, pp. 379-386, 2016.

[7] P. Kafarski, "Rainbow code of biotechnology," CHEMIK, vol. 66, no. 8, pp. 811-816, 2012.

[8] M. C. S. Barcelos, F. B. Lupki, G. A. Campolina, D. L. Nelson and G. Molina, "The colors of biotechnology: general overview and developments of white, green and blue areas," FEMS Microbiology Letters, vol. 365, no. 21, p. 1, 2018.

[9] Dhaka Tribune , "Bangladesh close to releasing Golden Rice," Dhaka Tribune, 28 October 2019. [Online]. Available: https://www.dhakatribune.com/bangladesh/agriculture/2019/10/28/bangladesh-close-to-releasing-golden-rice. [Accessed 13 July 2020].

Laura is a first year medical student who is also passionate about science. Having done some extensive research in STEM related topics, she has grown to enjoy the science aspect of STEM. Besides medicine and science, Laura is passionate about health and fitness, personal development, as well as horticulture, and enjoys reading, and baking too. Laura is a Science Communication Editor, as part of the Youth STEM Matters Volunteer Team.


Exchange of genetic information

Bacteria do not have an obligate sexual reproductive stage in their life cycle, but they can be very active in the exchange of genetic information. The genetic information carried in the DNA can be transferred from one cell to another however, this is not a true exchange, because only one partner receives the new information. In addition, the amount of DNA that is transferred is usually only a small piece of the chromosome. There are several mechanisms by which this takes place. In transformation, bacteria take up free fragments of DNA that are floating in the medium. To take up the DNA efficiently, bacterial cells must be in a competent state, which is defined by the capability of bacteria to bind free fragments of DNA and is formed naturally only in a limited number of bacteria, such as Haemophilus, Neisseria, Streptococcus, and Bacillus. Many other bacteria, including E. coli, can be rendered competent artificially under laboratory conditions, such as by exposure to solutions of calcium chloride (CaCl2). Transformation is a major tool in recombinant DNA technology, because fragments of DNA from one organism can be taken up by a second organism, thus allowing the second organism to acquire new characteristics.

Transduction is the transfer of DNA from one bacterium to another by means of a bacteria-infecting virus called a bacteriophage. Transduction is an efficient means of transferring DNA between bacteria because DNA enclosed in the bacteriophage is protected from physical decay and from attack by enzymes in the environment and is injected directly into cells by the bacteriophage. However, widespread gene transfer by means of transduction is of limited significance because the packaging of bacterial DNA into a virus is inefficient and the bacteriophages are usually highly restricted in the range of bacterial species that they can infect. Thus, interspecies transfer of DNA by transduction is rare.

Conjugation is the transfer of DNA by direct cell-to-cell contact that is mediated by plasmids (nonchromosomal DNA molecules). Conjugative plasmids encode an extremely efficient mechanism that mediates their own transfer from a donor cell to a recipient cell. The process takes place in one direction since only the donor cells contain the conjugative plasmid. In gram-negative bacteria, donor cells produce a specific plasmid-coded pilus, called the sex pilus, which attaches the donor cell to the recipient cell. Once connected, the two cells are brought into direct contact, and a conjugal bridge forms through which the DNA is transferred from the donor to the recipient. Many conjugative plasmids can be transferred between, and reproduce in, a large number of different gram-negative bacterial species. Plasmids vary in size, from a few thousand to more than 100,000 base pairs the latter are sometimes called megaplasmids.

The bacterial chromosome can also be transferred during conjugation, although this happens less frequently than plasmid transfer. Conjugation allows the inheritance of large portions of genes and may be responsible for the existence of bacteria with traits of several different species. Conjugation also has been observed in the gram-positive genus Enterococcus, but the mechanism of cell recognition and DNA transfer is different from that which occurs in gram-negative bacteria.


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Why Is DNA Replication Important?

DNA replication is important because it creates a second copy of DNA that must go into one of the two daughter cells when a cell divides. Without replication, each cell lacks enough genetic material to provide instructions for creating proteins essential for bodily function.

DNA is generally tightly packed into a structure called chromatin. It is double stranded and twisted into a structure called a double helix. In order to replicate, DNA must unwind. After unwinding, each side of DNA separates by unzipping down the middle, with the two unzipped strands serving as templates for creating new strands. At the end of replication, the two new segments of DNA each contain one old and one new strand.

Replication occurs at different rates in different types of cells. Some cells continuously divide and must constantly replicate their DNA. Other cells divide at a much slower rate and do not need to replicate their DNA as often. Some cells divide until the organ they make up reaches its normal size, and then they do not divide again.

DNA stands for deoxyribonucleic acid. Each strand of DNA is made up of a sugar, a phosphate and a nitrogenous base bonded together into a structure called a nucleotide. Many nucleotides bond together to form DNA.


The Role of Genes and Inheritance

Genes are located on rodlike structures called chromosomes that are found in the nucleus of every cell in the body. Each gene occupies a specific position on a chromosome. Because genes provide instructions for making proteins, and proteins determine the structure and function of each cell in the body, it follows that genes are responsible for all the characteristics you inherit.

The full genetic instructions for each person, known as the human genome, is carried by 23 pairs of chromosomes, and consists of around 20,000-25,000 genes.

At conception, the embryo receives 23 chromosomes from the mother's egg and 23 chromosomes from the father's sperm. These pair up to make a total of 46 chromosomes. Pairs 1 to 22 are identical or nearly identical the 23rd pair consist of the sex chromosomes, which are either X or Y. Each egg and sperm contains a different combination of genes. This is because when egg and sperm cells form, chromosomes join together and randomly exchange genes between each other before the cell divides. This means that, with the exception of identical twins (see How twins are conceived), each person has unique characteristics.

How gender is determined

Of the 23 pairs of chromosomes that are inherited, one pair determines gender. This pair is composed either of two X (female) chromosomes, in which case the baby will be a girl, or of one X and one Y (male) chromosome, in which case the baby will be a boy.

An egg always contains one X chromosome, while a sperm can carry an X or a Y chromosome. Whether your baby is a boy or a girl will therefore always be determined by the father. If a sperm carrying an X chromosome fertilizes the egg, the resulting embryo will be a girl. If a sperm with a Y chromosome fertilizes the egg, the resulting embryo will be a boy. In the male, both the X and Y chromosomes are active. In females, however one of the two X chromosomes is deactivated early in development of the embryo in order to prevent duplicate instructions. This could be the X chromosome from either the mother or the father.

Gene variations

Each gene within a cell exists in two versions, one inherited from each parent. Often these genes are identical. However, some paired genes occur in slightly different versions, called alleles. There may be two to several hundred alleles of a gene, although each person can only have two. This variation in alleles accounts for the differences between individuals, such as color of eyes or shape of ears. One allele may be dominant and "overpower" the other recessive one.

Genes usually exist in a healthy form, but sometimes a gene is faulty. Genetic disorders arise either when an abnormal gene is inherited or when a gene changes, or mutates. Genetic disorders may follow a dominant or recessive pattern of inheritance. They can also be passed on via the X chromosome. Such sex-linked disorders are usually recessive, which means that a woman can carry the faulty gene without being affected, because she has another healthy X chromosome to compensate. If a boy receives an affected X chromosome, he will be affected a girl will be a healthy carrier like her mother. An affected male could pass on the affected gene only to his daughters.


Genetics: Biosynthesis Pathway of A New DNA Nucleobase Elucidated (Biology)

DNA is composed of nucleobases represented by the letters A, T, G and C. They form the basis of the genetic code and are present in all living beings. But in a bacteriophage, another base, represented by the letter Z, exists. This exception, the only one observed to date, has long remained a mystery. Scientists from the Institut Pasteur and the CNRS, in collaboration with the CEA, have now elucidated the biosynthesis pathway of this base. This work has been published in the April 30th, 2021 issue of Science.

DNA, or deoxyribonucleic acid, is a molecule that serves as the medium for storing genetic information in all living organisms. It is a double helix characterized by alternating purine nucleobases (adenine and guanine) and pyrimidine nucleobases (cytidine and deoxycytidine). The bases of each DNA strand are located at the center of the helix and are bonded together, thereby linking the two DNA strands: adenine forms two hydrogen bonds with thymine (A–T), and guanine forms three hydrogen bonds with cytosine (G–C). This applies to all living beings, with one exception.

Cyanophage S-2L, an exception to conventional genetics.

Cyanophage S-2L is a bacteriophage, in other words a virus that infects bacteria. In this phage, adenine is completely replaced by another base, 2-aminoadenine (represented by the letter Z). The latter forms three hydrogen bonds with thymine (Z–T), instead of the usual two bonds between adenine and thymine. This higher number of bonds increases the stability of the DNA at high temperatures and changes its conformation, meaning that the DNA is less well recognized by proteins and small molecules

2-aminoadenine biosynthesis pathway elucidated

Since it was discovered in 1977, cyanophage S-2L has been the only known exception, and the biosynthesis pathway of 2-aminoadenine has remained unknown. Scientists from the Institut Pasteur and the CNRS, in collaboration with the CEA, recently elucidated this biosynthesis pathway and demonstrated its enzymatic origins. They achieved this by identifying a homolog of the known enzyme succinoadenylate synthase (PurA) in the genome of cyanophage S-2L. A phylogenetic analysis of this enzyme family revealed a link between the homolog, known as PurZ, and the PurA enzyme in archaea. This indicates that the homolog is an ancient enzyme that probably conferred an evolutionary advantage. The research was carried out using the Institut Pasteur’s Crystallography Platform.

The new Z–T base pair and the discovery of the biosynthesis pathway show that new bases can be enzymatically incorporated into genetic material. This increases the number of coding bases in DNA, paving the way for the development of synthetic genetic biopolymers.

Source

A third purine biosynthetic pathway encoded by aminoadenine-based viral DNA genomes, Science,April 30, 2021


Watch the video: Chapter 3 storage of genetic information (August 2022).