Structure and Roles of Nucleic Acids

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MODULE 2: GENETICS, VARIATION AND NATURAL SELECTION

SPECIFIC OBJECTIVES & EXPLANATORY NOTES

1. Structure and Roles of Nucleic Acids

1.1 illustrate the structure of RNA and
DNA using simple labelled
diagrams;

The genetic substance found in all organisms called DNA or deoxyribonucleic acid is a kind of nucleic acid. Nucleic acids consist of two long polymers of simpler units, called nucleotides; that are composed of three (3) main units as shown below: 1) A pentose sugar (deoxyribose in DNA, ribose in RNA)

2) A nitrogenous base (a purine or pyrimidine)
3) A phosphate group

These nucleotide monomers link via phosphodiester bonds and form long chain polynucleotide chains.

Basic structure of a polynucleotide:

These polynucleotide chains can bond together via the nitrogenous bases on their molecules. There are two main classes of nitrogenous bases found on nucleotide molecules: * The purines which consist of the bases: adenine (A) and guanine (G)

* The pyrimidines which consist of the bases: cytosine (C) and thymine (T) These bases form bonds with only one other type of base, particularly A-T or C-G parings. This is known as complementary base pairing.

These bases are held together via weak hydrogen bonds that stabilize DNA molecules.

Thus these linkages between polynucleotide chains form the double stranded structure of DNA.

In contrast, the substance called RNA or ribonucleic acid is another type of nucleic acid. It differs from DNA structurally as it is:
* a single stranded molecule
* contains the pentose sugar ribose
* the pyrimidine base uracil replaces the thymine base of DNA.

1.2 explain the importance of
hydrogen bonds and base pairing
in DNA replication;

In DNA molecules, the double helix structure is held together by hydrogen bonds between complementary base pairs. In order for bases to orient themselves in this way, one DNA strand will run in a 5’to 3’ direction whilst the other strand runs in this way but in an antiparallel direction.

The hydrogen bonds present between the nucleotides make sure complementary base pairing is always specific. They can be formed and broken relatively easily; which is key for allowing the strands to be unwound for their genetic information to be copied.

The complementary nature of the base pairings also means that on separation each strand can act as a template for the copying or replication of genetic coding of the complementary strand.

Below is a diagram of the basic stages involved in semi-conservative DNA replication:

In higher organisms, such as humans, the efficiency of DNA replication must be extraordinary. The procedure described above will only replicate about 50 nucleotides per second; therefore there must be many thousands of such replication sites in action during cell division, for the cellular division to be completed within a reasonable period of time. A given length of double stranded DNA may undergo strand unwinding at numerous sites in response to promoter actions; where the unraveled section or the “replication bubble” of the DNA helix has two replication forks, so assembly of new complementary strands may proceed in two opposite directions. The new DNA material formed from these sections fuse together to achieve full replication of the entire DNA double helix.

The main role of DNA in organisms is the long-term storage of genetic information.

Within the nucleus of cells, DNA is found highly coiled into condensed “X”-shaped structures called chromosomes. Chromosomal DNA consists of thousands of genes or specific DNA sequences that code for a specific protein or trait. They act as units of inheritance and are responsible for genetic expressions seen in organisms.

Therefore, replication of DNA is a vital process for heredity or passing genetic material from parent to offspring.

These genes can code for amino acids which are...
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