Option B.8 – Nucleic Acids

Option B.8 – Nucleic Acids

B.8.1 – Describe the structure of nucleotides and their condensation polymers (nucleic acids or polynucleotides)

Phosphate Group

A phosphate group (PO₄^3-)

Sugar

The five-carbon sugar is bound to the phosphate group. In DNA molecules, the nucleotides contain deoxyribose, which has one less -OH group, whilst RNA molecules have nucleotides containing ribose.

Base

The organic nitrogenous bases exist in two forms:

Purines – having two fused rings in their structure. These are adenine and guanine.
Pyrimidines – having a single ring in their structure. These are thymine, cytosine and uracil

These three elements come together in a condensation reaction. Ribonucleotides are found in RNA, being made of the phosphate group, ribose sugar and either cytosine, guanine, adenosine or uracil. Deoxyribonucleotides are found in DNA, being made of the phosphate group, deoxyribose sugar and either cytosine, guanine, adenosine or thymine.

Multiple nucleotides will condense to form long polynucleotides. They form phosphodiester links along the chain. These bonds form between the 3^l carbon and the phosphate group, which is attached to the 5^l carbon.

B.8.2 – Distinguish between the structures of DNA and RNA

Sugar

In DNA, the sugar is deoxyribose
In RNA, the sugar is ribose.

Base

Both RNA and DNA have the same bases, except for one. DNA contains thymine, whilst RNA contains uracil.

Structure

DNA exists as a double helix, whist RNA exists as a single-strand molecule.

B.8.3 – Explain the double helical structure of DNA

DNA is made up of two polynucleotides that have coiled around each other in a helix. The organic bases form hydrogen bonds with their complementary base, called base pairing:

Adenine bonds with thymine
Cytosine bonds with guanine

Note that a pyrimidine can only bond with a purine. In this structure, the bases are on the inside of the helix, with the sugar-phosphate backbone on the outside.

The two polynucleotide strands are antiparallel, since they go in opposite directions. The nucleotides can form an infinite number of combinations, allowing them to form all the different codes necessary.

DNA is stored in the nucleus of the cell.

B.8.4 – Describe the role of DNA as the repository of genetic information, and explain its role in protein synthesis

Since each base can only bind with a complementary base, the DNA sequence can act as a code for the copying of genes.

Transcription

In this process, the DNA double helix unwinds, and the code is copied to form an mRNA (messenger RNA) molecule. This is a single short strand that carries only a part of the genetic information found in the DNA. This allows for a necessary segment of the genetic code to be copied for use in protein synthesis. The formation of mRNA is based on the pairing of ribonucleotides with a complementary strand of DNA. The smaller mRNA is able to leave the nucleus for further use elsewhere.

Translation

The mRNA molecule then binds to the enzymes called ribosomes so that the code they carry can be translated. Another molecule, called tRNA (transfer RNA), carries amino acids to the ribosome. The base sequence along the mRNA determines the primary structure of the protein. Bases are in groups of three, called codons. In this process, bases are like letters, and codons are like words, used to transmit a message.

The tRNA has an anticodon that will match a certain codon. When the tRNA finds its matching codon, the amino acid it carries becomes part of the polypeptide chain. DNA is found in all living organisms, so it is regarded as the universal code.

B.8.5 – Outline the steps involved in DNA profiling and state its use

Every individual has different genes, and therefore different DNA. A person can be identified using a process called DNA profiling. This is done in the following steps:

A sample of DNA is taken. Restriction enzymes are used to cut the DNA into segments to identify the length of the short tandem repeats (STR) in the individual’s DNA.
Polymerase chain reaction (PCR) is used to amplify the DNA by replicating it. With this process, a tiny sample of DNA is sufficient to give accurate and clear data. The STRs are copied a few thousand times.
The DNA is put through gel electrophoresis to separate the strands according to how long they are. All DNA has a negative charge, and will move when an electrical field is applied. It is then treated with radiation, causing the DNA to appear as dark bands in the gel. The distances travelled by the data can then be compared with other samples to determine the identity of the individual.

This process has many uses:

Identifying criminals who have left traces (blood, hair, etc) at the scene of a crime
Determining the identity of a victim of an accident or crime whose bodies are not completely recovered
To identify the biological parent of a child where this is unknown
Studying evolution and populations