Structure of DNA (Deoxyribonucleic Acid)The complete structure of the DNA molecule was put forward by Francis Crick and James Watson in 1953. Their findings were based on the earlier studies of Rosalind Franklin (1920 - 1958).
According to Watson and Crick, DNA is built like a ladder twisted on itself or like a double spiral staircase. This shape is called the double helix.
DNA is a large molecule composed of a series of sub units called nucleotides.
Each nucleotide consists of three units:
(i) A sugar molecule
(ii) A phosphate group
(iii) One of four different nitrogen containing compounds called bases.
The four bases are Adenine (A), Thymine (T), Guanine (G) and Cytosine(C).
The deoxyribose molecule occupies the centre position of the nucleotide with the phosphate group on one side and a base on the other.
The nucleotides join together with a bond between the phosphate group of one nucleotide and the sugar of the next. The linked sugar and phosphate sub units form the side rails of the ladder. The bases face inwards forming the rungs of the ladder. The bases are held together by hydrogen bonds.
Adenine forms two hydrogen bonds with Thymine and Guanine forms three hydrogen bonds with Cytosine.
P - Phosphate group, D - Deoxyribose sugar, G - Guanine, C - Cytosine, T - Thymine, A - Adenine.
Guanine always bonds to Cytosine. Adenine always bonds to Thymine.
Differences between DNA And RNADNA
1. Contains the sugar deoxyribose
2. The bases are Adenine, Thymine, Guanine and Cytosine
3. Double stranded (double helix)
4. Found in the nucleus
1. Contains the sugar ribose
2. The bases are Adenine, Uracil, Cytosine and Guanine
3. Single stranded
4. Found in the nucleus and the cytoplasm
Protein SynthesisProteins are composed of amino acids and the structure and function of a protein is determined by the sequence of its amino acids. The sequence of amino acids is determined by the sequence of the bases in the DNA.
DNA ReplicationThis occurs during interphase of mitosis and meiosis.
MutationsA mutation is a change in genetic make up.
There are two main types:
1. Gene Mutations (point mutations):
This is a change in a single gene. Examples include:
2. Chromosome Mutations
[A] Change in chromosome structure:
This is when part of the chromosome breaks off, turns around, rejoins the chromosome again. This results in the sequence of the genes being reversed.
This is a change of genes between non-homologous chromosomes.
This is when part of a chromosome is lost.
[B] Change in chromosome number:
a. Addition or loss of one or more individual chromosomes.
This can occur during anaphase 1 of meiosis when two homologous chromosomes do not separate. As a result one of the daughter cells will have an extra chromosome while the other will be missing a chromosome. Humans normally have 46 chromosomes in each non-reproductive cell - 44 autosomes and 2 sex chromosomes. If they gain an extra chromosome they will have 47 while if they loose a chromosome they will have 45.
(i) Down's syndrome:
This is caused by an extra number 21 chromosome (47 chromosomes instead of 46). The symptoms include a skin fold in the inner corners of the eyes, small hands and fingers, large tongue and inadequate development of the brain.
(ii) Turner's Syndrome:
This is caused by the loss of an X-chromosome (45 chromosomes instead of 46). Females may suffer physical and sexual abnormalities.
b. Addition or loss of a complete set of chromosomes.
A gain of a whole set of chromosomes is called polyploidy. A polyploid cell has more than two of each chromosome.
Causes of Mutations
Mutagens are agents, which speed up mutations. A mutagen, which causes cancer, is called a carcinogen.
The main mutagens are:
1. Radiation, such as: