Genetic diversity can arise as a result of mutation or during meiosis

GENE MUTATIONS

A mutation is any change to the quantity or the base sequence of the DNA of an organism. 

A gene mutation is any change to one or more nucleotide bases, or a change in the sequence of the bases, in DNA. 

BASE DELETION 

• When a nucleotide (base) is lost from the normal DNA sequence. 

• One deleted nucleotide causes all triplets in a sequence to be read differently because each has been shifted to the left by one base, this is frame shift.

BASE SUBSTITUTION 

• When a nucleotide in a DNA molecule is replaced by another nucleotide that has a different base is known as a substitution. 

• The polypeptide produced will differ in a single amino acid. 

• The significance of this difference will depend upon the precise role of the original amino acid.

CONSEQUENCES OF DELETION MUTATION

1. One nucleotide / base removed from DNA sequence

2. This causes frameshift where all triplets in a sequence are read differently because each has been shifted to the left by one base. 

3. Changes sequence of codons on mRNA after point of mutation. 

4. Changes sequence of amino acids in primary structure of polypeptide.

5. Changes position of hydrogen / ionic / disulphide bonds in tertiary structure of protein

6. Changing the tertiary structure/ shape of the protein and leaving it potentially unable to carry out its function.

the order of DNA bases

The order of DNA bases in a gene determines the order of amino acids in a particular protein. 

MUTAGENIC AGENTS

Mutagenic agents increase the rate of gene mutation (above the rate of naturally occurring mutations).

• High energy radiation

• Ultraviolet light 

• Alpha particles 

DIPLOID CELLS 

• Diploid number of chromosomes (2n) 

• In humans 2n=46

• Each cell has two of each chromosome (a homologous pair) one set provided by each parent.

HAPLOID CELLS 

• Haploid number of chromosomes (n)

• In humans n=23

• Each cell contains one copy of each chromosome in a homologous pair

MEIOSIS 

• Meiosis forms gametes.

INTERPHASE

• DNA unravels and replicates to form 2 copies of each chromosome. 

• DNA condenses into double armed chromosomes - each with 2 sister chromatids joined at centromere. 

MEIOSIS I (first division, homologous pairs separate)

• Chromosomes arrange themselves into homologous pairs (one mum, one dad)

• Crossing over occurs in prophase 1. 

• Homologous chromosome pairs line up at middle of the cell double file.

• Independent segregation in metaphase 1 (random if mum or dads is on top)

• Pairs of chromosomes separate. 

• This halves the number of chromosomes (23 chromosomes, 46 chromatids). 

MEIOSIS II (second division, chromatids separate)

• Chromosomes made up of 2 chromatids (not homologous as only one parent) line up in the middle in single file. 

• The pairs of sister chromatidsseparate and pulled by spindle fibres to poles (centromere divided).

END RESULT

• Four haploid gametes are produced, each genetically different from each other. Each has (23 chromosomes, 23 chromatids, as all have centromere). 

GENETIC VARIATION AS A RESULT OF MEIOSIS 

RANDOM FERTILISATION

CROSSING OVER AND RECOMBINATION

INDEPENDENT SEGREGATION

RANDOM FERTILISATION

1. When two gametes (haploid, formed by meiosis) fuse together they form a zygote. 

2. All gametes are genetically different from each other so when they fuse together, they increase genetic variation. 

CROSSING OVER AND RECOMBINATION

1. In prophase I The chromosomes in a homologous pair come together, forming bivalents. 

2. They become twisted round each other (crossing over) at chiasmata. 

3. During this twisting process, tensions are created and portions of the chromatids break off.

4. Equivalent portions of homologous chromosomes are exchanged.

5. Recombination occurs where the broken portions rejoin with the chromatids of its homologous partner. 

6. In this way new genetic combinations of maternal and paternal alleles are produced. 

INDEPENDENT SEGREGATION

1. In a homologous pair, one chromosome is from each parent. 

2. When they align in metaphase I they are in double file (two rows) with random orientation meaning it is random which parent chromosome is on each side. 

3. In meiosis I, the homologous pairs separate so one chromosome from each pair ends up in the daughter cell. 

4. It is completely random which chromosome ends up in which daughter cell. 

CHROMOSOME MUTATIONS - NON-DISJUNCTION

A chromosome mutation is changes in the structure or number of whole chromosomes by non-disjunction. 

Non-disjunction is where chromosomes fail to separate in meiosis I or chromatids fail to separate in meiosis II.