A-Level Biology - 3.4.3 Genetic diversity can arise as a result of mutation or during meiosis

How could a mutation lead to a non-functioning enzyme?

changes the sequence of amino acids (primary structure of the polypeptide)

this would affect ionic/hydrogen/disulphide bonds formed, changing the tertiary structure

therefore the shape of the active site is different so the enzyme can't bind to the substrate

What do gene mutations involve?

a change in the DNA base sequence of chromosomes

How do gene mutations occur?

they arise spontaneously during DNA replication

Name two types of gene mutations

base deletion

base substitution

What is base deletion?

Where one or more bases are removed

What is base substitution?

Where a base is replaced by another type of base

Why do not all base substitutions cause a change in the sequence of encoded amino acids?

due to the degenerate nature of the genetic code

a different triplet of bases may code for the same amino acid

What is frame shift?

the genetic code is non-overlapping

so deletion mutations will always lead to a change in the sequence of amino acids, as the entire reading frame will shift to the left

What do mutagenic agents do?

Increase the rate of gene mutation

Give three examples of mutagens

radiation such as UV

certain chemicals

certain viruses

How do chromosome mutations occur?

arise spontaneously by chromosome non-disjunction during meiosis

What is chromosome non-disjunction?

chromosomes fail to separate correctly in meiosis

results in gametes with one more or less chromosome than normal

What is a diploid cell?

a cell with two sets of chromosomes

(46 in total)

What is a haploid cell?

a cell with one set of chromosomes

(23 in total)

Where does meiosis occur?

reproductive organs (ovaries and testes)

How many nuclear divisions occur during meiosis?

two nuclear divisions

What does meiosis produce?

four genetically different haploid daughter cells

Describe the parent cell in meiosis

a single diploid parent cell

Give two ways in which meiosis produces genetic variation

independent segregation of homologous chromosomes

crossing over between homologous chromosomes

Name the eight steps involved in meiosis

MEIOSIS I:

-prophase I

-metaphase I

-anaphase I

-telophase I (+ cytokinesis)

MEIOSIS II:

-prophase II

-metaphase II

-anaphase II

-telophase II (+ cytokinesis)

Describe the process of meiosis I

Prophase I:

-chromosomes condense (become visible) and align themselves in pairs of homologous chromosomes (2x2n)

-nuclear membrane breaks down

-crossing over can occur between homologous chromosomes

Metaphase I:

-homologous pairs line up next to each other and move to the equator of the cell

-spindle forms and attach to centromeres

Anaphase I:

-spindle fibres contract, separating homologous pairs and pulling them towards opposite poles of the cell

Telophase I and cytokinesis I:

-nuclei reform

-cell divides

Describe the process of meiosis II

Prophase II:

-nuclei break down

Metaphase II:

-chromosomes line up at equator

-spindle forms and attaches to centromeres

Anaphase II:

-centromeres divide

-chromatids are separated and pulled to opposite poles of the cell

Telophase II and cytokinesis II:

-nuclei reform

-cells divide

Describe how the process of meiosis results in haploid cells

DNA replication creates two identical chromatids

two nuclear divisions

first division: homologous pairs separated

second division: sister chromatids separated

produces 4 haploid daughter cells

When does crossing over occur?

occurs during prophase I

Describe the process of crossing over and explain how it increases genetic diversity

Homologous pairs of chromosomes associate

Chiasmata form

Equal lengths of non-sister chromatids (alleles) are exchanged

Producing new combinations of alleles

What is independent segregation?

The random division of homologous chromosomes into separate daughter cells during meiosis

in human body cells there are 23 homologous pairs, so 2^23 possible combinations of chromosomes

What expression is used to calculate the possible number of different combinations of chromosomes following meiosis?

2^n

where n is the number of homologous chromosome pairs

Apart from mutation, explain one other way genetic variation within a species is increased

Random fusion of gametes which produces new allele combinations.

Explain how the chromosome number is halved during meiosis

homologous chromosomes pair separates, one of each pair goes to each daughter cell

Explain why all cells of the body will have a chromosome mutation

The mutation occurred in a gamete that formed the zygote

All cells derived form that zygote by mitosis

Mitosis produces genetically identical cells