Explain the relationship between the structure and behaviour of chromosomes during meiosis

and the inheritance of genes.

9.3.3.2.4

Meiosis• Process that produces gametes or sex cells.• The word means ‘reduction division’.• Gametes have half the number of chromosomes

of body cells and are haploid cells (n).

• All body cells have 46 chromosomes and are diploid cells (2n).

Duplication(Chromosomes are made up of DNA)• The chromosomes are first duplicated to form

double chromosomes.• Each part of a double chromosome is called a

‘chromatid’. This cell is a diploid cell

2n

Duplication

Line up• Chromosomes randomly line up in their

homologous pairs.

Duplication

Line up (randomly arranged)

RANDOM Segregation• Homologous pairs separate and the cell

divides into 2.• When the pairs move apart, there is an equal

chance that the new cell will get either chromosome. This is called random segregation. Line up

Random segregation

or

Notice that at this step it went from diploid to haploid.

The chromosomes in different homologous pairs move independently into new cells- this is called independent assortment

Separation of chromatids

• Now cells divide again and separate the chromatids to form gametes.

.

or

or

Line up

Separation of

chromatids

Gametes!• Gamete cells: not identical to parent cell

Each cell is now a haploid cell

n=2

Explain the relationship between the structure and behaviour of chromosomes during meiosis and the

inheritance of genes.

• During meiosis the processes of crossing over (exchange of genetic material on arms of homologous pairs) and random segregation (random separation of maternal and paternal chromosomes into daughter cells) ensures genetic variation in gametes.

• The result is an increased number of different gamete combinations and hence genes that may be inherited.

9.3.3.3.1Process information from secondary sources to construct a

model that demonstrates meiosis and the processes of crossing over, segregation of chromosomes and the production of

haploid gametes.

crossing over

• Reshuffling of parts of a homologous pair of chromosomes.

Crossing over occurs when homologous chromosomes pair up

Crossing over

or

or

1st segregation into 2 cells

Final segregation into 4 gametes

A total of 8 possible different gamete combinations from crossing over of just one pair of chromosomes.

9.3.3.3.1Process information from secondary sources to construct a

model that demonstrates meiosis and the processes of crossing over, segregation of chromosomes and the production of

haploid gametes.

9.3.3.2.5Explain the role of gamete formation and

sexual reproduction in variability of offspring

Explain the role of gamete formation and sexual reproduction in variability of offspring.

• All gametes vary genetically because of meiosis. This is one source of variability in offspring (crossing over, random segregation).

• Sexual reproduction brings 2 haploid gametes together to form a diploid cell.

• Genetic variation is increased because it is sheer chance that determines which gametes will be involved in fertilisation.

How do cells produce genetically diverse gametes?

• Random segregation/independent assortment

• Crossing over

• Sexual reproduction

• Mutation!