4.2 & 10.1 Meiosis

Topic 4 & 10 Genetics

4.2 Meiosis 4.2.1 State that meiosis is a reduction division of a diploid

nucleus to form haploid nuclei. 4.2.2 Define homologous chromosomes. 4.2.3 Outline the process of meiosis, including pairing of

homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells.

4.2 Meiosis Limit crossing over to the exchange of genetic material

between non-sister chromatids during prophase I. Names of the stages are required.

4.2.4 Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).

The characteristics of Down syndrome are not required. 4.2.5 State that, in karyotyping, chromosomes are

arranged in pairs according to their size and structure.

4.2 Meiosis Limit crossing over to the exchange of genetic material

between non-sister chromatids during prophase I. Names of the stages are required.

4.2.4 Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).

The characteristics of Down syndrome are not required. 4.2.5 State that, in karyotyping, chromosomes are

arranged in pairs according to their size and structure.

4.2 Meiosis 4.2.6 State that karyotyping is performed using cells

collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.

4.2.7 Analyse a human karyotype to determine gender and whether nondisjunction has occurred.

10.1 Meiosis 10.1.1 Describe the behaviour of the chromosomes in the

phases of meiosis. 10.1.2 Outline the formation of chiasmata in the process

of crossing over. 10.1.3 Explain how meiosis results in an effectively

infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.

10.1.4 State Mendel’s law of independent assortment. 10.1.5 Explain the relationship between Mendel’s law of

independent assortment and meiosis.

FertilisationMale germ cell in

testis (diploid)Female germ cell in

ovaries (diploid)

46 46

Sperm-Gamete (haploid)

Egg-Gamete (haploid)

23 23

Fertilisation

zygote46

Embryo46

46Foetus

Mitosis

Mitosis

MeiosisMeiosis

(diploid)

Haploid and Diploid The nucleus of normal human body cells consist of 46

chromosomes or 23 pairs of chromosomes (2 of each chromosome).

This is referred to as the diploid number for humans (2n). Gametes, sex cells, only have one set of chromosomes (23). This is referred to as the haploid number for humans (n). In diploid cells, each pair of chromosomes have the same

genes, arranged in the same sequence (loci), but they do not necessarily have the same alleles of all of the genes.

They are therefore not identical but instead are homologous, Homologous pairs.

Reductive Division The number of chromosomes in a cell can be reduced

from diploid to haploid by the process of Meiosis. Meiosis is described as Reductive Division. Organisms that reproduce sexually have to halve their

chromosome number at some stage in their life cycle because the fusion of gametes during fertilisation double it again (restores the diploid number).

Meiosis Meiosis is a type of cell division which:

Results in the production of gametes (sex cells) Occurs in germ cells in the gonads - diploid Four gametes are produced from every germ cell Each gamete has half the number of chromosomes as the

original parent cell – haploid

Meiosis involves TWO divisions: Meiosis I Meiosis II

In sexual reproducing species, haploid cells must be formed by meiosis before fertilisation to ensure the diploid number of chromosomes in offspring is obtained.

Meiosis I

Ref: Advanced Biology, Kent

Meiosis II

Ref: Advanced Biology, Kent

MeiosisMeiosis I Homologous chromosomes pair up.

They are called a bivalent. Non-sister chromatids cross over at

points called chiasmata. They may exchange genetic

material – crossing over. Homologous pairs line up at

equator. Maternal and paternal

chromosomes of each pair line up independently of other pairs –independent assortment.

Homologous chromosomes separate and move towards opposite poles.

Two new cells form, each with half the original chromosome number.

Meiosis II New spindle apparatus forms. Chromosomes line up at the

equator in a single line. Centromeres divide and sister

chromatids move towards opposite poles.

Each cells divides, resulting in a total of four haploid cells.

Each cell formed is genetically unique due to crossing over and independent assortment.

Outline of meiosis.

Meiosis – Gamete Production Meiosis is the name given to a specialised for of cell

division which produces the gametes. In animals this process occurs in organs called the

Gonads. In mature human females, eggs are produced by a process

called Oogenesis in the Ovaries. In mature human males, millions of sperm are produced

daily by a process called spermatogenesis in the testes.

Meiosis – Gamete Production

Ref: Biology Key Ideas

Sources of Variation in meiosis

1. Crossing-Over (Recombination)

First meiotic division Homologous chromosomes come together

to form a bivalent The bivalent has four chromatids (2 from

each chromosome) These can intertwine and exchange

segments This is called crossing over

Methods of genetic recombination:1. crossing over

Homologous chromosomes align.

Crossing over The point at which two chromatids cross is

called a chiasma Crossing over changes the mixture of

genetic information carried on a chromatid It produces genetic recombination This makes an important contribution to the

genetic variability that sexual reproduction introduces to a species

2. Independent Assortment

Independent Assortment Meiosis gives rise to genetic variation. Variety in gametes is produced by how the bivalents line

up on the equator during Metaphase I.

Ref: Biology Key Ideas

Independent assortment Refers to the random separation and assortment

of non-homologous chromosomes during meiosis 2 chromosomes produces 4 combinations – 2n. Humans have 23 pairs of chromosomes – 223. 8 388 608 combinations If you double that, because of each gamete, the

total possible combinations is over 64 trillion. Independent assortment alone is the source of

significant variation

3. Random fusion of gametes 8 388 608 combinations of different sperm

(or eggs) due to independent assortment Fertilisation is a random fusion of gametes.

(random as to which egg fuses with which sperm).

The total possible combinations is over 64 trillion.

Mendel’s Law of Segregation When gametes are produced, each gamete must receive a

full complement of genes. For this reason, the factors/alleles must separate so that

only one factor/allele is present in each gamete. Mendel’s Law of Segregation states:

“The characteristics of a diploid organism are determined by alleles which occur in pairs. Of a pair of such alleles,

only one can be carried in a single gamete” Thus each gamete receives one complete set of alleles,

and hence chromosomes: ie: 23 chromosomes.

The two alleles of a gene are located on homologous chromosomes which move to opposite poles, causing segregation.

Non-disjunctions Non-disjunctions are a form of chromosome mutation. They occur when homologous chromosomes fail to

separate properly during meiosis. An extra chromosome is drawn to on pole, producing

gametes with an extra chromosome and gametes with one less chromosome.

This is referred to as Trisomy. Down’s syndrome is an example of trisomy 21. Down’s syndrome people have an extra chromosome 21.

A Non-disjunction Leading to Down’s Syndrome

Ref: Advanced Biology, Kent

Down’s Syndrome

Ref: Advanced Biology, Kent

A Down’s syndrome boy. A Karyotype of a Down’s Syndrome boy

Turner’s Syndrome

Ref: Advanced Biology, Kent

4.2 Meiosis 4.2.1 State that meiosis is a reduction division of a diploid

nucleus to form haploid nuclei. 4.2.2 Define homologous chromosomes. 4.2.3 Outline the process of meiosis, including pairing of

homologous chromosomes and crossing over, followed by two divisions, which results in four haploid cells.

4.2 Meiosis Limit crossing over to the exchange of genetic material

between non-sister chromatids during prophase I. Names of the stages are required.

4.2.4 Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).

The characteristics of Down syndrome are not required. 4.2.5 State that, in karyotyping, chromosomes are

arranged in pairs according to their size and structure.

4.2 Meiosis Limit crossing over to the exchange of genetic material

between non-sister chromatids during prophase I. Names of the stages are required.

4.2.4 Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).

The characteristics of Down syndrome are not required. 4.2.5 State that, in karyotyping, chromosomes are

arranged in pairs according to their size and structure.

4.2 Meiosis 4.2.6 State that karyotyping is performed using cells

collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities.

4.2.7 Analyse a human karyotype to determine gender and whether nondisjunction has occurred.

10.1 Meiosis 10.1.1 Describe the behaviour of the chromosomes in the

phases of meiosis. 10.1.2 Outline the formation of chiasmata in the process

of crossing over. 10.1.3 Explain how meiosis results in an effectively

infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.

10.1.4 State Mendel’s law of independent assortment. 10.1.5 Explain the relationship between Mendel’s law of

independent assortment and meiosis.