2B.4 Sexual reproduction

Aims to recall that gamete formation involves a reduction division (meiosis) and understand its significance as the division of a diploid nucleus to give haploid nuclei; to understand the behaviour of chromosomes during the first and second divisions of meiosis, including chiasmata formation to understand that haploid and diploid phases occur in the lifecycles of organisms.

Cell division:Involves the division of the nucleus then the division of the cytoplasm. The two types are mitosis and meiosis:

Sexual reproduction involves: Gamete formation Meiosis – halves the number

of chromosomes to form haploid gametes – takes place in the reproductive organs

Fertilisation – haploid male gamete fuses with haploid female gamete to form a diploid zygote.

Growth of the zygotes due to cells dividing by mitosis.

What are homologous pairs? A diploid cell will contain one set of

chromosomes from the male parent and one set from the female parent.

Before cell division there will be two copies of each chromosome (called homologous pairs) in the nucleus.

Two homologous chromosomes will share the following characteristics:

Same length Centromere in same position Same number of genes Genes arrange in the same order

Gametogenesis Gametogenesis – the production of sperm and ova in

animals The number of chromosomes reduces from the diploid

number (2n) to the haploid number (n) Replication of the DNA takes place during interphase –

but then there are two cycles of nuclear divisions: First meiotic division: separates the homologous pairs Second meiotic division : separates the chromatids. These two divisions result in the formation of 4 haploid

cells from 1 diploid cell. The gametes will not be identical and will contain a single set of chromosomes.

Summary diagram of meiosis

The stages of meiosis (see hand out)

Meiosis I: the homologous chromosomes separate:

Prophase I Metaphase I Anaphase I Telophase I

Meiosis II: the chromatids separate:

Prophase II Metaphase II Anaphase II Telophase II

Meiosis animation

How does meiosis lead to genetic variation?

1. Crossing over – during prophase I of meiosis the homologous chromosomes come together in pairs. Each chromosome is divided into 2 chromatids. The homologous chromosomes twist around each other – causing tension and possible breaks across the chromatids. During crossing over, fragments of chromatids often get swapped over – so this cutting and sticking of chromosomes causes genetic material to be exchanged

How does meiosis lead to genetic variation?

2. At metaphase I – when the chromosomes arrive at the equator they arrange themselves randomly.

Comparing mitosis and meiosis

Comparing mitosis and meiosis

Difference in DNA quantity between mitosis and meiosis

Difference in DNA quantity between mitosis and meiosis

IPMATThe stages of the Cell Cycle can be rememberedby IPMAT:

1. Interphase

2. Prophase

3. Metaphase

4. Anaphase

5. Telophase