Sexual Reproduction

Sexual reproduction

• Germline cells give rise to gametes.

• each gamete only receives half the amount of genetic material (n)

• After fertilisation, this amount is restored to the full number of chromosomes (2n)

Gametes

• Male gametes are usually very small in size. They are usually motile – they can make their own way to the ovum.

• Female gametes are larger than the male, and cannot move on their own.

Fertilisation

• Fertilisation occurs when the two gametes of the male and female fuse.

• Forms new, single cell = zygote.

• Cell undergoes mitosis = develop into embryo.

Meiosis

Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells – sperm in males, eggs in females

Mitosis vs Meiosis

Two divisions

• Meiosis I separates homologous chromosomes.

• Meiosis II separates sister chromatids (just like mitosis).

Crossing over

* In the early stages of meiosis, crossing-over happens.
* Homologous chromosomes (matching pairs of chromosomes) swap pieces.
* This increases the variation seen in offspring.

Assortment

* resulting zygote contains a combination of genes arranged in an order that has never occurred before and will never occur again.

Stages of Meiosis 1

Prophase I

Metaphase I

Anaphase I

Telophase I

Prophase I

• Chromatin shortens and thickens, and chromosomes appear.

• Each chromosome has two chromatids attached at the centromere.

• Homologous chromosomes match up and crossing-over occurs.

Metaphase I

• Random assortment occurs; homologs line up on equator.

• Nuclear membrane breaks down.

Anaphase I

* Each homologue separates and moves to opposite poles of the cell; being pulled by the spindle fibres.

Telophase I

• New nuclear membranes form.

• Chromosomes decondense.

• Cytoplasm divides (cytokinesis).

Stages of Meiosis 2

Prophase II

Metaphase II

Anaphase II

Telophase II

Prophase II

• Nuclear membranes break down.

• Chromosomes shorten and thicken.

• Centrioles move to poles.

• Spindle fibres form.

Metaphase II

* Chromosomes line up along equator, not in homologous pairs.

Anaphase II

* Each sister chromatid is separated at the centromere, and they move to opposite poles of the cells.

Telophase II

• Nuclear membrane reforms.

• Chromosomes uncoil and lengthen.

• Spindle disappears.

• Nuclear envelopes reform.

• Cytoplasm divides in cytokinesis; 4 haploid daughter cells (gametes) are formed.

Genetic Variation

• Each gamete is unique.

• Offspring will always be genetically different to the parents.

• This is due to crossing over and independent assortment.

When Meiosis goes wrong

Chromosomal abnormalities are a major cause of spontaneous abortions.

Costs and benefits of sexual reproduction