Studied by 5 people
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
Note 
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