Sexual reproduction

What are the specialised reproductive cells called

Gametes

How are gametes produced

Meiosis

What is produced when 2 gametes fuse

Zygote

What is the advantage of sexual reproduction

It introduces genetic variation within a population increasing the chances of the population surviving when the environment changes.

Haploids

Cells with 1 set of chromosomes. Represented by n.

Diploids

Cells with 2 sets of chromosomes. Represented by 2n.

How many chromosomes does a human diploid cell have

46

What kind of cells are gametes

Haploid

What is the role of meiosis

To half the number of chromosomes in gametes.

Why must the number of chromosomes in gametes be halved

To ensure that the chromosome number does not double in each subsequent generation when the gametes fuse at fertilisation.

Why are there 2 versions of every chromosome.

One set of chromosomes is inherited from the mother, and another set from the father.

How many daughter cells are produced in meiosis

4

Prophase I

Chromosomes condense and homologous chromosomes pair up to form a bivalent and swap equivalent sections of DNA at chiasmata in crossing over. Spindles form. Nuclear envelope disintegrates.

Metaphase I

Bivalents randomly line up across the cell equator (independent assortment of chromosomes). Spindle fibres attach to the centromeres.

Anaphase I

Homologous chromosomes separate with one chromosome from each pair going to opposite pole of the cell. Centromeres do not divide.

Telophase I

In animal cells, the two haploid sets of chromosomes are surrounded by new nuclear membranes. Chromosomes unravel and the cytoplasm divides. Plant cells usually pass directly from anaphase I to prophase II.

Prophase II

Chromosomes condense and become visible. The nuclear envelope disintegrates. Spindles form.

Metaphase II

Chromosomes randomly line up at the equator (independent assortment of chromatids). Spindle fibres attach to the centromeres.

Anaphase II

Centromeres divide and sister chromatids are pulled to opposite poles, centromere first.

Telophase II

Nuclear envelope reforms around the 4 haploid sets of chromosomes.

How is genetic variation achieved in meiosis

Crossing over in prophase I.

Independent assortment of chromosomes in metaphase I.

Independent assortment of chromatids in metaphase II.

Crossing over

During prophase I, homologous chromosomes pair to form a bivalent. Crossing over occurs between adjacent non-sister chromatids at chiasmata. Chromatids break and rejoin, exchanging equivalent sections of DNA, to form recombinant chromatids with a new combination of alleles.

Independent assortment of chromosomes

Random distribution of pairs of homologous chromosomes at equator during metaphase I. Determines subsequent independent segregation of chromosomes (with different alleles) to opposite pole in anaphase I.

Independent assortment of chromatids

Random distribution of chromosomes at equator in metaphase II. Determines independent segregation of chromatids (may be different due to crossing over) in anaphase II.

How is genetic variation achieved through sexual reproduction

Random mating within the population (between individuals with different alleles) and random fusion between genetically unique gametes.