6.3 Meiosis
First review @November 10, 2023
Practise Q's Done
Second review @November 12, 2023
meiosis is used to produce gametes (sex cells) that are haploid (contain half the number
of chromosomes as the parent cell)
this is because when the two sex cells (egg and sperm) fuse together to make a
zygote the zygote is then diploid
during meiosis there are two sets of cell divisions
meiosis is known as reduction division
Homologous chromosomes
each characteristic of an organism is coded for by two copies of each gene and the
nucleus contains matching sets of chromosomes— these are called homologous
chromosomes which are diploid
each chromosome in a homologous pair has the same genes at the same loci
alleles— different variations of the same gene, e.g. eye colour and hair colour
different alleles of the same gene will have the same locus
since homologous chromosomes have the same genes in the same positions, they will be
the same length and size when they are visible in prophase, the centromeres will also be
in the same positions
The stages of meiosis
meiosis I— first round of cell division out of two which is a reduction division, the
homologous chromosomes are separated into two daughter cells and so the cell becomes
haploid
prophase I— nuclear membrane breaks down, spindle fibres assemble and nucleolus
disappears. homologous chromosomes pair up to form bivalents— when the bivalents
are brought close together, the chromatids entangle and a process called crossing over
occurs
6.3 Meiosis 1
First review @November 10, 2023
Practise Q's Done
Second review @November 12, 2023
meiosis is used to produce gametes (sex cells) that are haploid (contain half the number
of chromosomes as the parent cell)
this is because when the two sex cells (egg and sperm) fuse together to make a
zygote the zygote is then diploid
during meiosis there are two sets of cell divisions
meiosis is known as reduction division
Homologous chromosomes
each characteristic of an organism is coded for by two copies of each gene and the
nucleus contains matching sets of chromosomes— these are called homologous
chromosomes which are diploid
each chromosome in a homologous pair has the same genes at the same loci
alleles— different variations of the same gene, e.g. eye colour and hair colour
different alleles of the same gene will have the same locus
since homologous chromosomes have the same genes in the same positions, they will be
the same length and size when they are visible in prophase, the centromeres will also be
in the same positions
The stages of meiosis
meiosis I— first round of cell division out of two which is a reduction division, the
homologous chromosomes are separated into two daughter cells and so the cell becomes
haploid
prophase I— nuclear membrane breaks down, spindle fibres assemble and nucleolus
disappears. homologous chromosomes pair up to form bivalents— when the bivalents
are brought close together, the chromatids entangle and a process called crossing over
occurs
6.3 Meiosis 1
