meiosis Flashcards
define ‘meiosis’
cell division which produces gametes (sex cells) that contain half the number of chromosomes making them haploid
why is meiosis a reduction division?
It enables the chromosome number to be halved as the parent cells are diploid but the daughter cells are haploid meaning they only have one chromosome from each homologous pair present
For humans, what is the diploid and haploid number?
diploid = 46
haploid = 23
How do you know what the haploid number is of a cell from a diagram?
Haploid number is how many different sizes of chromosomes are present in the cell.
The cell becomes diploid when the matching size pairs join
What is the gene loci?
The position on the chromosome where the gene is found
define ‘gene’
The DNA sequence which codes for a polypeptide which when folded into its tertiary protein structure, gives rise to a characteristic. E.g eye colour or hair texture
define ‘allele’
Alternative versions of a gene due to slight variations in DNA sequence of the polypeptide leading to a variation in the final protein
What are homologous chromosomes?
Chromosomes that share the same structural features e.g size, same genes at same loci position, but alleles may be different
What’s the state given to homologous chromosomes that carry the same alleles and the state when carrying different alleles?
-Same alleles = Homozygous state
-Different alleles = Heterozygous state
summarise prophase 1
- x shaped chromosomes condense
- homologous chromosomes pair up and form bivalents
- crossing over occurs at chiasmata
- nuclear envelope breaks down
- nucleolus disappears
- centrioles separate
- spindle fibres form
summarise metaphase 1
- bivalents line up in middle of cell
- this is independent assortment
summarise anaphase 1
- homologous pairs pulled apart to poles (random segregation)
- sister chromatids remain attached at centromeres
summarise telophase 1
- chromosomes assemble at either pole
- nuclear envelope forms to make two haploid nuclei
- chromosomes diffuse
- cytokinesis and cleavage furrow
summarise prophase 2
- chromosomes condense
- nuclear envelope breaks down
- centrioles separate
- spindle is formed
summarise metaphase 2
- chromosomes line up at equator
- independent assortment of chromatids
- the spindle is attached at centromeres
summarise anaphase 2
- centromeres replicate
- random segregation of chromatids
summarise telophase 2
- chromatids assemble at poles
- nuclear envelope forms
- 4 haploid nuclei
- cytokinesis
how does prophase 1 cause genetic variation
- chromatids from different homologous pairs interweave at chiasmata points and form bivalents
- produces variation as genetic material is exchanged between homologous chromosomes producing new combinations of alleles
how does metaphase 1 cause genetic variation?
- independent assortment = when homologous chromosomes move to the equator the alignment of each chromosome is random
- when homologous chromosomes are separated in anaphase 1, many different chromosome combinations can be formed in the daughter cells
how does metaphase 2 cause genetic variation
- independent assortment: chromosomes lines up at the cell equator but the side on which each sister chromatids is positioned is random
- this causes genetic variation as sister chromatids are no longer identical
