Meiosis Flashcards
What is a general definition of meiosis?
When many multicellular organisms reproduce sexually using the process of meiosis. These species produce gametes (sex cells) that contain half the number of chromosomes making them haploid (n)
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 (2n) is 46- 23 homologous pairs of chromosomes
-haploid (n) is 23- one chromosome from each of the homologous pairs
What does the process of meiosis and fertilisation do to the number of chromosomes in a cell?
-During meiosis = cells are haploid as contain one chromosome from each pair
-During fertilisation = cells are diploid as now have both pairs of chromosomes
How do you know what the haploid (n) 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
What is the definition of a gene? Give an example
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
What is the definition of an allele? Give an example
Alternative versions of a gene due to slight variations in DNA sequence of the polypeptide leading to a variation in the final protein. E.g round hair follicles which produce straight hair,, oval follicles for curly hair,, very flat follicles for tightly curled hair.
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
In meiosis, what happens in the stage prophase 1 in terms of key features/ cells appearance?
-duplicated (x shaped) chromosomes condense
-homologous chromosomes pair up to form bivalents
-crossing over occurs at chiasmata
-the nuclear envelope breaks down
-nucleolus disappears
-centrioles separate
-spindle forms
In meiosis, what happens in the stage metaphase 1 in terms of key features/ cells appearance?
-homologous pairs (bivalents) line up at middle of cell
-this is the independent assortment of chromosomes
In meiosis, what happens in the stage anaphase 1 in terms of key features/ cells appearance?
-the homologous pairs of chromosomes are pulled apart to opposite poles of cell (random segregation)
-the sister chromatids remain attached at the centromeres
In meiosis, what happens in the stage telophase 1 in terms of key features/ cells appearance?
-chromosomes assemble at either pole
-the nuclear envelope forms to make two haploid nuclei
-chromosomes become diffuse
-cytokinesis occurs
-cleavage furrow
In meiosis, what happens in the stage prophase 2 in terms of key features/ cells appearance?
-chromosomes condense
-nuclear envelope breaks down
-centrioles separates
-spindle is formed
In meiosis, what happens in the stage metaphase 2 in terms of key features/ cells appearance?
-chromosomes line up at equator
-Independent assortment of chromatids
-the spindle is attached at the centromeres
In meiosis, what happens in the stage anaphase 2 in terms of key features/ cells appearance?
-centromeres replicate
-shortening spindle fibre pull the chromatids apart
-random segregation of chromatids
In meiosis, what happens in the stage telophase 2 in terms of key features/ cells appearance?
-chromatids assemble at poles
-nuclear envelopes form to make 4 haploid nuclei
-chromosomes may become diffuse
-cytokinesis occurs
-meiosis 2 is complete when there are 4 distinct haploid nuclei
Explain how stage prophase 1 of meiosis causes genetic variation.
-Non sister chromatids (chromatids from different homologous pair) interweave at points called chiasmata forming bivalents.
-This produces variation as genetic material is exchanged between homologous chromosomes producing new combinations of alleles
Explain how stage metaphase 1 of meiosis causes genetic variation.
INDEPENDENT ASSORTMENT OF CHROMOSOMES:
-when homologous chromosomes move to the equator the alignment of each chromosome is random (position of each chromosome within bivalents is equally likely due to chance)
-there are 2^n different ways they could align (n=23 in humans)
-when homologous chromosomes are separated in anaphase 1, many different chromosome combinations can be formed in the daughter cells= genetic variation
Explain how stage metaphase 2 of meiosis causes genetic variation.
INDEPENDENT ASSORTMENT OF CHROMATIDS:
-chromosomes lines up at the cell equator but the side on which each sister chromatids is positioned is random
-there are 2^n different ways they could align up (in humans n=23)
-this causes genetic variation as sister chromatids are no longer identical (due to crossing over) therefore many chromatid combinations are possible in daughter cells
