Meiosis Flashcards
diploid and haploid:
- diploid 2n (2 sets of chromosomes)
- haploid n (one set)
somatic cells in humans:
- 23 pairs
- numbered by size
- only X and Y not paired
paired chromosomes vs sister chromatids:
- paired homologous duplicated chromosomes: similar but from different parents
- sister chromatids: exact same copy joined by centromere
meiosis general pathway:
- 2 cell divisions
- meiosis I and meiosis II
- 4 daughter cells
which have only half chromosomes of parent (n)
meiosis I:
- homologous chromosomes separate
- results in 2 haploid daughter cells w replicated chromosomes
meiosis II:
- sister chromatids separate
- results in 4 haploid daughter cells
stages of meiosis I:
- prophase I
- metaphase I
- anaphase I
- telophase I
- cytokinesis
stages of meiosis II:
- prophase II
- metaphase II
- anaphase II
- telophase II
- cytokinesis
daughter cells of meiosis:
- each genetically different from each other/ parent cell
- haploid set of chromosomes
result of meiosis I:
- 2 haploid cells
- chromosomes have sister chromatids (replicated)
result of meiosis II:
- 4 haploid daughter cells
- un-replicated chromosomes
differences btw mitosis and meiosis:
- chiasma: pair of duplicated homologous chromosomes attached
- during metaphase I will line up together
- homologs separated in telophase I
- all different genetic material in 4 daughter haploid cells
crossing over:
- recombination
- parents DNA shuffled
- approx 50% from each parent
homology:
- similarities in DNA sequences due to common ancestry
- same genes in same regions of chromosome
homologous recombination:
- paternal and maternal chromosomes line up
- chiasmata forms at regions of homology
- segments of DNA are exchanged
euploid:
having equal number of all chromosomes of haploid set
aneuploid:
having different number of chromosomes than is usual
when does aneuploid happen:
- error is meiosis
- most cases: miscarriage
- 1 in 160 babies
- 21, 18, 13
origins of genetic variation:
- behaviour of chromosomes in meiosis and fertilisation responsible for variation of each generation
mechanisms contributing: genetic variation
- independent assortment of chromosomes
- crossing over
- random fertilisation
- mutation (usually harmful)
independent assortment of chromosomes:
- each pair of chromosomes sorts maternal/parternal homologues into daughter cells independently of other pairs
- n= 23, 8 million combos
crossing over:
- recombinant chromosomes
- early in prophase I
- homologs pair up gene by gene (synapsis)
- homologous portions of non sis chromatids trade places
random fertilisation:
- any sperm can fuse with any ovum
- zygote 70 trillion diploid combos
autosomes:
- carry same series of genes along their length
- each one may carry slightly altered version (alleles)
- so cells may carry two alleles of same gene
cystic fibrosis:
- recessive disease
- mutant allele of gene CFTR, chromosome 7
- normal CFTR protein (wild type) controls movement of chloride ions across membranes of cells in gut/ lungs/ pancreas/ liver
- mutant allele deletion of 1 amino acid:
- excess mucous production blocks ducts (eg. bronchioles)
- most patients die from lung infections
cons of sexual reproduction:
- significant investment of time and energy (mating, producing gametes)
- slower than asexual
- risk STDs
- random mixing may have suboptimal results
