D2.1 - meiosis (5e) Flashcards
what is the role of meiosis in eukaryotes?
meiosis is a type of nuclear division, which is essential before cell division to avoid the production of anucleate cells
what does meiosis do to the chromosome number?
meiosis halves the chromosome number and generates genetic diversity
- this is achieved through random separation of the alleles on pairs of homologous chromosomes into different gametes
what is meiosis a part of?
meiosis is part of gametogenesis
- as part of a sexual life style two gametes fuse in fertilisation
- this explains the need to halve the number of chromosomes during meiosis, so that the doubling of chromosomes at fertilisation restores the original number of chromosomes in the somatic cells of the offspring
why is meiosis needed in sexual life cycles?
chromosome numbers must be halved during meiosis, so that the fusion of gametes during fertilisation does not double the number of chromosomes in the offspring, but instead restores the original number
what is the process of meiosis?
Meiosis I: homologous chromosomes are separated
- Prophase I
- homologous chromosomes pair up to form bivalents - there is crossing over between non-sister chromatids, causing an exchange of alleles
- chromosomes condense by supercoiling - Metaphase I
- pairs of homologous chromosomes move to the equator of the cell, attached to spindle fibres
- orientation of chromosomes before separation is random - Anaphase I
- homologous chromosomes are pulled apart by spindle fibres
- centromeres do not divide so the whole centromere and 2 sister chromatids go to opposite poles, halving the chromosome number - Telophase I
- centromere with 2 sister chromatids arrive at each pole and 2 haploid nuclei are produced
Meiosis II: sister chromatids separate
- Prophase II - chromosomes re-condense
- Metaphase II - single chromosomes move to the equator of the cell, attached to spindle fibres
- Anaphase II - sister chromatids separate
- Telophase II - 4 haploid nuclei are produced, each genetically different
what occurs in the first division of meiosis?
in the first division the pairs of homologous chromosomes are separated, so that the groups of chromosomes at each pole contain one chromosome from each homologous pair
- thus, at the end of the first division there are two haploid nuclei, each containing replicated chromosomes (consisting of two sister chromatids)
what occurs in the second division of meiosis?
in the second division the sister chromatids are separated, each forming a single-armed chromosome, and these are separated at each pole, forming a total of four haploid nuclei
- the four nuclei each contain a single-armed chromosome from one of every pair of homologous chromosomes
what does meiosis produce?
one diploid nucleus forms four haploid nuclei
4 chromosomes –> 2 chromosomes –> 2 chromosomes
8 chromatids –> 4 chromatids –> 2 chromatids
what does haploid and diploid mean?
haploid - 2 pairs of homologous chromosomes
diploid - doesn’t have a pair of homologous chromosome
what is non-disjunction?
an error in meiosis where chromosomes fail to separate correctly
when can non-disjunction occur?
- during meiosis 1 - homologous pair of chromosomes do not separate at anaphase I, so an entire bivalent moves to a pole
- this means that both chromosomes end up in the same nucleus during telophase I and 2 gametes have an extra chromosome while 2 gametes have missing chromosomes - during meiosis II - sister chromatids do not separate and both move to 1 pole, so they end up in the same nucleus during telophase II
- this means that out of the 4 gametes produced, 2 will have the normal amount of chromosomes, 1 will have an extra chromosome and 1 will have a missing chromosome
what will be caused by the fertilisation of a cell that has an extra chromosome?
trisomy - three chromosomes of 1 type
what is the trisomy that causes down’s syndrome?
trisomy 21
