grade 12 meiosis Flashcards
what is meiosis
a type of cell division whereby a somatic diploid cell undergoes two cell divisions, meiosis I and meiosis II and divides to form four different haploid sex cells.
what is the difference between diploid and haploid cells?
diploid cells have two sets of chromosomes, where each chromosome has a homologous partner. haploid cells only have one set of chromosomes, chromosomes in haploid cells have no homologous partners
what takes place before meiosis (DURING INTERPHASE)
DNA replication takes place. the result is two sets of chromosomes consisting of two identical chromatids joined together with a centromere.
what happens in the first meiotic division
the number of cells is doubled, but the number of chromosomes is not, this results in half as many chromosomes in each cell
what happens in the second meiotic division
the number of chromosomes does not get reduced
what happens in prophase 1
- chromatin network becomes shorter and thicker
-chromosomes become visible - chromosomes lie in pairs next to each other, which are called homologous chromosomes
- homologous chromosomes exchange segments of their chromosomes during crossing over
- chromatids touch at chiasmata on homologous chromosomes where crossing over takes place
- chromatids break at the chiasmata
-chromosomes of homologous pairs recombine - homologous chromosomes exchange segments of the chromatids.
what is the important of crossing over
- it promotes genetic variation
what happens during metaphase 1
- spindle fibres are spread over the whole cell
- homologous chromosomes arrange themselves randomly (random arrangement) and spindle fibres are attached to the centromeres of the chromosomes
what is the importance of random arrangement
it contributes to genetic variation
anaphase 1
-spindle fibres contract and pull homologous pairs apart
- one full chromosome (2 chromatids joined by a centromere) of each pair moves the opposite poles of the cell
telophase 1
- chromosomes reach the poles of the cell
- poles only have half of the original chromosome number
- cell membrane pinches in the middle and the cytoplasm divides (a.k.a cytokinesis)
what happens at anaphase II
- spindle fibres start to contract, pulling the centromeres apart, the centromeres divide
- chromatids are pulled to the opposite poles of each cell
what is the result of meiosis 1
two cells with half the chromosome (not 2n, but n)
what happens at telophase II
-single stranded (unreplicated) chromosomes reach the poles
- a new nucleus is formed
- the cell membrane of each cell pinces at the middle
- cytoplasm divides by cytokinesis
- four haploid(n) cells are formed
- each gamete only has half the chromosome number of the original cell (in humans its 23 chromosomes)
- 4 gametes are all genetically different.
genetic variation is introduced through…
- crossing over (prophase I)
the random arrangement of chromosomes at the equator (metaphase I & II)
similarities between mitosis and meiosis
- cell division occurs
- creates new cells
- starts with a parent cell
differences between mitosis and meiosis
mitosis
- only occurs in somatic cells
- creates body cells
- only one cell divison occurs
-creates 2 identical diploid cells
meiosis
- only occurs in reproductive organs
- creates gamates
- 2 cell divisions occur
- creates 4 halpoid cells which are all genetically different
chromosome definition
a threadlike structure made up of DNA and protein found in the nucleus of most living cells, carrying genetic information in the form of genes
chromatid definition
one of the two identical strands of a replicated chromosome
chromatid definition
one of the two identical strands of a replicated chromosome
centromere definition
region where the two chromatids of a chromosome are held together
homologous chromosomes definition
a pair of chromosomes of the same shape, size and having similar genes for each characteristic occupying the same position
bivalent
a pair of homologous chromosomes which lie next to each other and are physically in contact with one another at a point where crossing over might occur.
unreplicated chromosomes definition
an unreplicated ‘chromosome’ has a single double -stranded DNA molecule
