meiosis Flashcards
what is the purpose of meiosis?
to make gametes
what is meant by the term diploid?
2 copies of each chromosome (one maternal one paternal)
what is meant by the term haploid?
one copy of each chromosome
what are gametes
haploid cells used for reproduction
where are the gametes produced?
in the reproductive organs
how many divisions occur in meiosis
2 nuclear divisions
what are chromatids
copies of chromosomes attached at the centromere
what is a centromere?
a region of a chromosome that joins two chromatids
what are homogenous chromosomes?
chromosomes with the same loci- eg your maternal and paternal chromosomes number 1 are homologous
how is meiosis different to mitosis
unlike mitosis you do not need to be aware of the different stages. there is two main events
what is the first stage of meiosis?
DNA replication occurs to form 2 chromatids joined at the centromere. homologous pairs, each withy their sister chromatid attached line up randomly on the equator (independent assortment). Bivalents wrap around each other crossing over occurs at chiasmata. Spindle fibres from centrioles attach to centromere. Spindle fibres contract and shorten chromosomes move to opposite poles (still with their sister chromatid attached)cytokinesis, 2 daughter cells
what is the second stage of meiosis?
chromosomes line up on equator (not homologous chromosomes) spindle fibres from centrioles attach to the centromere. Spindle fibres contract and shorten, centromeres split and individual chromatids move to opposite poles. Cytokinesis 4 non identical daughter cells (haploid)
what are the sources of variation in meiosis?
indenpendent segregation, crossing over
independent segregation
maternal and paternal chromosome line up in homologous pairs on the equator in a random order. the daughter cells at the end of meiosis 1 will have a random selection of maternal and paternal chromosomes. Because this is random, it is unlikely to get gametes with the same combination of maternal and paternal chromosomes in other rounds of meiosis so the gamete are genetically unique thus contributing to genetic variation in the offspring.
crossing over
when homologous chromosomes pair up during prophase 1, the chromatids of each homologous pair becomes twisted around each other. tensions are created and portions of the chromosome break off at specific points called chiasmata. the Brocken portions may rejoin on the hoologous chromosome instead of the chromosome it came from. crossing over creates chromosomes with new combinations of alleles that have never existed together before Increases variation in the gametes to an infinite number of possible gametes that can be made.
