7B- meiosis Flashcards
the production of haploid gametes from diploid cells by meiosis, including the significance of crossing over of chromatids and independent assortment for genetic diversity
meiosis
a specialised form of cell division used to produce gametes in sexually reproducing organisms
gametes
reproductive cells that arise from germ line cells and contain half the genetic material (n) of a somatic cell. the gametes in animals are sperm and egg cells
zygote
the diploid cell formed by the combination of two haploid gamete cells
germ line cells
cells that are involved in the generation of gametes in eukaryotes
gonads
the organs that produce gametes from germline cells. in humans these are the testes and the ovaries
metaphase plate
the equator of a dividing cell where chromosomes will line up during metaphase
microtubules
long tube-like fibre proteins that form part of the cytoskeleton of a eukaryotic cell and help give the cell its structure. Microtubules are used for a variety of cell movements, including transport of cell organelles and the movement of chromosomes during cell division
cytokinesis
the division of the cytoplasm and formation of two daughter cells
crossing over
the exchange of genetic material between non-sister chromatids during prophase I of meiosis, resulting in new combinations of alleles in daughter cells
chiasma
the point/location of overlap between two non sister chromatids
independent assortment
the random orientation of homologous chromosomes along the metaphase plate during metaphase I
what is meiosis?
meiosis is a specific type of cell division where a single cell divides to produce 4 unique daughter cells/gametes each with half the number of chromosomes of the original.
it is a specialised form of cell division that produces gametes in sexually reproducing organisms
what are the gametes in humans?
in humans, the gametes present are sperm and egg (ova) cells that contain one copy of each chromosome (n). they fuse together during fertilisation to create two copies of each chromosome (2n) inside a developing zygote
what is the dividing cell in humans?
germ line cells. they are involved in the generation of gametes in eukaryotes
where are germ line cells found?
gonads are the organs that produce gametes from germ line cells and are the testes and ovaries
how many cellular divisions are there in meiosis?
2
meiosis I→ separates homologous chromosomes into 2 different cells meiosis II→ separates sister chromatids into 4 different cells
before division
there are a homologous pair of chromosomes
2n,2s
interphase
chromosomes replicate
homologous pair of replicated chromosomes
identical to that of mitosis, where the cell grows and duplicates all of its chromosomes to prepare for division
2n,4s
meiosis I
first cell division
each homologous chromosome is separated into a different haploid cell
n,2s and n,2s
meiosis II
second cell division
each sister chromatid is separated into one of four haploid cells
n,s x4- 4 HAPLOID DAUGHTER CELLS
input of mitosis (start of G1)
1 somatic cell, 2n
output of mitosis (end of cytokinesis)
2 identical somatic cells, 2n
input of meiosis (start of G1)
1 germ line cell (2n)
output of meiosis (end of cytokinesis(
4 genetically unique gamete cells (n)
prophase I
nuclear membrane breaks down as chromosomes condense and line up in homologous pairs
important for crossing over of genetic material between the homologues
crossing over
exchanging of genetic material from one homologue to the other during prophase I. homologous chromosomes cross over at the chiasma swapping part of their DNA with each other
this forms new recombinant chromatids each with its own unique combination of alleles, adding to the genetic diversity of the resulting haploid daughter cell. the sister chromatids that they inherit aren’t identical.
metaphase I
homologous chromosomes randomly line up on opposite sides of the metaphase plate with one copy (paternal/maternal) on either side
each chromosome attaches to microtubules of whichever pole its closest too, preparing to be separated
2 lines
independent assortment
random assortment of homologous chromosomes along the metaphase plate occur irrespective of the orientation of other pairs.
they line up on opposite sides of the plate, resulting in a combination of alleles in each daughter cell are randomised as what is inherited is dependent on which side of the plate each chromosome is positioned. around 8 million different combinations
anaphase I
homologous chromosomes move apart towards opposite poles of the cell
sister chromatids remain attached to one another at the centromere. chromosomes are pulled apart
telophase I
chromosomes arrive at opposite ends of the cell and the nuclear membrane cleaves
a cleavage furrow forms in preparation for cytokinesis
aim of meiotic division
to produce gametes that are genetically distinct from each other and from the parent cell from which they originated. this occurs due to intricate processes that contribute to increased genetic diversity in the resulting daughter cells
prophase II
two cells prepare- nuclear envelope breaks down causing chromosomes to condense. spindle fibres form in preparation to pull apart the sister chromatids
metaphase II
chromosomes align along the metaphase plate of the cell and microtubules from opposite poles of the cell prepare to pull the chromosomes apart
1 line
anaphase II
sister chromatids are separated and pulled towards opposite poles of the cell by microtubules that are attached at each centromere. chromatids are pulled apart
telophase II
individual chromatids are at each pole of the cell and so separate nuclear membranes begin to form around each set as the chromosomes decondense and unravel
cytokinesis
splits each of the chromosome sets at opposite ends of the cell into 4 new individual cells, resulting in haploid daughter cells.
meiosis before interphase
46 chromosomes and 46 chromatids
meiosis after interphase
46 chromosomes and 92 chromatids
after meiosis I
23 chromosomes and 46 chromatids
after meiosis II
23 chromosomes and 23 chromatids
how many chromosomes in human body cells and gametes
46 chromosomes in human body cells and 23 chromosomes in gametes
how to count chromosomes
the amount of centromeres present
mitosis before interphase
46 chromosomes and 46 chromatids
mitosis after interphase
46 chromosomes and 92 chromatids
during mitosis
46 chromosomes and 92 chromatids
after mitosis (per daughter cell)
46 chromosomes and 46 chromatids
