Meiosis and Mitosis Flashcards
What happens in metaphase 1?
1) Pairs of homologous chromosomes align either side of the equator randomly
2) Each chromosome faces opposite pole
3) Independent assortment - random combination of chromosomes from each parent.
What happens in early telophase 1?
Sometimes nuclear envelope reforms around haploid group of chromosomes and the chromosomes decondense but sometimes the chromosomes stay in condensed form
What is a bivalent?
The association of the two chromosomes of a homologous pair at prophase I of meiosis
What is a chiasma?
The site at which the chromosomes exchange DNA in genetic crossing over
What happens in late telophase 2?
1) Chromosomes lengthen and can no longer be distinguished
2) Nuclear envelope and nucleolus should reforms
What’s the difference in anaphase in meiosis 1 and 2?
In meiosis 1 chromosomes are separated but in meiosis 2 chromatids are separated
Number of daughter cells from meiosis 1 is 2 but 4 in meiosis 2
What happens in late prophase 1 ?
1) chromosomes coil up and become shorter and thicker
2) centrioles separate and move to poles of cell, begin to form spindle
3) Chromosomes associate in homologous pairs to form a bivalent and wrap around each other
4) crossing over - segment of DNA exchanged
5) nuclear envelops disintegrates and nucleolus dissapears
What’s the difference in prophase in meiosis 1 and 2?
In meiosis 1 it follows DNA replication and crossing over takes place but this does not happen on meiosis 2
What happens in late prophase 2?
The centrioles separate and organise a new spindle at right angles to the old spindle
What does meiosis form?
Four genetically distinct haploid gametes
What happens in metaphase 2 ?
1) Chromosomes line up along the equator and spindle fibres attach to each centromere
2) independent assortment happens
What’s the difference in metaphase in meiosis 1 and 2?
In meiosis 1 the homologous pairs align either side of the equator whereas in meiosis 2 the chromosomes align on the equator
In meiosis 1 it is independent assortment of homologous chromosomes and in meiosis 2 it is independent assortment of chromatids
What happens in anaphase 1?
1) chromosomes from each bivalent split
2) spindle fibres shorten pulling one of the pair to each pole centromere first
3) each pole gets a random combination of maternal and paternal
What happens in anaphase 2?
Centromeres divide and the spindle fibres shorten pulling chromatids to opposite poles
What happens in early telophase 2?
Chromosomes reach the poles
Why is variation important?
It produces selection pressures - evolution is based on it
To survive variation is essential
What is mitosis?
A type of cell division which produces genetically identical daughter cells with each cell containing the same number of chromosomes as the parent nucleus
What is mitosis needed for?
Growth
Cell replacement and regeneration
Asexual Reproduction
Describe processes in interphase in mitosis?
DNA replicates, chromosomes are not visible as chromatin is still dispersed
cell grows in size
organelles replicate
proteins (e.g. histones) and enzymes are synthesised
Describe prophase in mitosis?
chromosomes condense (2 chromatids held together by a centromere)
nucleolus and nuclear envelope disappear
centrioles move to opposite poles of the cell and start to form spindle fibres
Describe metaphase in mitosis?
chromosomes arrange themselves on the equator of the spindle
spindle fibres attach to the chromosomes by their centromeres
Describe anaphase in mitosis?
centromeres divide in two
spindle fibres shorten and pull the chromatids to the opposite poles of the cells, centromere first
chromatids now called daughter chromosomes
Describe telophase in mitosis?
daughter chromosomes reach the poles, then uncoil and lengthen (become chromatin)
nuclear envelope and nucleolus reform
2 new nuclei are formed
spindle fibres disintergrate
Describe cytokinesis in mitosis?
DNA appears as chromatin again as chromosomes have uncondensed
the division of the rest of the cell (cytoplasm and organelles) after the nucleus divides
in animal cells the cytoplasm pinches in
in plant cells a cell plate forms
What happens after mitosis and cytokinesis?
The cell returns to interphase and continues to grow and perform regular cell activities
What is cancer in terms of mitosis?
uncontrolled mitosis
- cancerous cells divide repeatedly forming a tumour (an irregular mass of cells)
What does cancer do to the body?
cancerous cells prevent normal cells/organs from functioning normally
What are cancers thought to be started by?
When changes occur in the genes that control cell division
What is meiosis?
cell division that occurs during sexual reproduction when it’s important that haploid gametes are produced
What does meiosis produce?
four genetically different cells from 2 consecutive divisions (gametes)
contain half the original number of chromosomes
Define diploid?
Cells or organisms that have two copies of each chromosome in their nuclei, has homologous pairs of chromosomes
Define haploid?
A cell or organism that has one set of chromosomes/one copy of each chromosome, doesn’t have homologous pairs of chromosomes
What is the diploid number in human?
46 (23 homologous pairs, one from mother and one from father)
What are homologous chromosomes?
pair of chromosomes that have the same genes at the same loci (1 maternal and 1 paternal)
What happens in interphase?
chromosomes are not visible
DNA replicates
Protein synthesis
ATP production
new organelles formed
mitochondria and chloroplasts replicate
nuclear envelope and nucleolus still present
What happens in early prophase I?
chromosomes condense from chromatin
each chromosome consists of 2 sister chromatids held together by a centromere
centrioles move to opposite poles of the cell
each homologous pair of chromosomes come together to form a bivalent
What happens in late prophase I?
crossing over takes place between the non-sister chromatids in the bivalent
the exchange of genetic material (alleles) between homologous chromosomes occurs at the chiasmata forming between chromatids
nuclear envelope and nucleolus disappear
What happens in metaphase I?
bivalents arrange themselves on the equator of the spindle
spindle fibres attach to centromere
chromosomes arrange themselves randomly arrange themselves (random/independent assortment)
What happens in anaphase I?
spindle attaches to the centromere of each homologous chromosomes and pulls whole chromosomes to opposite poles as spindle fibres shorten
What happens in telophase I?
(in animals cells) organelles and cytoplasm divide (cytokinesis)
two new haploid cells are formed (each cell has one copy of each homologous pair)
chromosome number per cell is halved
chromosomes are genetically different from those in the original cell
nuclear envelope reforms
plants cells go straight to meiosis II, cytokinesis doesn’t take place
What happens in prophase II?
centrioles replicate
new spindle fibres form at right angles to the first
nuclear envelope and nucleolus disappear
What happens in metaphase II?
chromosomes align themselves randomly on the equator of the (90º to original)
each chromosome is a pair of chromatids
this phase produces genetic variation due to the random assortment of chromatids on the equator
What happens in anaphase II?
centromeres divide
- chromatids pulled to opposite poles of the cell by the attached spindles
What happens in telophase II?
cytokinesis begins
four haploid cells result
each daughter cell produced in genetically different
nuclear envelope and nucleolus reappear
chromatids lengthen and become chromatin
Describe processes that happen to produce genetic variation
during independent assortment of homologous chromosomes, maternal and paternal chromosomes are mixed up
during sexual reproduction, there is a random fusion of haploid gametes to form zygote
during crossing over, parts of homologous chromosomes may be exchanged producing new allele combinations
Difference between mitosis and meiosis: number of nuclear divisions
mitosis: 1 nuclear division
meiosis: 2 nuclear division
Difference between mitosis and meiosis: number of cells formed
mitosis: 2 daughter cells formed
meiosis: 4 daughter cells formed
Difference between mitosis and meiosis: ploidy of parental cells/nuclei
mitosis & meiosis: 23 pairs of chromosomes (2n, diploid)
Difference between mitosis and meiosis: ploidy of daughter cells/nuclei
mitosis: diploid
meiosis: haploid (produces cells with half the original number of chromosomes)
Difference between mitosis and meiosis: genetic nature of daughter cells/nuclei
mitosis: genetically identical daughter cells formed
meiosis: genetically different daughter cells
Difference between mitosis and meiosis: pairing of homologous chromosomes
mitosis: no pairing of homologous chromosomes
meiosis: pairing of homologous chromosomes
Difference between mitosis and meiosis: crossing over
mitosis: no crossing over
meiosis: crossing over occurs
Difference between mitosis and meiosis: segregation of homologous chromosomes
mitosis: homologous pairs aren’t separated
meiosis: homologous pairs are separated
Where is DNA found and what is its function?
Found in the nucleus and controls all cell activities
What is chromatin?
Long and thread-like DNA in a non-dividing cell
What is a chromosome?
doubled, coiled, short DNA in a dividing cell
chromosomes only become visible when chromatin condenses prior to cell division
the 2 copies of the chromosome are called sister chromatids and are joined at the centromere
Why does chromatin condense?
DNA changes from chromatin to chromosome for more efficient division
What is a chromatid?
One of the 2 identical copies of a chromosome, joined at the centromere prior to cell division
What is the centromere?
Specialised region of a chromosome where 2 chromatids join and to which the microtubules of the spindle attach at cell division
What is meant by homologous?
The chromosomes in a homologous pair are identical in size and shape and they carry the same gene loci, with genes for the same characteristics.
One chromosome of each pair come from each parent, X and Y are not homologous
What are the 4 stages in the cell cycle?
Mitosis
G1 phase
S phase
G2 phase
What happens in G1 phase?
first growth phase
protein synthesis
cytoplasm increases
number of organelles increases
What happens in S phase?
DNA replicates if cell is going to divide
What happens in G2 phase?
second growth phase
- proteins necessary for cell division are synthesised
