1.6- Cell division Flashcards
How many chromosomes do humans have?
46 (23 pairs of homologous chromosomes)
When do chromosomes become visible?
when chromatin condenses prior to cell division, after each DNA molecule has replicated and made an exact copy of itself
What are chromatids?
when a single thread of DNA becomes 2 identical threads
What is the centromere?
the region where the sister chromatids join
How many daughter cells does mitosis produce?
2 (identical)
What is a cell cycle?
Dividing cells undergo a regular pattern of events
What are the 4 stages of mitosis?
Prophase
Metaphase
Anaphase
Telophase
What phase is crucial to the cell cycle?
interphase
What happens during interphase?
Replication of DNA
Replication of organelles which have their own DNA – (mitochondria and chloroplasts)
Making new organelles
Synthesis of ribosomal material
Synthesis of ATP.
Synthesis of proteins.
Increase in cell size
Where does mitosis take place in plants?
meristem
What is seen at interphase?
nothing
chromosomes have not condensed and the chromatids are not visible
What happens in prophase in mitosis?
Prophase is the first stage of mitosis
During prophase the DNA condenses (becomes shorter and thicker) forming chromosomes
Chromatids become visible. In animal cells the centrioles move to opposite poles of the cell
Protein microtubules form from each centriole and the spindle develops, extending from pole to pole. Towards the end of prophase the nuclear membrane disintegrates and the nucleolus disappears
Pairs of chromatids can clearly be seen lying free in the cytoplasm.
What happens in metaphase in mitosis?
During metaphase the chromosomes arrange themselves at the equator of the spindle
The chromosomes become attached to the spindle fibres at the centromere
Contraction of the spindle fibres draws the individual chromatids apart
the chromosomes are now maximally condensed
What happens in anaphase in mitosis?
Anaphase is a very rapid stage
The centromere splits and the spindle fibres contract
The chromatids separate and are pulled to opposite poles of the cell; the centromeres lead the way
What happens in telophase in mitosis?
Telophase is the final stage of mitosis
The chromatids have now reached the poles of the cells and are referred to as chromosomes again
The chromosomes uncoil and lengthen
The spindle breaks down
The nucleolus reappears and the nuclear membrane reforms
What is cytokinesis?
In animal cells cytokinesis occurs by constriction of the centre of the parent cell from the outside inwards (cytokinesis means cytoplasm splitting).
In plant cells, a cell plate forms across the equator of the parent cell from the centre outwards and a new cell wall is laid down
What is mitosis used for?
growth, the repair of tissues
replacement of dead or worn out cells
Asexual reproduction
What is an advantage of asexual reproduction?
the ability to increase in numbers quickly to take advantage of an ideal environment
What is an disadvantage of asexual reproduction?
the lack of genetic variation, leading to an inability to adapt if the environment changes
How do you calculate the mitotic index?
(No of cells in prophase + metaphase + anaphase + telophase/Total number of cells) x 100
What is uncontrolled mitosis?
cancer
What can damage/mutate cells?
radiation
viruses
some chemicals
How do tumours form?
Cancerous cells divide repeatedly, out of control
Where does meiosis take place?
reproductive organs
What does meiosis result in?
the formation of gametes with half the normal chromosome number; this is the haploid number
What happens during prophase 1?
first stage of meiosis
DNA condenses (becomes shorter and thicker) forming chromosomes
Chromatids become visible
In animal cells the centrioles move to opposite poles of the cell
Protein microtubules form from each centriole and the spindle develops, extending from pole to pole
Paternal and maternal chromosomes associate as homologous pairs (this process is called synapsis); each pair is called a bivalent
Towards the end of prophase the nuclear membrane disintegrates and the nucleolus disappears
What do bivalents consist of?
4 strands, consisting of 2 chromosomes, each with 2 chromatids
These chromatids wrap around each other and then partially repel each other, but remain joined at certain parts called chiasmata
What is crossing over?
The swapping of pieces of chromosomes
At the chiasmata the chromatids may break and recombine with a different but equivalent chromatid
Is crossing over a source of genetic variation?
yes
What happens during metaphase 1?
the homologous chromosomes arrange themselves randomly on the equator of the spindle
Chance determines how the homologous chromosomes are arranged on the equator
This random distribution and consequent independent assortment of chromosomes produces new genetic combinations
The homologs remain in their bivalent pairs at this stage
What happens during anaphase 1?
rapid stage
The spindle fibres contract
The chromosomes in each bivalent separate and are pulled to opposite poles of the cell
because of the random arrangement of homologous pairs at metaphase I this means that each pole has a random mixture of paternal and maternal chromosomes
What happens during telophase 1?
marks the end of the first meiotic division
The chromosomes have reached opposite poles
The nuclear envelope reforms around each group of haploid chromosomes
The chromosomes remain in their condensed form. In animal cells cytokinesis occurs after telophase I
Meiosis II follows on immediately
What happens during prophase 2?
begins with the 2 daughter cells produced by the first meiotic division
if there was a nuclear envelope formed, then it will begin to break down again during this stage
the centrioles have replicated and are moving toward the poles
again a spindle apparatus is growing outward from the centrioles, at right angles to the old spindle
What happens during metaphase 2?
the chromosomes line ups the equator/metaphse plate, with each chromosome attached to a spindle fibre by its centromere
independent assortment happens because the chromatids of the chromosomes can face wither pole
What happens during anaphase 2?
the centromeres divide and the spindle fibres shorten, pulling the sister chromatids to opposite poles
What happens during telophase 2?
the chromosomes reach opposite poles
cytokinesis occurs
the two cells produced by meiosis 1 divide to form 4 haploid daughter cells, and nuclear envelope forms
the spindle fibres disintegrate
meiosis is now complete
How many division in mitosis?
1
How many division in meiosis?
2
How many chromosomes are in the daughter cells of mitosis ?
same as parent cell
How many chromosomes are in the daughter cells of meiosis ?
half of parent cell
