module 2.6: cell division and specialisation Flashcards
what happens in the G1 phase
cells grow and increase in size
transcription of genes (mRNA made)
protein synthesis occurs
organelles duplicate
what happens in the G1 checkpoints
G1 cyclin-CDK complexes promote the production of transcription factors needed to produce enzymes for DNA replication
G1 checkpoint ensures that the cell is ready for DNA synthesis
what happens in S phase
DNA replicates, producing pairs of identical sister chromatids
what happens in the S checkpoints
active S cyclin-CDK complexes ensure all DNA is replicated once
what happens in G2 phase
cells grow
spindle fibres begin to form
growth of organellels
short gap before mitosis
what happens at the G2 checkpoint
G2 checkpoint ensures that the cell is ready to enter M phase
what happens in the M phase
cell growth stops
nuclear divisiom consisting of stages: prophase, metaphase, anaphase and telophase
what are the M checkpoint
mitiotic cyclin-CDK complexes promote the production of the spindle and the condensation of chromosomes
metaphase checkpoint ensure that the cell is ready to complete mitosis
what are the reasons for mitosis
growth of the organism
repair of tissues
replacement of old cells
asexual reproduction
what happens in prophase
nuclear envelope breaks down
chromosomes condenses
spindle fibres attach the centromere on the chromosomes
what happens in metaphase
chromosomes are held on the spindle at the middle of the cell
each chromosome is attached to the spindle on either side of its centromere
what happens in anaphase
chromatids break apart at the centromere and are moved to opposite ends of the cell by the spindle
what happens in telophase
nuclear envelopes reform around the chromatids that have reached the 2 poles of the cell
each new nucleus has the same number of chromosomes as the original, parent cell
the nuclei are genetically identical to each other
chromosomes uncoil
cell surface membrane undergoes cytokinesis
spindle fibres break down
what is meiosis
It produces four cells that are:
not genetically identical
gametes - sex cells used for sexual reproduction
haploid (contain half the normal number of chromosomes)
what do these gametes contain one of
they contain one chromosome from each pair of homologous chromosomes
what is a homologous chromosomes
they have:
the same shape and size
the centromere in the same position
the same genes in the same positions on the chromosomes
what happens in prophase I
- chromosomes condense — they supercoil to become shorter and thicker
- homologous chromosomes pair to form bivalents containing four chromatids
- the chromatids in each bivalent break and rejoin to form chiasmata or crossovers — this is where sections of the non-sister chromatids can be exchanged
- the nuclear membrane breaks up to form small membrane sacs
- the centriole replicates and migrates to opposite poles of the cell and forms the spindle fibres
what happens in metaphase I
- microtubules attach from the centrioles to the centromere of each chromosome.
- the bivalents move to the equator of the cell.
- orientation of each bivalent on the equator is random — maternal or paternal chromosomes could be facing either pole
what happens in anaphase I
- the microtubules shorten to separate the homologous chromosomes and pull them towards opposite poles
- each chromosome still consists of two chromatids
what happens in telophase I
- the chromosomes reach opposite poles
- the nuclear membrane reforms around each set of chromosomes to produce two nuclei
- these nuclei are haploid as they have one chromosome from each homologous pair (but there are still two sister chromatids)
- the cell membrane pinches in to form two cells — this is cytokinesis
what happens in prophase II
- the nuclear membranes break up again
- the centrioles replicate again and migrate to opposite poles of the two new cells
- chromosomes condense
- spindle fibres develop
what happens in metaphase II
- microtubules attach between the centrioles and the centromere of each chromosome
- the chromosomes move to the equator and align randomly
- nuclear envelope disappear
what happens in anaphase II
- sister chromatids move to opposite poles
what happens in telophase II
- the nuclear membranes reform
- cytokinesis occurs to produce four genetically different haploid cells