ocr Biology - 2.6 Cell Division

OCR, A level, Biology, 2.6, Chapter 6, Module 2, Cell, Division, A, Cell cycle, Mitosis, Meiosis, Organisation, specialisation, stem cells

the cell cycle

series of events that take place when one parent cells divide into two daughter cells

name the stages in the cell cycle

interphase, mitosis, cytokinesis

cytokinesis

division of the cytoplasm following nuclear division, resulting in two new daughter cells

interphase

phase of the cell cycle where the cell is not divided. divided into growth and synthesis phases (g1, s, g2)

mitosis

a type of nuclear division that produces daughter cells which are genetically identical to each other and the parent cell

• cell growth stops

• PMAT nuclear division

• cytokinesis

what does the length of the cell cycle depend on?

type of cell and availability of nutrients

how long is the average human cell cycle?

24 hours

what happens in G1?

• cells grow and increase in size

• transcription of genes to produce RNA occurs

• organelles replicate

• checkpoint at end ensures the cell is ready for S phase, otherwise enters G0

what happens in G0?

• cell leaves cycle permanently or temporarily and is no longer able to divide

• due to either differentiation, DNA damage or senescence

• some cells may undergo apoptosis

apoptosis

programmed cell death

Senescence

• process of deterioration with age

what happens during the synthesis (S) phase?

• DNA replicates

• gene replication is in order of genes which are active in all types of cell to fewer types of cell

• when all chromosomes have been duplicated, each one consists of a pair of identical sister chromatids

why is the S phase rapid?

exposed DNA base pairs are more susceptible to mutagens, so it reduces the chances of random mutations

what happens during G2?

Cell continues to grow

Energy stores are duplicated

DNA is checked by kinases

At the end of G2, DNA must have no errors

Proteins trigger formation of spindle

End of G1 checkpoint

most important as it checks for:

• cell size

• nutrients

• growth factors

• DNA damage

S phase checkpoint

ensures genes are expressed in a specific order

end of G2 checkpoint

• check cell size

• check DNA replication

• check DNA damage

• triggers condensation of DNA

metaphase checkpoint

• spindle assembly

• check if all chromosomes are attached to spindle

why have checkpoints?

• prevent uncontrolled cell division

• to detect and repair DNA damage

• keep cell cycle progressing in one direction

• ensures DNA is only replicated once during each cell cycle

where does mitosis occur in plant cells?

meristems (roots and shoots)

importance of mitosis

• growth of organisms

• repair of tissues

• asexual reproduction

early prophase

early:

• chromosomes become more distinct (coil up, shorten, thicken, take up stain more intensely)

• centriole divides

• nucleolus becomes less prominent

late prophase

• chromosomes are seen as 2 chromatids joined by a centromere

• centrioles migrate to opposite poles of the cell

• nucleolus continues to shrink and disappears

• nuclear envelope disintegrates

• centrioles produce spindle fibres which attach to the centromere of chromosomes

metaphase

• each centromere is attached to both poles

• chromosomes pulled to the equator of the cell

anaphase

• spindle fibres shorten

• centromere divides

• chromatids are pulled to opposite poles of the cell by centromere first

• each half of the cell receives one chromatid from each chromosome

telophase

• chromatids reach the poles of the spindle, begin to uncoil and become less distinct

• nuclear envelope starts to reform

• chromatids are now called daughter chromosomes

what happens as the cell divides (cytokinesis) in plant and animal cells

• animal cells starts by invagination of the edges of the cell

• plant cells a cell plate is formed within the cell where the spindle was then a new cell membrane and wall are laid down

meiosis

type of nuclear division resulting in the formation of cells containing half the number of chromosomes of the parent cell

loci

position of a gene on a chromosome

chromosome

a linear DNA molecule wrapped around histone proteins in the nucleus

fertilisation

the fusion of male and female gamete nuclei

diploid

a cell that has two complete sets of chromosomes - 2n

haploid

a cell that has only one set of chromosomes - n

zygote

cell produced after fertilisation of two gametes during sexual reproduction

homologous chromosomes

matching chromosomes containing the genes at the same places (loci). may contain different alleles

what happens during fertilisation (meiosis)

• fusing of two cell nuclei

• each cell contains half the genetic material

• restores the diploid number in gamete

• introduces variation

what happens in prophase 1

• condensation of chromatin : shorten, thicken, take up stain more intensely, become visible

• homologous chromosomes pair up and crossing over may occur

• centriole divides and starts migrating to opposite poles of the cell

• nuclear envelope and nucleolus disappear

• spindle forms at end (made of microtubules)

what is crossing over

two non-sister chromatids wrap around each other and may swap sections so that the alleles are shuffled. this happens during prophase 1

what happens before meiosis occurs?

• DNA is replicated in S phase of interphase

• each chromosome consists of 2 sister chromatids

what happens in metaphase 1

• pairs of homologous chromosomes (bivalents) randomly attach themselves at the equator of the spindle by their centromere (independent assortment)

• pairs of homologous chromosomes face opposite poles

what happens during anaphase 1

• spindle fibres shorten and separate whole chromosomes

• centromeres do not divide

• the crossed over pairs of chromosomes separate (resulting in allele shuffling)

what happens during telophase 1

• chromosomes reach the end of the spindle

• following telophase, in animal cells, the nucleolus and envelope reforms, the cell divides by cytokinesis. each cell has half the original number of chromosomes

• in plant cells, it often jumps from A1 to P2

what happens during prophase 2

• centriole divides

• chromosomes coil and condense

• nucleolus and nuclear envelope disappear

• spindle forms