17 - cell cycle Flashcards
what does the cell cycle involve?
cells growing, performing routine activities needed to survive, and dividing to create new cells.
cells arise only by the division of existing cells, why os this important?
- cell division is essential to all life
- it enables multi-cellular organisms to grow and repair damage
- is the basis of reproduction in every organism.
what are chromosomes?
structures composed of DNA and proteins that carries the genetic information of an organism:
- provide continuity between one generation and the next.
when do chromosomes become clearly visible?
they become clearly visible when the chromatin condenses during the cell cycle.
if the chromosomes are separated out, they can be arranges into matching pairs, according to what?
size, position of the centromeres and banding pattern
what are pairs of chromosomes called?
homologous pairs
the results are referred to as a karyotype.
how many chromosomes do human cells usually have?
46 chromosomes:
- 22 homologous pairs of autosomal chromosomes (autosomes)
- 1 pair of sex chromosomes (allosomes)
where do the chromosomes within the pair come from?
one chromosome in each pair comes from the individuals mother, the other from the father.
what are cells that have the normal two sets of chromosomes called?
diploid. (2n)
haploid cells contain a single copy (n)
…
on copies of the same chromosome, with the same banding pattern, the same gene will be at the same locus within the chromosome but possibly different alleles.
what is a centromere?
chromosomes have a centromere which keeps the pairs aligned and acts as an attachment site during cell division.
- short arms - ‘p arm’
- long arms - ‘q arm’
it is the point where mitotic spindle attached during prometaphate.
what are Idiograms?
When a chromosome is stained, is appears with a characteristic pattern of light and dark bands.
Idiograms are used to describe the location of genes on each chromosome.
what is mitosis?
Before dividing, the cell must duplicate its components — most importantly,
its genome — so that it can split into two complete daughter cells.
There are two forms of nuclear division: mitosis and meiosis
In mitosis, there is one nuclear division stage resulting in two identical nuclei
Mitosis ensures that each successive cellular generation has the same genetic
information as the previous generation
(n.b. unless some form of mutation occurs)
what are the new cells from mitosis used for?
growth, maintenance and repair of a cellular organsim
what is a chromatid?
a longitudinal DNA subunit of a chromosome. The two
chromatids of a duplicated chromosome are held together at a region of
DNA called the centromere.
what is meiosis?
Meiosis is the basis of sexual reproduction.
- gametes have only a single set of chromosomes e.g. they have half the number of chromosomes of somatic cells
- meiosis produces these haploid (n) gametes.
in meiosis the nucleus divides twice, producing 4 haploid nuclei.
the haploid gametes produced by meiosis fuse during fertilisation and create a diploid cell.
meiosis involves ‘recombination’ during which homologous chromosomes can exchange genetic material with one another.
what is the cell cycle?
the cell cycle is an organised sequence of events that occurs from one cell division to the next in a proliferating cell.
what are the three main stages of the cell cycle?
- interphase
- mitosis
- cell division (or cytokinesis)
what must the cell pass through that ensure conditions are favourable for division?
checkpoints
what happens during interphase?
DNA replicates; RNA is synthesised constantly/proteins actively produced as the cell grows; and the centrioles duplicate.
how long does interphase typically last in mammalian cells?
20-24 hours
what three stages can interphase be divided into?
- G1 (gap 1)
- S (synthesis)
- G2 (gap 2)
cells may also enter a resting phase, G0.
what happens during the G0 phase?
a cell may leave the cycle and stop proliferating.
the cell will continue to perform all other functions associated with its cell type. this may be a temporary resting period or more permanent.
some mature cells (e.g. skeletal muscles, neurons) remain in G0 indefinitely.
what happens in the G1 phase?
cell increase in size in G1 and actively synthesise proteins required for growth and subsequent division.
what is the S phase?
To produce two daughter cells, the complete set of genetic information
in the cell must be duplicated. DNA replication occurs during this synthesis phase.
what happens during the G2 phase?
during the gap between DNA synthesis and mitosis, the cell will continue to grow and produce new proteins.
when does G1 begin?
when the cell senses growth factors or other mitogens.
what checkpoint is there during G1?
During G1, a critical checkpoint ensures environmental conditions are favourable for replication.
- If conditions are not favourable, the cell may enter G0
what does the cell cross in G1 that is the point of no return?
The cell crosses a restriction point ca. 8-10 hours into G1
- this is the point of no return, the cell commits to divide or die
what do checkpoints in G2 do?
At end of G2, checkpoint to ensure DNA is structurally intact & properly replicated
- The cell may pause at this point to allow time for DNA repair, if necessary
what do cyclin-dependant Kinases (CDKs) do?
The main proteins involved in cell cycle regulation are a family of proteins known as cyclins and their associated cyclin-dependent kinases (CDKs):
various combinations of these two components regulate orderly progression
through the cell cycle
They signal by phosphorylating other proteins e.g. controlling the activity of various transcription factors.
Their concentrations vary throughout cell cycle.
Hartwell, Hunt and Nurse received the 2001 Nobel Prize for their description of
the cyclin and cyclin-dependent kinase regulatory mechanisms
when does nuclear division take place?
during mitosis (M phase)
what five stages is mitosis divided into?
prophase (when the chromosome condenses)
prometaphase (when the nuclear envelope breaks down)
metaphase (when the chromosomes are lined up along the midline of the cell)
anaphase (when the chromosomes separate)
telophase (when the chromosomes are encapsulated into two new nuclei)
what are the two types of chromatin?
heterochromatin - tightly packed, not very active in transcription.
euchromatin - less tightly packed, gene rich and usually active in transcription.
explain prophase
begins when the chromatin starts condensing into chromosomes, which become visible.
each chromosome structure comprises two sister chromatids, attached at the centromere, with the same genetic information.
how do microtubules of the cytoskeleton disassemble in prophase?
- the building blocks of these microtubules are used to grow new microtubules called spindle fibres that extend between the pair of centrosomes (interpolar microtubules)
- other microtubules attach to the plasma membrane (aster microtubules) or radiate outwards into the cytoplasm.
explain prometaphase
in this stage the nuclear envelope breaks down so there is no longer a recognisable nucleus.
some mitotic spindle fibres (kinetochore microtubules) elongating from the centrosomes attach to kinetochores:
- kinetochores are protein structures that associate with the centromere on each sister chromatid
other elongating spindle fibres, instead of attaching to chromosomes, overlap each other at the centre of the cell.
what happens during metaphase?
begins as the chromosomes move to a narrow central zone at the equator of the cell (the metaphase plate)
- orchestrated by mechanical forces applied via the spindle fibres.
explain anaphase
the centromere splits and the kinetochore microtubules shorten pulling the individual chromatids apart.
- the astral microtubules and the interpolar microtubules pull the poles further apart and exert additional pull on the chromosomes.
explain telophase
Daughter chromosomes arrive at the poles and nuclear membranes reform. The chromosomes uncoil and the spindle fibres that
have pulled them apart disassemble.
explain cytokinesis
Spindle fibers that were not attached to chromosomes begin breaking down until only the portion of overlap is left. A contractile ring is formed and cleaves the cell into two daughter cells.
- microtubules then reorganise into a new cytoskeleton for the return to interphase.
what are telomeres?
repetitive sequences of DNA are found at the end of chromosomes.
telomeres shorten with each cell division.
- once its telomeres reach a critical length, a cell can no longer divide.
- in cancer and stem cells, telomerase repairs telomerase and cells do not stop dividing.
