cell cycle Flashcards
what is the cell cycle ?
- a highly ordered sequence of events that take place in a cell
what does the cell cycle result in ?
division of the cell
formation of 2 genetically identical daughter cells
in eukaryotic cells what are the 2 main phases of the cell cycle ?
- interphase
- miotic phase
what is interphase ?
long periods of growth and normal working separate divisions
which phase of the cell cycle does the cell spend most time in ?
interphase
what happens during interphase ?
- cell carries out all of its major functions
- produces enzymes / hormones
- preparing for cell division
- DNA replicated and checked for errors
- protein synthesis
- mitochondria grow and divide
- chloroplasts grow and divide
- normal metabolic processes of cells occur
what are the 3 stages of interphase ?
- G1
- S
- G2
what is G1 ?
- first growth phase
- proteins are produced
- organelles replicate
- cell increases in size
what is S ?
synthesis phase
Dna is replicated
what is G2 ?
- second growth phase
- cell increases in size
- energy stores are increased
- duplicated DNA is checked for errors
what is the miotic phase ?
- period of cell division
what are the 2 main stages of cell division ?
- mitosis
- cytokinesis
what happens during mitosis ?
nucleus divides
what happens during cytokenisis ?
cytoplasm divides and 2 cells are produced
what is G0 ?
- name given to the phase when the cell leaves the cycle either temporarily or permanently
why will the cell enter G0 ?
- differentiation
- DNA damaged
- cells become senescent
why does differentiation result in cells leaving the cell cycle ?
- when a cell becomes specialized it can no longer divide as it will carry out its function indefinitely and won’t enter the cycle again
if a cells DNA becomes damaged why can’t it enter the cycle ?
- it is no longer viable as it is damaged so enters a period of permanent cell arrest
why do senescent cells no longer enter the cell cycle ?
- it can no longer divide as cells generally only divide a number of times before this happens
as you age what happens to the number of senescent cells in your body ?
it increases
due to age related diseases such as cancer and arthritis
give an example of cells which can enter G0 but are stimulated to go back into the cell cycle and start dividing again ?
lymphocytes during an immune response
what is controlled in the cell cycle ?
- to ensure a cell only divides when :
- it’s the right size
- replicated DNA is error free
- chromosomes are in correct positions
why is the cell cycle controlled ?
- to ensure that 2 identical daughter cells are created from the parent cell
what are checkpoints ?
control mechanisms of the cell cycle
what do checkpoints do ?
- monitor and verify whether the processes at each phase of the cell cycle have been accurately completed before the cell can progress onto the next phase
where do checkpoints occur ?
- G1
- G2
- Spindle assembly checkpoint
what happens at the G1 checkpoint ?
- end of G1 phase, before S phase
- if cell satisfied requirements it is triggered to begin dna replication
- if not it enters G0 resting state
what happens at G2 checkpoint ?
- end of G2 phase before mitosis
- checks if DNA is replicated without error
- if passed, proceeds to begin mitosis
what happens at the spindle assembly checkpoint (metaphase checkpoint) ?
- at point in mitosis when all chromosomes should be attached to spindles and have aligned
- mitosis won’t proceed until this has passed
which enzymes brings about the passing of a cell cycle checkpoint ?
kinases
what is kinases ?
a class of enzyme which catalyze the addition of a phosphate group to a protein (phosphorylation)
what does phosphorylation caused by kinases do to checkpoint proteins ?
- changes their tertiary structure, activating them at certain points if the cell cycle
how is kinases involved in cell cycle regulation activated ?
- by binding to a variety of checkpoint proteins called cyclins
what are cyclins ?
- checkpoint proteins
what happens when the correct cyclin binds to the appropriate kinase ?
a cyclin dependent kinase complex forms
what are cyclin dependent kinase complexes activated by ?
enzymes
why is it important that CDK complexes catalyze the activation of cell cycle proteins (phosphorylation) ?
- to ensure a cell progresses through the different phases of its cycle at the appropriate times
how is a cell signaled to move into the next stage of the cycle ?
- different enzymes break down cyclins when they aren’t needed
how is cancer caused ?
uncontrolled cell division
what is a tumour ?
an abnormal mass of cells
what are the 2 types of tumors ?
benign
malignant
what are benign tumors ?
tumors which stop growing and don’t travel to other locations in the body
what is a malignant tumor ?
a tumor which continues to grow unchecked and uncontrolled
which type of tumor is the basis of cancer ?
malignant
what are tumors the result of ?
- damage or spontaneous mutation of genes which encode the proteins that are involved in regulating the cell cycle
this includes checkpoint proteins
give an example as to how cancer could from via checkpoint proteins ?
- if over expression of a cyclin gene results from mutation the large quantity of cyclins produced would disrupt the regulation of the cell cycle
- results in uncontrolled cell division
- cancer
how can CDK be used to treat cancer linked to checkpoint proteins ?
- they can be used as a target for chemical inhibitors
- if the activity of CDKs can be reduced if may stop cell division and cancer formation
what does mitosis actually refer to ?
nuclear division
what does mitosis ensure ?
- both daughter cells produced when a parent cell divides are genetically identical
after mitosis what should be produced ?
- 2 new daughter cells which have an exact copy of the DnA present in the parent cell and the same number of chromosomes
when is mitosis necessary ?
- when daughter cells must be identical
when must the daughter cells be identical ?
- growth
- replacement and repair of tissues in multicellular organisms
- asexual production
give examples of multicellular organisms who need help with tissue repair via mitosis
fungi
plants
animals
what is asexual reproduction ?
production of genetically identical offspring from one parent
give examples as to who uses asexual reproduction ?
- plants
- fungi
- animals
- amoeba species
how do prokaryotic organisms, eg bacteria reproduce ?
- asexually by binary fission
before mitosis DNA in nucleus is replicated during interphase. What is each DNA molecule converted into ?
- 2 identical dna molecules called
CHROMATIDS
how are the 2 chromatids joined together ?
- at a region called the centrometer
why is it necessary to keep the chromatids together during mitosis ?
- so they can be precisely manouvered and segregated equally one into the 2 new daughter cells
how many stages of mitosis are there ?
4
what are the 4 stages of mitosis called ?
prophase
metaphase
anaphase
telophase
how can the phases of mitosis be indentified ?
light microscope
give an example as to how dividing cells can be obtained ?
growing root tips of plants
how can the root tips of plant cells
be separated ?
such a chemical
how can root tips of plant cells be viewed on a microscope ?
- cells can be squashed to form a single layer of cells on a microscope slide
- stains which bind to dna make chromosomes clearly visible
what happens during prophase ?
- chromatin fibres coil and condense to form chromosomes
- the nucleolus disappears
- nuclear membrane begins to break down
what are are chromatin fibers made up of ?
- various proteins
rna
dna
what is the nucleolus ?
an area of the nucleus responsible for rna synthesis
what do protein microtubules form in prophase ?
- spindle shaped structures
what are the spindle shaped structures necessary for ?
- linking the poles of the cell
- moving the chromosomes into the correct positions before division
what happens to centrioles in animal and plant cells, during pro phase ?
- migrate to opposite poles of the cell
what are centrioles ?
- cylindrical bundles of proteins that help in the formation of spindle
what do spindle fibers do in prophase ?
- attach to specific areas on centrometers and start to move the chromosomes to the center of the cell
by the end of prophase what happens to the nuclear envelope ?
it has disappeared
what happens during metaphase in mitosis ?
- chromosomes are moved by spindle fibers to form a plane in the center of the cell and are held in position
what is the metaphase plate ?
a plane in the center of the cell
what happens during anaphase ?
- centrometers holding together the pairs of chromatids in each chromosome divide during anaphase
- chromatids are separated and pulled to opposite poles of the cell
what happens during telophase ?
- chromatids have reached the poles and become chromosomes again
- 2 sets of chromosomes assemble at each poll
- nuclear envelope reforms
- chromosomes start to uncoil and the nucleolus forms
- cell division / cytokenisis begins
what is cytokenisis ?
- actual division of the cell into 2 separate cells
what is cytokenisis ?
- actual division of the cell into 2 separate cells
when does cytokenisis begin ?
telophase
during cytokenisis what happens to animal cells ?
- cleavage furrow forms around middle of cell
- cell surface membrane is pulled inwards by the cytoskeleton until it’s close enough to fuse around the middle forming 2 cells
are plant cells able to form a cleavage furrow ?
no as they have cell walls
how does cytokenisis happen in plants ?
- vesicles from golgi apparatus assemble where the metaphase plate was formed
- vesicles fuse with each other and cell surface membrane dividing cell in 2
what happens to plant cells after they have divided in 2 and have completed cytokenisis ?
new sections of cell wall form along the new sections of membranes
what would happen if the dividing cell wall began to form before the daughter cells separated ?
osmotic lysis would occur
normal cells have 2 chromosomes of each type, what is this termed as ?
diploid
cells have 2 chromosomes of each type, where do these chromosomes come from ?
1 inherited from each parent
what are sex cells known as ?
gametes
what happens in sexual reproduction ?
2 gametes, one from each parent, fuse to produce a fertilised egg
what is the fertilised egg known as ?
zygote
what is the origin of all the cells that an organism develops ?
zygote
why must gametes only contain half the standard number of chromosomes in a cell ?
chromosome number of an organism would double with every round of reproduction
what form of cell division forms gametes ?
meiosis
how many daughter cells are produced in meiosis ?
4
how many times does the nucleus divide in meiosis ?
twice
how many chromosomes does a gamete contain after meiosis ?
- half the chromosome number of the parent cell - it is haploid
what is meiosis known as ?
reduction division
how are characteristics for organisms coded ?
by 2 copies of each gene, 1 from each parent
how many sets of genes does does the nucleus of the organisms cells contain ?
2 full sets
what are homologous chromosomes ?
matching sets of chromosomes - diploid
what are alleles ?
different versions of the same gene
what are alleles also known as ?
genetic variants
give an example of an allele ?
genes for eye colour are still the same type as they code for the same thing but the colour can be different, meaning there are different versions of that gene
how to homologous chromosomes appear ?
- same length
- same size
- centrometers in same positions
- as they have the same genes
what happens in meiosis I ?
- first division:
- reduction division
- pairs of chromosomes are separated into 2 cells
what is produced from meiosis I ?
- each cell will contain 1 full set of genes instead of 2 = haploid
what happens during meiosis II ?
- similar to mitosis
- pairs of chromatids present in each daughter cells are separated forming 2 more cells
what is the result of meiosis II ?
4 haploid daughter cells are produced
what happens during prophase I (meiosis)
- chromosomes condense
- nuclear envolope disintegrates
- nucleolus disappears
- spindle formation begins
what happens to the chromosomes in prophase I which doesn’t happen in mitosis ?
- homologous chromosomes pair up
- forming bivalents
- crossing over occurs
what is it called when homologous chromosomes pair up ?
bivalents
how does crossing over take place in prophase I ?
- chromosomes are large molecules and moving through the cytoplasm as they are brought together results in the chromatids entangling
what happens during metaphase I of meiosis ?
- same as in mitosis
- homologous pairs of chromosomes assemble along metaphase plate instead of individual chromosomes
what is independent assortment ?
- when the orientation of each homologous pair on the metaphase plate is random which means the maternal or paternal chromosomes can end up facing either pole
when does independent assortment occur ?
metaphase I in meiosis
what can independent assortment lead to ?
- different combinations of alleles facing the poles
- genetic variation
what happens during anaphase I of meiosis ?
- homologous chromosomes are pulled to opposite poles, and the chromatids stay joined together
where can an exchange of dna occur during anaphase I of meiosis ?
- sections of sister chromatids which become entangled during crossing over, break off and rejoin
what is chiasmata ?
the point at which chromatids break off and rejoin
when sister chromatids break off an rejoin, and exchange occurs, what is this called ?
recombinant chromatids
what happens with recombinant chromatids ?
genes are exchanged between chromatids
how does genetic variation occur at anaphase I of meiosis ?
- genes exchanged in recombinant chromatids may be different alleles of the same gene
- combination of alleles on recombinant chromatids are different from allele combination on original chromatids
causing genetic variation as sister chromatids are no longer identical
what happens during telophase I of meiosis ?
- similar to mitosis
- chromosomes assemble at each pole and nuclear membrane reforms
- chromosomes uncoil
- cell undergos cytokenisis and divides into 2 cells
what is produced at the end of meiosis I ?
2 haploid daughter cells
what happens during prophase II (meiosis II)
- chromosomes (still consisting of 2 chromatids) condense and become visible
- nuclear envelope breaks down
- spindle formation begins
what happens during metaphase II ?
- individual chromosomes assemble on metaphase plate
why aren’t chromatids identical at metaphase II stage of meiosis ?
crossing over
what is produced again in metaphase II ?
- independent assortment
- genetic variation
what happens in anaphase II ?
- chromatids of individual chromosomes are pulled to opposite poles after division of the centrometers
what happens during telophase II ?
- chromatids assemble at poles
- chromosomes uncoil and form chromatin again
- nuclear envelope reforms
- nucleolus becomes visible
- cytokenisis
what is formed by cytokenisis from telophase II of meiosis ?
4 haploid daughter cells
why will the cells be haploid (meiosis)
reduction division
what will the 4 haploid daughter cells produced from meiosis be genetically different to ?
each other and from parent cells
why will the 4 haploid daughter cells produced from meiosis be different from each other and parent cells ?
crossing over
independent assortment
what is mitosis important for ?
- growth
- repair
- asexual reproduction
- development of embryos
why is meiosis important ?
- creates genetic variation
- ensures offspring has correct number of chromosomes
