Cell Division Flashcards
(37 cards)
The CELL CYCLE is divided into 2 main parts:
M (MITOTIC) PHASE :
- mitosis takes place
- the cell divides into two
- relatively short period
INTERPHASE:
- longer period
- the cell is carrying out the functions of that particular cell type
FIRST STAGE of interphase
G1 PHASE:
The cell replicates organelles ~ requires proteins so a high level of transcription & translation take place.
The size of the cell increases ~ ensures when the cell divides the two daughter cells are the correct size.
SECOND STAGE of interphase
S (SYNTHESIS) PHASE:
- All of the chromosomes are replicated
- The cell synthesises a great deal of DNA
THIRD STAGE of interphase
G2 PHASE:
- Cell replaces the energy stores used during the S phase.
- Cell continues to carry out transcription & translation.
- Any damaged chromosomes are REPAIRED.
- Cell continues to grow
M (mitotic) phase
MITOSIS:
- Refers to the DIVISION of the NUCLEUS.
- The chromosomes are separated into two nuclei.
CYTOKINESIS:
- Refers to the DIVISION of the CELL.
- Cell divides into two
G0 phase
Sometimes cells can EXIT the cell, either permanently or temporarily.
FULLY DIFFERENTIATED CELLS:
- Permanent
- e.g adult neurones
SENESCENT CELLS:
- Permanent
- e.g damaged DNA , cell can no longer divide
CELLS IN G0 TEMPORARILY:
- e.g B memory cells
- These can be triggered to re-enter the cell cycle during infection.
CHECKPOINTS in the cell cycle
- The cell cycle is very TIGHTLY REGULATED
- These checkpoint are important and if failed, the cell leaves the cell cycle.
G1 CHECKPOINT:
- Cell has grown to the correct size
- Any DNA damage
- cell enters S phase
G2 CHECKPOINT:
- Cell has grown to the correct size
- Ay DNA damage
- cell enters mitotic phase
METAPHASE/SPINDLE CHECKPOINT:
- Checks chromosomes are assembled correctly on the mitotic spindle.
- Cell completes mitosis and proceeds through CYTOKINESIS.
DURATION of cell cycle
- The whole cell cycle can take a long time
- Most Mammalian cells ~ 24 hours
- Other cells can take much longer ,e.g moths
MITOSIS
- Start with ONE DIPLOID cell and end up with TWO IDENTICAL DIPLOID CELLS.
- Takes place when we want TWO identical copies of a cell.
USES:
- Growth & repair
- Asexual reproduction
- Part of a larger process called the CELL CYCLE.
CHROMATIN
- During INTERPHASE, we cannot see chromosomes in the nucleus.
- They are still there , but NOT VISIBLE as distinct structures.
- Instead we see DARK MATERIAL called CHROMATIN.
- CHROMATIN ~ chromosomes have a loose, open structure so DNA is accessible for transcription and replication
CHROMOSOME REPLICATION during:
Interphase - mitosis
- A chromosome undergoes DNA replication to give TWO IDENTICAL molecules of DNA.
- These are called SISTER CHROMATIDS and are joined at a point called the CENTROMERE.
- This process happens for EVERY chromosome ~ in humans that means for 23 pairs.
- After interphase, the nucleus divides into two by mitosis.
Mitosis & the FIVE stages
A type of CELL DIVISION that forms TWO IDENTICAL diploid cells from ONE diploid cell.
5 Stages:
- Prophase
- Metaphase
- Anaphase
- Telophase
- Cytokinesis
PROPHASE - mitosis
- The chromosomes CONDENSE and become VISIBLE.
- Each chromosome consists of TWO SISTER CHROMATIDS joined at the CENTROMERE.
- The NUCLEOULUS DISAPPEARS
- NUCLEAR MEMBRANE begins to BREAK DOWN.
- A pair of CENTRIOLES move to either POLE of the cell.
- PROTEINS begin to form SPINDLE FIBRES which attach to the CENTROMERE.
- This causes the chromosomes to move to the CENTRE of the cell.
METAPHASE - mitosis
- The SPINDLE FIBRES completely form.
- Chromosomes are LINED UP along the EQUATOR of the cell.
ANAPHASE - mitosis
- The CENTROMERES DIVIDE into TWO.
- SPINDLE FIBRES begin to SHORTEN
- This PULLS the SISTER CHROMATIDS to OPPOSITE POLES of the cell.
TELOPHASE - mitosis
- The chromatids reach the POLES of the cell.
- Are now referred to as CHROMOSOMES again.
- The SPINDLE FIBRES BREAK DOWN
- The NUCLEAR MEMBRANE REFORMS
- Chromosomes UNCOIL back to their CHROMATIN state.
- NUCLEOLUS REAPPEARS.
CYTOKINESIS - mitosis
- The CENTRAL CELL MEBRANE is pulled INWARDS by the CYTOSKELETON.
- This creates a GROOVE or FURROW which gradually DEEPENS.
- Eventually the membranes FUSE to form TWO INDEPNDENT CELLS.
How does MITOSIS differ in PLANT CELLS.
- CENTRIOLES do not play a role in forming the spindle fibres as they are not present.
- FURROWS are not formed in the cell membrane during cytokinesis.
INSTEAD:
- VESICLES from the golgi apparatus, form membrane structures down the centre of the cell.
- These FUSE together to form a central cell membrane , dividing the CYTOPLASM into TWO.
- A new CELLULOSE WALL also forms down the centre.
Diploid Vs Haploid
Diploid ~ a cell with PAIRS of chromosomes.
Haploid ~ a cell with INDIVIDUAL chromosomes.
Meiosis KEY POINTS
Only takes place in SEX ORGANS to produce GAMETES:
- Mammals ~ sperm + eggs
- Plants ~ pollen + eggs
- One DIPLOID CELL forms four HAPLOID GAMETES.
- Type of REDUCTION DIVISION ~ the number of chromosomes halves
Why is it important that gametes are haploid cells?
- During fertilisation, gametes fuse together
- This forms a fertilised egg or ZYGOTE.
- As gametes are haploid, the zygote has the DIPLOID number of chromosomes.
- All gametes made by meiosis are GENETICALLY DIFFERENT.
Homologous pair of chromosomes
- One chromosome from FATHER and other from MOTHER.
- Both of these chromosomes have the exact SAME GENES
- However, you can inherit DIFFERENT ALLELES from each parent.
- Exchange of DNA in a homologous pair
- Parts of the chromatids can BREAK OFF and exchange between the homologous chromosomes.
- The exchange of DNA between maternal and paternal chromosomes produce RECOMBINENT CHROMOSOMES.
- Because they have exchanged DNA , they can exchange alleles.
What is the major source of GENETIC VARIATION in meiosis?
CROSSING OVER:
- The exchange of alleles
- The chiasmata can form at multiple points
- This means a large number of alleles can be exchanged.
INDEPENDENT ASSORTMENT:
- When homologous chromosome pairs line up on the equator in metaphase 1
- We cannot predict whether the paternal or maternal chromosome will end up in which gamete
FERTILISATION:
- Male and female gametes fuse RANDOMLY with each other
