Cell Division Flashcards
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
Before meiosis begins …
The cell will have been through INTERPHASE:
- Chromosomes are copied
- Organelles are copied
- Chromosomes are not visible as distinct structures
Prophase 1
- The chromosomes condense and become visible
- The homologous chromosomes link together.
- The chromatids of the two chromosomes wrap around each other , known as CROSSING OVER ~ alleles are exchanged
CHIASMATA ~ points where chromosomes are joined.
BIVALENT ~ name given to the attached pair of chromosomes
Metaphase 1
- The pairs of homologous chromosomes are now lined up on the EQUATOR of the spindle apparatus.
- INDEPENDENT ASSORTMENT of the the homologous chromosomes occurs.
Anaphase 1
- The homologous chromosomes SEPARATE
- This happens as the CHIASMATA BREAK
- NO separation of the centromere
Telophase 1
- The chromosomes have now reached the poles of the cell
- The nuclear membrane reforms
- The chromosomes uncoil back to their CHROMATIN STATE
Cytokinesis 1
- Forms TWO HAPLOID CELLS
- They no longer contain pairs of homologous chromosomes
Prophase 2 & Metaphase 2
PROPHASE 2
- The chromosomes CONDENSE and become visible again
- The nuclear membrane BREAKS DOWN
- SPINDLE FIBRES begin to DEVELOP
METAPHASE 2
- Chromosomes are lined up on the EQUATOR of the spindle apparatus
- INDEPENDENT ASSORTMENT of sister chromatids
Anaphase 2
- The CENTROMERE of each chromosome DIVIDES
- Spindle fibres SHORTEN
- The CHROMATIDS are pulled towards opposite POLES of the cell
Telophase 2 & Cytokinesis
- Chromatids reach the poles of the cell ~ now called CHROMOSOMES
- Nuclear membrane REFORMS
- Chromosomes UNCOIL back to their chromatin state
- Each cell undergoes CTOKENESIS to produce two HAPLOID cells
How to calculate:
The number of genetically different gametes produced by independent assortment
2^n
n= number of homologous chromosome pairs
In humans ~ 2^23
Stem cells & their sources
- UNSPECIALISED cells able to express all of its genes and divide by mitosis
- PLURIPOTENT ~ can form any cell type
SOURCES:
- Embryonic stem cells ~ present in early embryo
- Stem cells in umbilical-cord blood
- Adult stem cells ~ found in developed tissues
- Induced pluripotent stem cells ~ developed in laboratories
Stem cells in developmental biology
- Study how these cells develop to make particular cell types
- learn how each cell type functions and see what goes wrong when they are diseased
- Studying if they can find out if they can extend the capacity that embryos have for growth and tissue repair, into later life
Repair or replacement of damaged tissues using stem cells
BONE MARROW STEM CELLS ~ develop liver cells and to treat liver disease
iPS ~ programmed to become pancreatic beta cells to treat mice with type 1 diabetes
NERVOUS TISSUE ~ used to treat Alzheimer and Parkinson disease or to repair spinal-cord injuries
