1.6 Cell Division Flashcards
How do you form chromosome?
-the DNA molecule winds around proteins called histones to form nucleosomes.
-they then supercoil to form the dense chromosome structure.
-this replicates before the cell divides to make a double stranded chromatid.
How many chromosomes in a typical human cell?
-In a typical human cell there are 46, but each species has a different chromosome number.
What is a karyotype?
a photograph of the chromosomes in a cell
3 main phases of the cell cycle
1- interphase
2- mitosis
3- cytokinesis
interphase is further split into 3 phases:
-G1 (gap 1)
-S phase (synthesis)
-G2 (gap 2)
G1 phase
-The cell increases in size and develops
-Normal cell processes occur
-Cell makes proteins needed for cell division
-ATP is produced for the energy intensive process of cell division about to occur
S phase
-The genetic material is copied
-Chromosomes replicate and become double stranded chromatids
G2 phase
-New organelles are made e.g. new mitochondria or chloroplasts
-More cytoplasm is produced as the cell grows further ready for division
-DNA is checked for errors
Cyclins
-Cyclins are a groups of regulatory proteins that control the progression of the cell cycle
-Different cyclins control when each next stage of the cell cycle occurs.
-Cyclins bind to enzymes called cyclin-dependent kinases.
-These then become active.
-They cause phosphate groups to attach to proteins.
-These proteins then become active and carry out tasks specific to one of the phases of the cell cycle.
How many types of cyclins are there?
4 main types = D,E,A,B
-unless these reach a THRESHOLD CONCENTRATION, the cell does not progress to the next stage
When does the DNA molecule number change?
-The number of DNA molecules increase only when DNA replicates, i.e. only in S phase
-The number of DNA molecules decrease only when the cell divides – so after cytokinesis, NOT in Telophase
Mitosis is split into 4 stages:
Prophase
Metaphase
Anaphase
Telophase
Prophase
-In prophase the centrioles begin to move to opposite poles.
-The nucleolus breaks down followed later by the nuclear envelope.
-The duplicated chromosomes are clearly visible with the two sister chromatids joined by a centromere.
-Spindle starts to form
Metaphase
-The centrioles form the spindle which is made from microtubules.
-The microtubules attach to the centromere of each chromatid pair.
-They push and pull them so that they line up along the equator (metaphase plate) of the cell.
Spindle
-The spindle fibres are polymers made from tubulin monomers.
-The removal of tubulin monomers causes the spindle fibres to shorten
Anaphase
-The centromeres split and the sister chromatids become individual chromosomes again.
-They are pulled centromere first towards opposite poles.
-This is done by the microtubule fibres of the spindle contracting and shortening which uses up the ATP stored in interphase.
Telophase
-The spindle starts to break down.
-Nuclear envelopes and nucleoli reform.
-Chromosomes become less dense.
Cytokinesis
After PMAT the cell has 2 complete nuclei but is sharing the cytoplasm so has to go through cytokinesis
Cytokinesis in animal cells
-In animal cells the spindle degenerates at telophase
-In animal cells the plasma membrane is pulled inwards around the equator of the cell to form a cleavage furrow .
-This is done using a contractile ring of protein made up of actin and myosin fibres.
Cytokinesis in plant cells
-Plant cells lack centrioles so although they have a spindle it is not generated by centrioles.
-In plant cells the spindle remains as a new cell wall is formed across the equator of the cell.
-A middle lamella or cell plate forms first and the cellulose is deposited here by both of the cells on either side to form the cell walls.
Chemotherapy drugs
Because cancer is uncontrolled cell division – many chemotherapy drugs target a phase in the cell cycle to interfere with it.
Preparing a root tip squash
A dye called acetic orcein can be used to stain chromosomes in dividing cells such as the meristem at the tip of a root in an onion.
Meristems
-Regions of growth – and therefore rapid cell division (mitosis).
-They are found at the apices (tips) of the shoot and root
-There is also another meristematic region that enables the plant to grow outwards in width, this is called the cambium
What is mitosis used for?
- asexual reproduction
- growth
- repair
Asexual reproduction
-Asexual reproduction involves one parent, there are no males and no females.
-The one individual can reproduce itself exactly.
-They are genetically identical and therefore clones.
-It is very fast and simple as the cells just divide by mitosis to achieve it.
-However, as all the offspring are identical there is no variation which does make them vulnerable to changes in the environment.
Ways of achieving asexual reproduction
- Budding – A small bud develops on the side of parent organism. Once the new nucleus moves into it, it breaks off and grows to normal size. Example: Yeast which are single celled fungi
-Binary Fission – Carried out by bacteria, the parent undergoes mitosis and splits down the middle to make two even sized offspring . - Sporulation – Fungi and plants such as moss reproduce by spraying out asexual spores. These travel in the air and wind and will develop into a full organism .
- Regeneration – Some organisms like this axolotl can replace whole body parts other organisms like starfish can grow new adults from fragments of their original body this is called fragmentation.
- Vegetative reproduction – This occurs in flowering plants in various ways. A new adult plant grows from part of the original plant. Examples include runners like this strawberry plant, bulbs, tubers like in potatoes, suckers and corms
Growth
-Growth is the permanent increase in number, size or mass of cells and the best way to do that is by mitosis.
-You started life as one cell, a zygote. You are now billions of cells all produced from that one first cell by mitosis.
-Organisms grow in different ways, animals stop at maturity, plants continue throughout life in regions called meristems.
Repair
-Even if you are fully grown your cells are still dividing by mitosis for repair – repeated cell renewal.
-You lose cells all the time, in the next 60 seconds for example you will shed about 40,000 skin cells. These need replacing.
-Tissues become damaged and need healing all the time.
Mitotic index
- represents the proportion of cells undergoing mitosis calculated by:
cells in mitosis / total number of cells in field view = mitotic index
Unrestricted cell growth
-If a cell goes into unrestricted cell growth – either because of a genetic mutation or environmental mutagenic influence it can lead to a cancerous growth.
-Proto-oncogenes that normally control growth can mutate to become Oncogenes – which are involved in causing uncontrolled cell division and the growth therefore of a tumour.
Sexual reproduction
-involves a cell from the male and a cell from the female fusing together (fertilisation)
- produces varied offspring- For a species to survive long term you want the offspring to be varied so that not all of them are vulnerable if the environment changes.
Gametes
- are produced in the sex organs of plants and animals.
- The male tends to produce a lot of small mobile gametes and the female few larger ones
- all other cells besides gametes are known as somatic cells
Plant gametes
male gamete = pollen
male sex organ = anthers
female gamete = ovules
female sex organ = ovaries
Animal gametes
male gamete = sperm
male sex organ = testes
female gamete = ova (eggs)
female sex organ = ovaries
Plant organs that carry out mitosis are:
the root and shoot tips
plant organs that carry out meiosis are the:
anthers and ovaries
Meiosis
-The cell divides twice not once.
-You end up with 4 haploid cells not 2 diploid.
-Chromosomes line up in homologous pairs.
-Independent assortment happens.
-Crossing over occurs.
Homologous chromosomes
-They are termed homologous because they are the same size and shape and carry the same genes in the same order at the same gene locus (position of the gene on the chromosome)
-The alleles (different forms of a gene) can be the same or can be different.
Prophase 1
=In prophase the centrioles begin to move to opposite poles.
-The nucleolus breaks down followed later by the nuclear envelope.
-The duplicated chromosomes are clearly visible with the two sister chromatids joined by a centromere.
different to mitosis =
-HOMOLOGOUS CHROMOSOMES PAIR UP WITH EACH OTHER.
-CROSSING OVER OCCURS.
Crossing over
-Crossing over is when homologous chromosomes swap parts of their chromosomes.
-Chiasmata form where the chromosomes break.
-This adds further genetic variation to the offspring
Metaphase 1
-The centrioles form the spindle which is made from microtubules – polymers of tubulin monomers.
-The microtubules attach to the centromeres of each homologous pair.
-They push and pull them so that they line up along the equator (metaphase plate) of the cell IN THEIR HOMOLOGOUS PAIRS!
Independent assortment at metaphase 1
- This is because each pair can orient themselves differently.
-This creates millions of options as to which (maternal or paternal) chromosomes end up in the gametes .
Anaphase 1
-One of EACH PAIR of DOUBLE CHROMATIDS are pulled towards opposite poles.
-This is done by the microtubule fibres contracting which uses up the ATP stored in interphase.
Telophase 1
-Nuclear membrane reforms and cell begins to divide.
-The two cells formed are now haploid cells with 23 double chromatids in each.
Prophase 2
-The second division of meiosis now works exactly as mitosis does for both the cells.
-So the centrioles begin to move to opposite poles and the nucleolus breaks down followed later by the nuclear envelope.
Metaphase 2
-The spindles join to the centromeres of the double chromatids .
-They line up down the equator of each cell.
Anaphase 2
The centromeres divide and the individual chromosomes are pulled to the poles of each cell.
Telophase 2
-Nuclear envelopes reform and the chromosomes return to normal state.
-Cytokinesis occurs to make 4 haploid cells.
Importance of meiosis
1- It introduces GENETIC VARIATION to the offspring cells (through random assortment, crossing over at the chiasmata of the bivalents etc)
2- It produces haploid (n) gametes, which allows the diploid (2n)state to be restored in the zygote at fertilisation.
i.e. IT PREVENTS DOUBLING OF THE CHROMOSOME NUMBER AT FERTILSATION.
Importance of mitosis
1- It produces daughter cells that are genetically identical to parents so brings genetic stability
2- It is important for growth
3- It is important for repair and replacement of missing cells/tissue
4- Repair to wounds or damaged tissues
5- Asexual reproduction by plants eg bulbs/tubers, bacteria and yeast
6- Allows large numbers of offspring to be produced so you can quickly colonise an area
7- It causes cancers through proliferation of cells
Down’s syndrome
- Down’s syndrome is an example of polysomy.
- You end up with three copies of chromosome 21.
- This affects both mental and physical development .
- Life expectancy is reduced but most people with Down’s syndrome live to over 30.
