Cell Division

Mitosis

● Mitosis is the division of the nucleus to ensure that the daughter cells have the correct, identical, sets of chromosomes
● Successful division requires the replication of DNA and all membrane bound organelles during interphase; successful nuclear division during the mitotic phase in mitosis; and the successful division of the cytoplasm to form to form the new cel (along with successful membrane and and cellulose cell wall formation respectively based on the type of cell)
● Where mitosis could be observed:
1. Embryos in animals and plants which is followed by fetal development. During fertilisation a gamete is formed during fusion of the two parent cells, this gamete divides by mitosis to form new organisms (animals)
2. Germination, which only occurs in plants, uses mitosis to enable rapid growth of the plumule and radical in a seed. Mitosis occurs at an extremely quick rate during germination as the radical and plumule are essential for seedling survival
3. During growth of animals mitosis occurs through the animal to enable the animal to grow into a fully grown organism. For plants, mitosis occurs throughout a plants life in specific growing regions known as meristems
4. Mitosis also occurs as cells gradually wear out during life predominantly in the cells of the skin and bone marrow where mitosis actively takes place in order to replace skin cells and red blood cells, both of which have a somewhat short lifespan (3 weeks, 115 days)
5. Mitosis is also vital for repair of tissue cells both internally and externally
● Ifa cell is not during one of the stages of mitosis then the chromosomes are not

necessarily easily identifiable as DNA is loosely coiled threads amassed in the nucleus while DNA is replicated. Once a cell cell enters the mitotic phase the DNA becomes tightly packed and becomes “shorter and thicker” and also more dense. During the process of condensing of chromosomes they take up any stains which makes them clearer under observation.
● Mitosis is a continuous process and has 4 stages plus the a 5 the part of the mitotic phase of the cell cycle which is cytokinesis
1. Prophase - chromosome pairs have replicated and condensed and thickened and are thus now visible. Each chromosome is a chromatid pair joined by a centromere. The nucleolus will disappear and the centriole begins to separate forming a spindle of microtubules.
2. Metaphase - The nuclear membrane begins to break down during metaphase and the centrioles move to opposite poles of the cell and the spindle fibres stretch between the centrioles and chromatids. These Chromatids jostle and move to the metaphase plate/the center of the spindle, the centromere attaches the chromatid to the spindle

Anaphase - The centromeres will begin to separate and the spindle fibre proteins contract pulling the chromatids along the fibres toward the opposite cell pole. Note: the centromeres lead causing the chromatids to appear in a V-Shape
Telophase - The chromatids reach the poles of the cell and now are known as chromosomes. Nuclear membranes start to form around the chromosome groups. Nucleoli form in the nuclei and centrioles then reform in the cells, the divisions of the cytoplasm begins which stimulates cytokinesis
[CYTOKINESIS] - The final stage of cellular division is the final splitting of the cytoplasm with each organelle it contains. The cell surface membrane reforms around each new cell. Note: plant cells also have to include the formation of the new cellulose cell wall and so the process has further steps. The cell wall builds from the centre of the cell with some of the spindle fibres left in the cytoplasm guiding the golgi vesicles to line up in the middle of the single cell with two nuclei. They form vacuoles which fuse with surface cell membranes which separates the contents of the two new cells. Cellulose plates then get laid down and fuse completing a cell wall

Mutations:

● When a cell divides the DNA is replicated which, with every replication, leaves room for potential error as there are huge amounts of base pairs involved.
● There are many checkpoints throughout the cell cycle to assure no errors are present in the DNA replication process or the DNA itself. Some cells can pass these checkpoints even with errors, although this is rare, yet mechanisms are in place to remove cells which have gone further in the cycle prior to error detection.

● Our immune systems can mark faulty cells to allow phagocytic cells to engulf and digest them. Cells also have a self-digesting system known as apoptosis within the cell itself it recognizes DNA and releases enzymes from lysosomes which then destroy themselves
● Mutations are changes in genetic material
● Random breaks are expected during mitosis and meiosis although these are usually

repaired by maintenance enzymes. There are, of course, some mutations which pass all

checkpoints and these can range from a single base to a whole chromosome.
● Mutations are common in the body cells or somatic cells during mitosis or during meiosis

when gametes are formed. Mutations may have no effect at all, although some mutations result in a different protein being formed which gives cells or organisms an advantage or disadvantage in particular environments, other such mutations can cause issue or genetic disease which can result in uncontrolled cell growth which is cancer
● Types of Mutations:
1. Point Mutations - the mutation of one or a small number of bases within a gene.
  
  These are changes in the codons of DNA and affect the amino acid produced
  
  which affects the proteins intern and so affects the whole cell
2. Chromosomal Mutation - these are changes in the position of the genes within a
  
  chromosome, these can produce problems in individuals as they result in the
  
  formation of a different protein which won't function properly
3. Whole Chromosome Mutation - mainly occur in meiosis and when a whole
  
  chromosome is lost or duplicated in gametes
● Cancer: The cell cycle is controlled by a number of chemical signals made as a

response to different genes. These are reinforced at checkpoints. Mutations are irreversible and are passed on to the daughter cells during the process of mitosis. Mutations are especially likely to be passed on if the cells which regulate mutations become mutated
● Oncogenes have the ability to change normal cells to cancer cells but will only occur if something happens to switch them on as this causes them to become “accelerator” genes of cancers
● Suppressor genes act as prevention mechanisms to prevent cell division. Mutations in suppressor genes will result in cells dividing uncontrollably. A common example of this is the human tumor cells often having a defective p53 gene, which is one of the most important tumour suppressor genes
● Caretaker genes help to keep DNA in a state of repair and thus prevent a build up of damaged DNA, mutations here can accelerate cancer formation. Mutations in these genes prevent repair of cancer causing mutations
● If cell division is uncontrolled the cell tend to be smaller and undifferentiated and tumours can form which is a mass of undifferentiated cells. Tumours can be malignant or cancerous, malignant tumours grow and spread into healthy tissue around them, during this some cells can detach and travel to other parts of the body in the blood and eventually lodge in another organ where they may begin to grow and invade further healthy tissues. This process is called metastasis. Non-malignant tumours are less dangerous but still pose other threats and can have severe consequences

● Carcinogens are agents which increase the likelihood of mutations occurring in dividing cells. Ionising radiation is an example of a carcinogen along with many other chemicals as well as UV light from the sun. Carcinogens are naturally present in the environment although some occur in industrial/medical settings and others may be involved with leisure activities such as tobacco etc.

❏ MeiosisMutationsoccurtoo,theyoccurinasimilarwaywithDNAreplicating inaccurately during interphase
❏ There are however, some further meiotic mutations which can occur and can be more severe as these mutations are passed onto offspring, mitotic mutations are not.
❏ Mutations are as a result vital to evolution, without mutations organisms would never progress as all phenotypes would be identical.
❏ If a mutation gives offspring an advantage for a certain environment the process of natural selection will essentially enable that mutation to be passed on to further off spring and eventually spread over a whole species. Disadvantaged organisms, equally, will die out very quickly preventing it from being passed to offspring and spreading
❏ Types of Mutation:
1. Nondisjunction occurs when chromatids do not successfully separate and so a
  
  daughter cell may have two copies of the same set of DNA and another daughter cell may have none. If either of these cells were to be fertilised the zygote formed would have three chromosomes or one. Having more companies of a chromosome is known as polysomy while having one chromosome is monosomy. Monosomy often means a fetus will not survive (e.g sex chromosomes , chromosome 21)
2. Translocation occurs during recombination whereby a piece from one chromosome breaks off and rejoins to another chromosome entirely. If this process is somehow balanced as genes swap places there will be relatively few effects to the individual. If however, translocation is imbalance leaving a chromosome short and another long serious consequences are lilley ot be seen such as Burkitts’s lymphoma between chromosomes 8 & 14
Meiosis:

● The number of chromosomes in both daughter cells is the same as the parent cells
● There are many stages of Meiosis as well:

1. Prophase I - like chromosomes form made of two chromatids joined at the centromere

2. Metaphase I - Spindle forms and the chromosome line up on the metaphase plate

3. Anaphase I - Homologous pairs pulled to opposite ends of the cell but the centromere does not split

4. Telophase I - Nuclear membrane will reform around the haploid nuclei
5. Prophase II - Nuclear membrane breaks down again, chromatids will still be

joined by the centromeres and no DNA replication or crossing over
6. Metaphase II - Spindles form and chromatids line up at metaphase plate again

Anaphase II - Centromeres divide and chromatids are pulled to opposite ends of the cell
Telophase II - Nuclear envelopes reform and chromosomes decondense which give haploid nuclei
Cytokinesis - completes the division process giving four haploid cells

● When gametes are formed the chromosome number needs to be halved. Nuclei of the gametes must be haploid so when they join in fertilisation the zygote formed is diploid. Meiosis only takes place in the sex organs.
● In meiosis the contents of the cell is replicated while the cell is in interphase, the cell then enters meiosis when the genetic material is replicated
● Meiosis each chromatid pair stays extremely close together forming a four chromatid unit
● The centromeres do not split in the first meiosis division, and so chromatid pairs move to

opposite ends of the cell prior to a second division, which is now akin to mitosis. Cytokinesis gives four haploid daughter cells meaning each has half the chromosome number as the parent cell with each nucleus differing from each other. These develop into gametes and hence every gamete is different
● Meiosis too is a continuous process
● Once meiosis is completed in the nucleus cytokinesis will occur which is what leads to

the formation of four haploid cells containing different allele combinations
● Gametes form through gametogenesis
● The variation of the haploid daughter cells is vital to natural selection and survival of the

fittest. Two key mechanisms are below
1. Each cell forming gametes contains two chromosome sets, one mother and one
  
  father cell, it is the mixture of the chromosomes that mean a gamete is completely random. Independent assortment/random assortment of chromosomes refers to the random nature of the mixture of chromosomes.
2. Recombination occurs during prophase 1 of meiosis as there are four chromatids lying close together. Multi-enzyme complexes cut and rejoin bts of the maternal and paternal chromosomes, they are broken at points called chiasmata. This changing of the alleles on the two chromatids leads to genetic variation