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DNA and Genetics 1

What phases are in interphase? G1, S and G2 phase.
What is the phase of cellular division called? The Mitotic phase.
When does the G1 phase occur and what is it? The G1 (Gap 1) phase is the phase following mitosis. During G1 the cell grows, it increases its supply of proteins, organelles etc. The name indicates a phase where the cell has a gap between mitosis and DNA replication.
What is the G0 phase? G0 phase is the 'quiescent' or resting phase, the cell enters this phase from G1 and may do so temporarily or permanently. During G0 phase cells are not actively growing and dividing but they are often still highly active carrying out cellular functions.
What cells can temporarily enter G0 phase? Lymphocytes (anti-body producing cells) which remain in that phase until they encounter an antigen.
What happens during the S phase? During this phase the cell starts DNA replication (2n to 4n), by the end of S phase each chromosome has been duplicated top produce two sister chromatids (4n). During this phase histone production is increased.
What happens in G2 phase? This is a phase of rapid cell growth, preparing the cell for division. Microtubules begin to reorganise during this phase to produce the spindle.
What happens in M phase? Phase M includes both nuclear division and cytokinesis (division of the cytoplasm). Phase M therefore represents full cell division.
What is cytokinesis Cytokinesis is the process by which the two daughter cells become completely separated. It involves equal division of the nuclei, cytoplasm, organelles and cytoplasm into two daughter cells.
Progression through the cell cycle is controlled by what? Cyclins and cyclin dependent protein kinases (CDK)
What do CDKs contain? The catalytic activity needed to modulate downstream events, but to be fully active these proteins need to combine with cyclins.
Upon receiving a pro-mitotic extracellular signal what levels increase within the cell? Cyclin G1.
What in turn happens after Cyclin G1 level increase? Increasing amount will become bound to CDK4. *This in turn prepares the cell to enter S-Phase*.
What does promoting the expression of transcription factors cause? They in turn promote the expression of *S cyclins* and proteins.
CDK4 also promote the degradation of what? The *degradation of inhibitors* preventing S-phase by targeting them for ubiquitination.
During the S phase the levels of what increase? S-phase promoting factor (SPF)
SPF includes what? Cyclin E, cyclin A and CDK2.
After replication is complete and the cell centers G2 phase, the levels of what fall? Levels of cyclin E fall, whilst the levels of cyclin B begins to rise.
A critical complex activated during mitosis is a ubiquitin ligase known as? The anaphase-promoting complex (APC) which promotes the degradation of mitotic cyclins.
Where are the DNA damage checkpoints? In G1, during replication (S-phase) and after replication (in G2).
What does the G1 checkpoint involve? A protein that appears to hold the cell in G1 until any DNA damage is repaired, preventing replication of damaged DNA. In the case of irreparable damage, this protein can intruct apoptosis.
What does the G2 checkpoint do? The G2 checkpoint appears to monitor the Okazaki fragments on the lagging strand of replicated DNA. The cell will not move into the M phase until all the fragments have been incorporated into a continuous DNA strand.
What do the M-phase checkpoints do? Assess formation of the spindles (the integrity of the spindles) that pull the chromosome dyads apart and if there are any error detected the cell cycle will stop in M phase, preventing cytokinesis. Apoptosis can be triggered if damage is severe.
When does mitosis begin? When the M-phase promoting factor forms, which includes cyclins A and B and CDK1.
Explain Prophase. The chromatids shorten and condense. The centrioles move apart to opposite poles of the nucleus. Mitotic spindles grow out from the centrioles. Two kinetochores appear on the centromeres. The nuclear membrane disintegrates.
Explain metaphase. The dyads, still attached by spindle fibres to the poles, line up across the metaphase plate.
Explain anaphase. M-phase promoting factor activates the anaphase promoting factor (APC/C) by phosphorylation, to begin anaphase. APC/C destroys cyclin B and *securin* (inhibits *separase*). The pairs of kinetochores now separate and migrate to the opposite poles.
Explain Telophase. A nuclear membrane forms around each cluster. The chromosomes 'relax'.
Explain cytokinesis in animal cells. A belt of the protein *actin* forms around the middle of the cell, between the reformed nucleii, and 'tightens' until it 'pinches' the cell into two daughter cells.
Explain cytokinesis in plant cells. A membrane-bound *cell plate* forms on the site of the metaphase plate. New cell wall material is synthesised along that plate.
What is cytokinesis? Separation of the cytoplasm.
How many homologous pairs (diploid cells) are in almostr ever cell in humans? 23 homologous pairs (46 chromosomes)
What are sperm or eggs also known as? Gametes and they are haploid (1n).
What are the specialised diploid cells in the gonads known as? Gamete mother cells. They can undergo a process to produce 4 haploid gametes which is termed meiosis.
Explain Prophase I. Homologous chromosomes pair up to form *bivalents*. Each bivalent consists of four chromatids with two centromeres. Spindle fibres emerge and kinetochores form. In this case one kinetochore forms on each chromosome dyad. The nuclear membrane disperses.
Explain Metaphase I. The spindle-attached bivalents align along a metaphase plate.
Explain Anaphase I. The chromosome dyads are pulled apart. Conditions are prepared for the production of haploid cells.
Explain Telophase I. The nuclear membrane reforms and cytokinesis commences. These cells are *haploid*.
What does meiosis II consist of? Prophase II; Metaphase II; Anaphase II and Telophase II.
What does loci mean? Position on the chromatid.
What is an allele? Alternative versions of the same gene.
What is independent assortment? Formation of random combinations of chromosomes in meiosis
What chance is there of two gametes being identical? 1 in 2^23 chance of two gametes being identical.
What genes will display independent assortment? Only genes on different chromosomes will display independent assortment; linked genes (genes physically close by on the same chromosome) will not display independent assortment.
What is a further source of variation introduced by meiosis? Chiasmata.
When does chiasmata occur? During prophase I, when the homologous chromosomes pair together.
What does chiasmata (singular = chiasma) mean? Cross-overs. Points at which some of the genetic material from the maternal and paternal versions of the same chromosome become entangled. They are then separated at anaphase I. The chiasmata often lead to exchange of genes (between maternal and paternal)
Created by: Cleggat
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