Cell Cycle

• Why do cells divide?

  Recall: large cells have a ↓ SA:VOL ratio & are therefore ↓ efficient than smaller cells.
• Any time new cells are required, mitosis is required:

  • Growth: Multicellular organisms ↑ size by ↑ their no. of cells through mitosis
  • Asexual reproduction: eukaryotic organisms may reproduce asexually by mitosis 
  • Tissue Repair: Dmg tissue can recover by replacing dead/ damaged cells
  • Embryonic development: fertilised egg (zygote) will undergo mitosis & differentiation in order to develop into an embryo
• The cell cycle is the series of events through which cells pass to divide and create 2 identical daughter cells.

  Cell cycle = cell division + mitosis (prophase, metaphase, anaphase, telophase)
• Interphase:

  It consists of the parts of the cell cycle that don’t involve cell division.

  G1:

  • ↑ vol of cytoplasm
  • Organelles produced
  • Proteins synthesised

  S (Synthesis):

  • DNA replicated 

  G2  (Gap 2)

  • ↑ vol of cytoplasm
  • Organelles produced
  • Proteins synthesised
• Cells spend the majority of their time in interphase. It is a very active phase of the cycle. This is when the cell carries out its normal functions eg Metabolic reactions (e.g. respiration to produce ATP) are necessary for the life of  the cell; Protein synthesis -  proteins and enzymes are necessary to allow cell grow; Organelles numbers are ↑ to first support the enlarged cell; DNA is replicated to ensure a second copy is available to enable mitosis. 
• Mitosis is the division of the nucleus & cytokinesis is the division of the cytoplasm and hence the cell. Though mitosis is similar for animal and plant cells, cytokinesis is very different. The division of the cell into two daughter cells (cytokinesis) occurs concurrently with telophase.
• Supercoil chromosomes (why?)

  Human cells are 10μm in diameter & nucleus <5 μm in diameter, but human chromosomes are 15mm - 85mm in length, meaning chromosomes need to be stored compactly to fit within the nuclei of cells. Problem becomes more obvi during mitosis when chromosomes need to be short & compact enough so that they can be separated & moved to each end of the cell.
• Supercoil chromosomes (how?)

  Strain is placed on a DNA helix by overwinding/ underwinding of the helix → DNA molecule coils back on itself becoming shorter & wider (in eukaryotes histones aid the process)
• Cyclins are a family of proteins that control the progression of cells through the cell cycle. Cells cannot progress to the next stage of the cell cycle unless the specific cyclin reaches its threshold. Cyclins bind to enzymes aka cyclin-dependent kinase, which become active & attach phosphate groups to other proteins in the cell → triggers other proteins to become active & carry out tasks (specific to one of the phases of the cell cycle).
• Tumours:

  They’re abnormal growth of tissue that develop at any stage of life in any part of the body. A cancer is a malignant tumour & is named after the part of the body where the cancer (primary tumour) first develops. Mutagens are agents that → gene mutations. Not all mutations → cancers, but anything that causes a mutation has potential to cause cancer. A mutation is a Δ in an organism's genetic code &  mutation in the base sequence of certain genes can → cancer.
• Mutagens can be:

  • Carcinogens: chemicals that cause mutations 
  • High energy (high v) radiation such as X-rays
  • Short-wave (high v) ultraviolet light
  • Some viruses
• Mutation in oncogenes:

  If a mutation occurs in an oncogenes it can become cancerous. Oncogenes control the cell cycle & cell division.

  Process:

  1. Mutation in a oncogene
  2. Uncontrolled cell division 
  3. Tumour formation
  4. Malfunction in the control of the cell cycle
• Other factors that also ↑ probability of tumour development: 

  • The greater the number of cells, the greater the chance of a mutation
  • The longer a life span the greater the chance of a mutation
  • Genetic dispositions (Most common cause of hereditary breast cancer is an inherited mutation in the BRCA1 or BRCA2 gene)
  • Several mutations must occur in the same cell for it to become a tumour causing cell. The probability of this happening in a single cell is extremely small. 
• How a primary tumour can become a secondary tumour: (Metastasis)

  1. A primary tumour is a malignant tumour growing at the site where the abnormal growth first occurred.
  2. Cancerous cells from pri tumour can detach
  3. Some cancerous cells gain the ability to penetrate the walls of lymph/ blood vessels & hence circulate around the body
  4. Circulating cancerous cells invade tissues at diff locations & develop into sec tumours via uncontrolled cell division 
• Mitotic index:

  It’s the ratio btw no. of cells in mitosis in a tissue & ttl no. of observed cells. Mitotic index = ttl no. of cells in mitosis/ ttl no. of cells

  Cancer is a disease in which some of the body’s cells grow uncontrollably and spread to other parts of the body. Hence, cancer cells will have a higher mitotic index as compared to normal cells
• Meiosis is a reduction division of the nucleus to form haploid gametes. 
• Interphase

  In the S-phase of the interphase before meiosis begins, DNA replication takes place. Chromosomes are replicated & these copies are attached to each other at the centromere. The attached chromosome & its copy are aka sister chromatids. Following S-phase, further growth and preparation take place for meiosis. 
• Homologous chromosomes:

  Somatic cell nuclei (body cells) are diploid (2n). They contain a homologous (same structure) pair of each chromosome. One is paternal (from papa) and the other is maternal (mama). Homologous chromosomes are = size & struc. They carry the same genes at the same loci. Alleles carried at each locus may vary. 22 of the human chromosome pairs are homologous and the last pair, sex pair, is non-homologous (obviously). 
• The homologous pair associates during prophase I, through synapsis to make a bivalent (pg. 65 diagram shows this). Crossing-over might take place btw non-sister chromatids  in prophase I, leading to recombination of alleles (blw diagram shows this). 
• Crossing over ↑ genetic variation through recombination of linked alleles. 
• In humans the avg no. of chiasmata/ bivalent is just over two. Chiasma occurs btw the sister chromatids & therefore should appear in the central space. 
• Crossing-over btw non-sister chromatids → recombination of alleles (Prophase I). Random orientation of homologous chromosomes means there are 2n possible orientations in metaphase I (Metaphase I & II). That is 223 different combinations in gametes. Cus both crossing-over & random orientation occur during meiosis the result is effectively infinite genetic variation in the haploid gamete. For a new organism to arise sexually meiosis must occur in both parents followed by fusion of the gametes (fertilisation).
• Meiosis in a single individual produces near infinite variation, but genetic variation is further ↑ by:

  • Meiosis occurs in 2 individuals
  • Alleles from 2 organisms combine in new ways
• ↑ genetic variation → ↑ resilient population that is more likely to withstand envi Δ such as a disease. Genetic variation is essential for successful change by evolution.
• Meiosis like all processes is sometimes subject to mistakes. For eg homologous chromosomes can fail to separate at anaphase aka non-disjunction.
• The result of non-disjunction is gametes that either have 1 chromosome too many or 1 too few. If the gamete is involved in fertilization → an organism with too few/ many chromosomes (duh). Non-disjunction of many human chromosomes is so serious that either the zygote is not viable/ individual does not survive.
• Trisomy: organisms with 1 extra chromosome 

  Trisomy 21: Down Syndrome (indi has 3 chromosomes 21 & 47 chromosomes in ttl.)

  Monosomy: organisms with 1 less chromosome 
• Karyogram is a diagram or photograph of the chromosomes present in a nucleus (of a eukaryotic cell) arranged in homologous pairs of ↓ length. The chromosomes are visible in cells that are undergoing mitosis – most clearly in metaphase. Stains used to make the chromosomes visible also give each chromosome a distinctive banding pattern. A micrograph is taken & chromosomes are arranged acc to their size, shape & banding pattern. They are arranged by ↓ length.
• Karyotype is a cell property described by no. & type of chromosomes present in the nucleus (of a eukaryotic cell). Karyogram is a diagram that shows, or can be used to determine, the karyotype. 
• The risk of a child having a trisomy such as Down Syndrome ↑ greatly in older mothers. It is ≈ advisable for mothers in high risk category to choose to have a prenatal (before birth) test. Amniocentesis or chorionic villus samples can be taken and from them a karyotype can be constructed. Data from a positive test can be used to decide the best course of action like abortion.