IQ2: Cell Replication

• • Cell replication aka mitosis occurs when a parent cells divides to produce two daughter cells. 
• Why do eukaryotes replicate?

  Growth and development: when cells divide and replicate, new cells are formed, cells become more specialised for different purposes (increases the size of the organism)

  Maintenance and repair: cells die (injury and damage) therefore identical new cells are needed
• • Unicellular organisms replicate to reproduce, creating new, genetically identical individuals. 
• What are the cell cycle stages?

  1. Interphase: cell doubles in mass and duplicates its entire components
  2. Mitosis: Nucleus divides
  3. Cytokinesis: cytoplasm divides
• 1. G1 is a gap phase for cell growth before DNA replication. During this phase, cell enlargement takes places as more organelles are produced by the cell and the cytoplasm increases in volume. 
• 1. S (synthesis) phase is where DNA replication occurs. This needs to occur prior to mitosis as both daughter cells needs a copy of DNA.
• 1. G2 is another gap phase after replication, when enzymes in the cell check the duplicated chromosomes for any errors and correct these 
• • Mitosis ensures that daughter cells receive the same number and form (exact copies) of chromosomes as the parent cell. 
• In a non-dividing cell, the DNA wound around protein is known as chromatin.
• • Chromosomes replicate during the S phase, where a chromatid is copied (sister chromatids) and is joined together at the centromere. These sister chromatids are then separated and move to opposite poles during mitosis.  
• Prophase: chromosomes begin to condense (shorten and thicken). Each chromosome can be seen as two chromatids held together at the centromere.The centrioles move to opposite sides to form poles. The nuclear membrane breaks down. Spindles start to form.
• • Metaphase: chromosomes line up across the equator of the cell. Each chr is attached to the spindle fibres by a centromere. Each chromosome consists of two identical sister chromatids.
• • Anaphase: sister chromatids separate and are pulled by their centromeres to opposite poles of the cell. The sister chromatids are now known as daughter chromosomes. 
• • Telophase: daughter chromosomes gather at opposite poles of the cell, spindle breaks down. Nuclear membrane reappears. Two daughter nuclei with chromosomes identical to each other are formed.
• Cytokinesis is the division of cytoplasm and begins while the nucleus is completing its division. This process separates the newly formed daughter nuclei and ensures that each cells has only one nucleus
• • Meiosis is a cell division process that is required for sexual reproduction and creating genetic variation.
• Features of meiosis

  • Produces four daughter cells that are genetically unique
  • Occurs only in eukaryotes
  • a.k.a. Reduction division as it reduces the number of chromosomes in the daughter cells in half of that in somatic cells
• The first stage of meiosis is called prophase 1. It involves homologous pairing (homologues) and crossing over between non-identical pairs of chromosomes. Crossing over occurs when there is an exchange of DNA segments between homologous chromosomes during prophase 1. This results in new combinations of genes on the chromosomes.
• In metaphase 1, homologous pairs align along the centre of the cell. In anaphase 1, homologous pairs move towards opposite ends of the cell. By the end of telophase 1, each cell contains two sets of chromosomes but they have not yet been separated into individual chromosomes.
• Prophase 2 is similar to prophase 1 except that the chromosomes consist of single chromatids rather than paired chromatids. Chromatin condenses into visible chromosomes. Centrioles migrate to opposite poles of the cell. Spindle fibers form from microtubules extending outwards from the centrosomes.
• Anaphase 1: Sister chromatids remain attached but homologous pairs separate and move towards opposite ends of the cell.
• Metaphase 2 is where the sister chromatids line up at the equator of the cell. The spindle fibres attach to the kinetochores of the chromosomes.
• Prophase II is similar to prophase I except no homologous pairing or crossing over takes place. The nuclear envelope disappears and the centrosome divides to form two centrioles which organise microtubules into spindles.
• Telophase 1: Chromosomes reach the end of the cell and begin to uncoil into chromatin. A nuclear envelope forms around each set of chromosomes. Cells divide by cytokinesis.
• Anaphase II is where sister chromatids separate from each other and migrate to opposite sides of the cell. Cohesin proteins break apart allowing separation of sister chromatids.
• • Crossing over occurs during prophase I- this is where genetic material is exchanged between the maternal and paternal chromosomes.
• • Independent assortment during metaphase I- order in which chromosomes line up- producing different variations 
• Random segregation: parental genes must separate randomly and equally into gametes during meiosis so there is an equal chance of the offspring inheriting either allele. No allele is favored or has an advantage over another.
• • The common feature between mitosis and meiosis is that both are processes of cell division to form new, additional cells.
• Watson and Crick Model

  They proposed the structure of DNA:

  Sugar phosphate backbone that is antiparallel (strands run in the opposite directions)

  Nitrogenous bases where one end is attached to the backbone and the other end is attached to another base by a hydrogen bond
• • Both DNA and RNA are made up of a repeating series of nucleotide units made from a nitrogenous base and a sugar (deoxyribose) phosphate backbone. 
• • RNA is single stranded and DNA has a double stranded helix structure. 
• • DNA carries the genes (genetic information) needed to assemble functional protein molecules from amino acids. 
• • A nucleotide is:
  • Phosphate group
  • A five carbon sugar (deoxyribose)
  • One of four nitrogen containing bases: adenine (A), cytosine (C), guanine (G) or thymine (T). There are two types: purines with a double ring molecular , and pyrimidines, with a single ring. The purine bases are adenine and guanine, and the pyrimidines bases are cytosine and thymine (and uracil (U) in RNA where it replaces T)
• • DNA replication is the production of two identical double stranded molecules of DNA from one original double helix molecule.
• Process of replication

  1. DNA double helix unwinds
  2. DNA unzips - two strands separate
  3. Nucleotides are added against each single strand
  4. Replication errors are identified and corrected
• The process of replication involves an enzyme called helicase which breaks hydrogen bonds between complementary bases on opposite strands of the DNA molecule. This causes the DNA double helix to unwind into two separate strands.
• Primase initiates the first stage of synthesis by promoting the free nucleotides to pair up with the complementary strand
• • The replication process leads from the 5’ (said as 5 prime) towards the 3’ end of DNA. 
• • As each new complementary strand forms, the replication fork moves along and the completed pairs of DNA begin to rewind into double helices under control of DNA polymerase.