Please enable JavaScript.

Coggle requires JavaScript to display documents.

Week 3 (Scientist and Scholar Theme (DNA Replication and Chromosomes…

- - - - The four phases of the cell cycle are G1, S, G2, and M. G1, S, and G2 fall under 'interphase' whilst M falls under mitosis.
        
        In G1, the cell grows and thrives. It is in this stage that it is still very much expanding itself.
        If the cell chooses, it can also enter an intermediate phase, G0. In this phase, the mechanisms for cellular reproduction are broken down by the cell and it is simply operating by its own processes. If the cell would like to then continue to mitosis, then the reproductive mechanisms must be rebuilt.
        In the S phase, the cell begins to get ready for mitosis. It is in this phase that the DNA in the cell starts to condense and form chromosomes and the centrosomes duplicate.
        Finally, in the G2 phase, the cell continues to grow and it is at this point that it is ready for mitosis.
        
        In the M phase, there are five steps that occur. Firstly, in prophase, the duplicated centrioles now start to move towards the poles of the cell. Microtubules are then formed by the centrioles and these stretch out towards the chromosomes. In prometaphase, the microtubules attach to the chromosomes at their centromeres (these are called kinetochore microtubules) and some microtubules simply span across the cell (called non-kinetochore microtubules). In metaphase, the chromosomes are now in the middle plane of the cell (called the metaphase plate). As they are aligned, in anaphase, the microtubules pull on the chromosomes until they are on the poles of the cell. In telophase, the nuclear membrane and nucleolus, which had disintegrated at the start of mitosis, have now reformed. Finally, cytokinesis takes place and a cleavage furrow forms between the cells that allows them to separate.
      - The two daughter cells formed are genetically identical to the parent.
      - Mitochondria replicate themselves in the cell, however how other organelles replicate is not understood well. The precise and equal allocation of organelles is not necessary.
  - - - DNA Helicase - Unwinds the DNA strand (requires ATP)
      - DNA Gyrase - Creates nicks in the DNA strand to allow for there to not be any twisting or tension.
      - Single-Stranded Binding Proteins - Keep the two DNA strands separate
      - RNA Primase - Allows for RNA primers to be attached to the new DNA strand. This is the starting point of replication.
      - DNA Polymerase - Allows for the DNA strand to be replicated from 5' to 3' and forms Okazaki fragments on the lagging strand. This is through adding complementary nucleotides. It also proofreads DNA and repairs it.
      - DNA Ligase - Seals up any breaks in the sugar-phosphate backbone.
    - - The leading strand has continuous replication (5'-3') and synthesises towards the fork. The fork keeps opening as replication continues.
        The lagging strand has discontinuous replication as the DNA polymerase protein moves across the 5'-3' Okazaki fragments. The synthesis is away from the fork and the RNA primer is replaced with DNA and the fragments are joined together by DNA ligase.
    - - The human genome consists of 22 autosomal pairs and 1 pair of sex chromosomes. The somatic cells of the human body are diploid (contain 46 chromosomes) and the gametes are haploid (contain 23 chromosomes).
      - The chromosome has a petite (p arm) and a long arm (q arm). The centromere is the site of spindle fibre attachment and is necessary for proper segregation of the chromosomes. The telomere on the other hand defines each end of the chromosome, is responsible for stability and contains the TTAGGG sequence which ends the chromosome. Finally, the chromatid is the copy of the duplicated chromosome.
      - The complementary strand is what is the copied strand of the template strand. If the template strand has the sequence, 5'-ATTCGG-3', then the complementary strand will have the same prime locations, but complementary base pairs that are flipped. Thus, the code of the complementary strand will be 5'-CCGAAT-3'.
  - - - Terminal differentiation on the other hand is when the cell has differentiated and reached the end of its cycle. Specifically, the cell has lost the ability to proliferate.
    - - Summary: The spermatozoon fertilises the oocyte, in the ampulla of the Fallopian tube. This allows for meiosis to restart in the oocyte and finally finish. This fertilised egg is called the zygote, and is formed 1-2 days after fertilisation. The zygote then undergoes very fast cellular division and by 3-4 days, the morula forms. This morula contains 16 cells and also is the last stage where the zona pellucida (protective layer of the oocyte) exists. Then, the early blastocyst forms (4 days), which is the stage where the zona pellucida disintegrates. This stage occurs after the intermediate stage of the blastomere. The early blastocyst is now in the uterus and after the trophoblast (outer layer of cells), inner cell mass (inner group of cells), and the blastocoel (inner fluid-filled cavity of the blastocyst). When the blastocyst is matured, it implants into the uterine wall (endometrium) and this process is able to begin the transfer of blood and nutrients from the mother to the now developing embryo (after 6 days).
      - In the process of embryonic cell development, the morula contains totipotent unspecialised stem cells, then when there are 16 cells present, the first differentiation and determination stages occur, and with the blastocyst, there are to populations of pluripotent cells.
      - Gastrulation is the process that gives rise to the three primary germ layers. At the 7.5 day blastocyst, the syncytiotrophoblast breaks down the endometrium and the cells of the embryonic disc (contains the hypoblast and epiblast) are separated from the amnion by a fluid-filled space.
      - The 12 day blastocyst has completed implantation and the extraembryonic mesoderm is forming a discrete layer under the cytotrophoblast.
      - Finally, the 16 day embryo (notice now it is called the embryo). has the cytotrophoblast and associated mesoderm become the chorion. The chorionic villi are elaborating. The embryo now has all three germ layers, the yolk sac, and an allantois, which forms the basis of the umbilical cord.