Online Learning 4

LEARNING GOALS

1. Describe the molecular structure of nucleic acids - DNA and RNA

2. Differentiate between leading strand and lagging strand synthesis in DNA replication

3. Describe the multiple stages in the process of cell division

4. Compare and contrast each stage of the cell cycle and their checkpoints in controlling cell division

5. Explain how cell division defects may lead to cancer

DNA

• Key molecule in living cells

• A polymer of individual building blocks

• Unique properties enable it to store the information required to make living cells

• Ancient molecule

• Structure the same in all 3 domains of life

• Video

  • DNA in double helix

  • If nucleic acids are polymers, nucleotides are monomers

  • Nucleotides

    • Adenine

    • Thymine

    • Cytosine

    • Guanine

    • Adenine and guanine are purines

    • Thymine, uracil (RNA) and cytosine are pyrimidines

    • In DNA

      • Adenine binds to thymine

      • Cytosine binds to guanine

    • In RNA

      • Adenine binds to Uracil

      • Cytosine binds to guanine

  • Because the 5th carbon on the pentose sugar is left on the leading edge this is called the 5' end of the strand

  • DNA replicated via a semi-conservative model - each strand in double helix serves as a template for new complementary strands

  • Replication occurs in the sample all the time - changes could to downstream genetic sequence which ultimately could result in lethal consequences

  • When DNA strands begin to separate this is called a replication fork/bubble

    • Multiple of these replication "bubbles" can be active at once due to size of chromosomes

  • Replication always in 5' to 3' direction

  • DNA has several enzymes present to prevent the formation of knots, coiling pr even super coiling during replication

    • Helicase

      • A protein which unzips the DNA helix to give single stranded DNA

      • Increases coiling ahead of the replication fork though.

    • Topoisomerase

      • Prevents this by transiently nicking both strands allowing the two strands to rotate around each other and then later stitches them back together

    • Single Strand Binding Proteins

      • Prevent unwound DNA from rewinding itself

      • Provide stability until work on the region of DNA is complete

    • Primase

      • Adds primers which allowing DNA polymerases to attach to the open strand and begin replication

    • DNA Polymerase III

      • Binds to RNA primers and begins the extension of the complementary strand

    • DNA polymerase I

      • Removes the RNA primers and fills in the space with DNA

    • DNA ligase

      • Stitches the okazaki fragments together to form a continuous strand

  • Leading strand

    • DNA strand that goes in the 5' to 3' direction

    • Synthesis of leading strand generally smooth and continuous

    • Process

      • Begins after unzipping DNA

      • DNA primase synthesises an RNA primer - this allows DNA polymerase III to bind and begin extension of a complimentary strand

      • DNA polymerase I then removes the original RNA primer and replaces it with DNA

  • Lagging strand

    • DNA strand that goes in the 3' to 5' direction

    • More complicated process

    • DNA can only be copied in 5' to 3' direction and lagging strand is in a 3' o 5' direction

    • Copied in chunks - not continuous

    • Process

      • DNA primase adds several primers throughout the exposed region of the lagging strand allowing DNA polymerase III to fill in the gaps between the primers

        • Filled regions called 'Okazaki fragments'

      • DNA polymerase I removes the RNA primers but nocks are present between each fragment

      • DNA ligase the responsible for stitching each Okazaki fragment together - covalently bonding them to form a single continuous strand

Cell division

• Unicellular

  • Replication of the organism

  • Division follows physical growth of the cell and most commonly involves splitting the cell in two equal halves - "binary fission"

  • Rate of cell division depends on nutrient availability and environmental conditions

• Multicellular

  • Necessary for growth and repair of the organism - occurs in embryonic development, growth to maturity and maintenance of adult tissues

  • Many cells are nondividing or infrequently dividing in the adult

  • DNA replication and cell division have to be tightly controlled according to the needs of the tissue.

• Video

  • Cell division crucial first step in life

  • Cells duplicate genetic material before they divide, ensuring that each daughter cell receives an exact copy of DNA

  • DNA molecules in eukaryotic cells are packaged into chromosomes

  • Eukaryotic chromosomes consist of chromatin - complex of DNA and protein that condenses during cell division

  • In animals

    • In animals

      • Somatic cells - two sets of chromosomes (one from each parent), all have the same chromosomes

      • Gametes have only one set of chromosomes

        • Produced  via special type of cell division called meiosis

  • Prokaryotic cell division

    • Divide by process called binary fission - asexual reproduction where a cell expands and then divides into two

    • DNA is free floating in cytoplasm and has no membrane-bound organelles

    • Very important to replicate the DNA so daughter cells have identical genetic info

  • Eukaryotic cell division

    • Usually have more DNA than prokaryotes

    • DNA is condensed into packaged chromosomes

    • Coordinated process that occurs via the cell cycle and mitosis

    • Chromosomes

      • Each duplicated chromosome has 2 sister chromatids which separate during cell division

    • Eukaryotic cell cycle consists of two phases

      • Interphase (G1, S and G2)

      • Mitotic phase (M)

        • Consists of mitosis - division of the nucleus

        • Cytokinesis

          • Division of cytoplasm

        • May last 30-60 mins

        • All other activities put on hold

        • 5 phases

          • Pre-mitosis: Interphase G2

            • Cell is preparing to enter mitosis

              • DNA duplicated in the previous S phase

              • Duplicated DNA still in form of chromatin

          •  Prophase (Pairs)

            • DNA condenses from chromatin into X-shaped chromosomes

            • Centrosome divides and with microtubules form the mitotic spindle

          • Prometaphase

            • Kinetochores are protein structure on chromosomes that bind to microtubules

            • Chromosomes are anchored to the Mitotic spindle via kinetochores

          •  Metaphase (middle)

            • The mitotic spindle moves to chromosomes to the middle of cell

          • Anaphase (Apart)

            • Microtubules pull sister chromatids apart

            • As the microtubules contract, the chromatids are pulled to opposite ends of the cell.

          • Telophase (two)

            • The two sets of chromosomes separated during anaphase are used to create new nuclei

            • Cell splits into 2 via cytokinesis

          • Cytokinesis

            • Actin filaments congregate near Metaphase plate to form a ring around inside of cell

            • Pinching action separates cytoplasm into two separate cells

            • The mechanisms of cytokinesis are different across different organisms

            • Animal cells form a cleavage furrow during cytokinesis where plant cells form a cell plate

        • DNA gets divided equally into daughter cells

        • Most other components get divided evenly because they are dispersed through the cell

        • One exception is mitochondria - they divide independently of cell DNA and so must divide prior to mitosis in order to maintain numbers in the new cells

        • Comparatively short part of cell cycle

        • Phase at which the cell physically divides

        • Can be subdivided into a number of phases each with its own characteristics and function

        • End result is two daughter cells with identical sets of DNA

    • Humans have 46 chromosomes, 23 different chromosomes because there are 2 sets of chromosomes.

    • One set of 23 chromosomes inherited from mother, other set of 23 chromosomes inherited from father

    • Haploid cells - 1 of each chromosome - germ cells

    • Diploid cells - 2 of each chromosome - body cells

    • G1 - after cell division cell is undergoing normal metabolic activity and growth

    • Enter S phase if it is appropriate time to divide where DNA is replicated, by end of this phase the amount of DNA has doubled as every chromosome has been replicated

    • G2 phase is period of prep for cell division

Cell Cycle

• Video

  • Cell cycle checkpoints

    • G1 checkpoint

      • Does the cell need to reproduce again right now and do we have the resources to do so?

    • G2 checkpoint

      • Was DNA damaged during S phase?

    • M checkpoint

      • Are chromosomes attached to spindle

    • Passage of a cell through checkpoints requires activation of a two-subunit protein complex

      • A regulatory subunit - termed a cyclin

      • A catalytic subunit with kinase activity when bound to cyclin - a cyclin-dependent kinase (CDK)

      • Cyclin bound to CDK is an activated protein complex - the maturation promoting factor (MPF)

    • Cyclin production begins in S phase

    • Cyclin accumulates in S and G2 phases

    • Cyclin binds with CDK forming the protein complex MPF

    • MPF activates mitosis proteins, passes G2 checkpoint

    • Mitosis is completes. MPF is broken down and cyclin is degraded

    • Cyclin levels peak in G2 and M phase

    • CDK levels remain constant

    • MPF activity spikes in M phase

    • We only discussed G2 checkpoint? G1, and M checkpoints done through different cyclins binding to different CDKs and resulting cyclin-CDK combinations can bypass G1, G2, and M checkpoints respectively

Cancer

• Primarily characterised by uncontrolled proliferation

• If genes that actively promote cell division (protooncogenes) are not turned off at the right times they become oncogenes and lead to cancer

• If genes that normally inhibit cell division from happening all the time (tumour suppressor genes) stop working, this can also lead to cancer.

• Protooncogenes

  • Overproduction due to translocation of the gene to an area where it is highly expressed for example under the control of a strong promoter

  • Aberrant amplified gene

    • Too many copies of the gene product

  • Point mutation in an oncogene or its controlling element

  • Excess product made

  • Hyperactive product made

• Tumour suppressor genes

  • Translocation of gene

  • Gene deletion

  • Point mutation

  • All causes of potential not working of tumour suppressor genes