DNA Synthesis Ppt

The Final will be Cumulative

Parents cell divides to create 2 “identical” daughter cells

The DNA of the parent cell must be replicated in rider for each daughter cell to inherit a full complement of chromosomes

The method of DNA synthesis in Eukaryotes is called Semiconservative replication, first proposed by Watson & Crick

• semiconservative replication is the replication of 2 strands of DNA from one parent strand

• This process uses a single strand from the parent as a template

• The new strands each retain half of the parent’s strand

• The site where DNA replication is initiated is known as an origin of replication

• Multiple sites of origin are needed in eukaryotes, because DNA is much longer, and the recess is slower due to presence of nucleosides

• Sites of replication are called replicants

• Origins of replication recruit proteins that initiate unwinding and replication of DNA

• The 1st step is the binding of a protein complex, the origin recognition complex (ORC), to a replication origin in

• The 2nd step is the binding of minichromosomal maintenance proteins (MCM) that include several DNA helicases that facilitate DNA replication by unwinding the double helix

• At this point, the complete group of DNA bound proteins is called a pre-replication complex

• Replication does not begin until several more proteins, including enzymes that catalyze DNA synthesis are added

• Unwinding Process:

  • the unwinding process involves a number of DNA helicases

  • These enzymes break the hydrogen bonds that hold the 2 DNA strands together

  • Once the strand separation has begun, single stranded DNA binding proteins (SSB) attach to the exposed single strands to keeps the DNA unwound

  • Topoisomerases prevent tangling of the unwound DNA by creating swivel points in the DNA molecule

• DNA synthesis begins at both replication forks and proceeds in opposite directions creating a replication bubble

• DNA polymerase is needed for the replication of DNA

• This enzyme works its way along each replication fork

• It’s also capable of roof reading the new strand of DNA to ensure accuracy

• The adding of a new nucleotide strand complementary to the template strand begins with an enzyme called primate (a specific type of RNA polymerase)

• The initial sequence of nucleotides is actually RNA, not DNA

• This RNA sequence is later removed by DNA polymerase I

• Why not use DNA in the 1st place?

  • This is related to the need for error correction

DNA Damage and Repair:

• DNA damage is a change in normal base pair sequencing

• These alterations are called mutations, which may or may not be beneficial

• Evolution is driven by the genetic variability used by mutations

  • Mutations can arise spontaneously, without exposure to mutagens

    • A mutagen may be physical, chemical, or biological agents

  • Spontaneous mutations arise in one of 3 ways:

    • Mispairing of bases with transient formation of a Tautomer

    • Slippage during replication

    • Damage to individual bases

  • Chemical mutagens have chemical structures that closely resemble one of the 4 DNA nucleotides

    • DNA polymerase cannot distinguish the different and inadvertently insert the analogues into a DNA strand instead of a G,C,T,A

    • Radiation damages DNA by causing a chemical reaction that can break bonds or form unwanted bonds

DNA Repair System:

• A variety of mechanisms have evolved to repair DNA

  • Base Excision repair

    • Corrects single damaged bases of DNA

  • Nucleotide excision repair

    • An enzyme “cuts out” the damaged region and DNA polymerase fills in the gap with the correct sequence

  • Mismatch repair

    • Target mismatched base pairs

  • Double-strand break repair

    • Splits strands back together in the correct sequence

Benzopyrene

• Most Carcinogenic substance known

• are covalently bonded to Guanine bases

• Binding distorts the DNA by perturbing the Double-helical DNA structure

  • Specifically targets the protective P53 gene

    • P53 is a tumor suppressor

    • This gene is a Transcription factor that Regulates the cell cycle

The Cell Cycle and Mitosis ppt

Cell division is required for growth of the organism, and the replacement of dead or dying cells

The Cell Cycle begins when a single parent cell divides itself into 2 new cells, and ends when one of these new cells also divides into 2 cells

The overall length of the cell cycle is called the Generation Time (18-24 hours in dividing mammalian cells)

The Cell Cycle has 2 major phases:

• Interphase

  • Normal cellular business of the cell

• M Phase

  • Involves 2 overlapping events

    • Mitosis (nuclear division)

    • Cytokinesis (Cytoplasmic division)

      • Final phase that produces 2 daughter cells

Interphase:

• Takes up the majority of the cell cycle

• Most cellular contents are synthesized continuously during this phase

• DNA replication

• 3 parts:

  • G1 phase (8-10 hours)

  • S phase (6-8 hours)

    • Time of DNA synthesis

  • G2 phase (4-6 hours)

• The replicated chromosomal structure consists of 2 identical copies called sister chromatids

  • these 2 identical copies of each chromosome are held together by a Centromere

• The kinetochore is the part of the centromere t which militia spindle microtubules attach

• When attached, these microtubules are called Kinetochore Microtubules

Mitosis: 5 stages:

• Prophase

  • Chromatin condenses into chromosome that are visible under the microscope

  • The replicated DNA still exits as sister chromatids

  • Nucleoli begin to disperse

• Prometaphase

  • Fragmentation of the nuclear envelope

  • Centrosomes complete their movement towards opposite sides of the nucleus

  • Spindle microtubules make contact with chromosomes

  • Chromosome still exists as sister chromatids

• Metaphase

  • Fully condensed chromosomes align at the metaphase plate

  • The tugging of sister chromatids toward opposite poles begins

• Anaphase

  • The 2 sister chromatids of each chromosome are abruptly separated and begin to move towards opposite poles

  • The chromatids are moved through 2 different mechanisms

    • Anaphase A

      • Pulled apart by shortening kinetochore microtubules

    • Anaphase B

      • Poles themselves

• Telophase

  • Daughter chromosomes arrive at the poles of the spindle

  • Chromosomes begin to uncoil

  • Nucleoli begin to develop

  • Spindles disassemble

  • Nuclear envelope forms around the 2 romps of newly separated chromosomes

Cytokinesis:

• the phase of the cell cycle in which the cytoplasm and its contents are divided to form 2 individual daughter cells

• This phase is not tightly coupled with mitosis, and may begin in late anaphase or early telophase as the nuclear envelope and nucleoli are reforming, and the chromosomes are decondensing

• The division of the cytoplasm is called Cleavage

  • Cleavage begins with an indentation or puckering of the cell surface, which then deepens to form a cleavage furrow

  • The furrow continues to deepen until it reaches the plasma membrane on the opposite side of the cell, effectively cutting the cell in half

  • Cleavage results from a belt-like bundle of Actin Microfilaments that form just beneath the plasma membrane in the center of the cell

    • These fibers create a contractile ring, which tighten and pinch the cell in half

Regulation of the cell cycle:

• the length of the cell cycle varies between tissues

• This varying length is most often dependent of the time each cell spends in G1, it delays may occur elsewhere in the cycle

• Readily dividing cells include:

  • Cells that give rise to spermatozoa

  • Stems cells that give rise to blood cells, epithelia, etc.

• Slowly dividing cells

  • Cells in connective tissue

• Cells that ne’er divide

  • Muscle cells and neurons

  • Remain arrested in an offshoot of G1, called G0

• Cells that don’t divide unless stimulated to do so:

  • Hepatocytes, Lymphocytes, etc.

• The cell cycle in controlled by a number of factors

  • These factors ensure that certain cellular events are completed before the ell moves on to the next series of events

• Cell Cycle control ensures the following:

  • Cellular events are carried out I the appropriate sequence

  • Specific cellular events are completed before the cell moves on to the next series of events

  • The cell responds to external conditions that indicate a need for proliferation

• The control of the cell cycle is accomplished by groups of molecules acting at key transition points

  • G1-S transition= restriction point

  • G2-M = transition

  • Metaphase-anaphase = Transition