DNA Structure & Replication - Enzymes in DNA Replication ¢â‚¬¢ The principle enzyme involved in DNA replication

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  • DNA Structure

    & Replication

  • DNA Structure

    • Chromosomes are made of DNA.

    • The DNA is wound around proteins called

    Histones.

    • When chromosomes are unraveled, the

    DNA strands are called Chromatin.

    • DNA is made of Nucleotides.

    – Deoxyribose (5 carbon sugar)

    – Phosphate group

    – Nitrogen Base (A, T, G, C)

  • DNA Structure

    • Sugar –

    Phosphate

    Backbone held

    together by strong

    Phosphodiester

    Bonds (covalent)

    • The Nitrogen

    Bases are held

    together by

    Hydrogen Bonds

  • 6

    DNA Nucleotides

    O=P-O O

    Phosphate Group

    N Nitrogenous base (A, G, C, or T)

    CH2

    O

    C1C4

    C3 C2

    5

    Sugar (deoxyribose)

    O

    copyright cmassengale

  • 7

    Nitrogenous Bases • Double ring PURINES

    Adenine (A) Guanine (G)

    • Single ring PYRIMIDINES Thymine (T) Cytosine (C)

    T or C

    A or G

    copyright cmassengale

  • Table 1. Nitrogen Base Make-Up of Different Organisms’ DNA

    (in Percentages)

    Organism A T G C

    Mycobacterium

    tuberculosis

    15.1 14.6 34.9 35.4

    Yeast 31.3 32.9 18.7 17.1

    Wheat 27.3 27.1 22.7 22.8

    Sea Urchin 32.8 32.1 17.7 17.3

    Marine crab 47.3 47.3 2.7 2.7

    Turtle 29.7 27.9 22.0 21.3

    Rat 28.6 28.4 21.4 21.5

    Human 30.9 29.4 19.9 19.8

  • 9

    Base-Pairings

    • Purines ONLY pair with Pyrimidines

    • Three hydrogen bonds required to bond Guanine & Cytosine

    CG

    3 H-bonds

    copyright cmassengale

  • 10

    T A

    •Two hydrogen bonds are required to bond Adenine & Thymine

    The hydrogen bonds between A=T and G=C create the twist in the double helix. Each twist of DNA is about 10 bases long.

    copyright cmassengale

    2 H-bonds

  • 11

    DNA

    P

    P

    P

    O

    O

    O

    1

    2 3

    4

    5

    5

    3

    3

    5

    P

    P

    P O

    O

    O

    1

    2 3

    4

    5

    5

    3

    5

    3

    G C

    T A

    copyright cmassengale

  • DNA • One strand

    begins with the 5’

    end of the sugar

    (5’ to 3’ =

    Lagging Strand)

    • The other strand

    begins with the 3’

    end of the sugar

    (3’ to 5’ =

    Leading Strand)

    • Strands are

    ANTIPARALLEL

    (opposite)

  • 13

    Deoxyribose Sugar • Carbons are numbered clockwise 1’ to 5’

    CH2

    O

    C1C4

    C3 C2

    5

    Sugar (deoxyribose)

    copyright cmassengale

  • Hydrogen

    bonds

    Nucleotide

    Sugar-phosphate

    backbone

    Key

    Adenine (A)

    Thymine (T)

    Cytosine (C)

    Guanine (G)

  • Chromosome

    Supercoils

    Coils

    Nucleosome

    Histones

    DNA

    double

    helix

  • 16

    DNA replication PURPOSE • Every cell in your body has the SAME DNA

    • DNA has to be copied before a cell divides

    • DNA is copied during interphase

    • New cells will need identical DNA strands

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  • DNA replication PURPOSE

    DNA replication is SEMI-CONSERVATIVE.

    Each daughter molecule of DNA is made of ONE

    ORIGINAL strand and ONE NEW strand.

  • DNA Replication

    • During DNA replication:

    – The DNA molecule separates into 2 strands.

    – 2 new complementary strands are produced following the rules of base pairing.

    • Each strand of the double helix of DNA serves as a template for the new strand.

  • How DNA Is Replicated

    • DNA replication is carried out by a

    series of enzymes.

    • The enzymes “unzip” (separate the

    strands) of a molecule of DNA.

    • The unzipping occurs when the

    hydrogen bonds between the base

    pairs are broken.

  • How DNA Is Replicated

    • Each strand serves as a template for

    the attachment of complimentary bases.

    – Example: GTAAGC produces

    CATTCG (complimentary)

    • Each DNA molecule resulting from

    replication has one original strand and

    one new strand.

  • Enzymes in DNA Replication

    • The principle enzyme involved in DNA

    replication is called DNA polymerase.

    • It attaches (or polymerizes) individual

    nucleotides to produce DNA.

    • DNA polymerase also “proof-reads” each

    new DNA strand, helping to maximize the

    odds that each molecule is a perfect copy

    of the original DNA.

  • Summary of DNA Replication

    1. Enzymes break the Hydrogen bonds between the nitrogen bases and separate the original strands of DNA.

    2. Another enzyme (DNA polymerase) bonds complimentary bases to each original strand.

    3. DNA polymerase then proofreads the DNA for mistakes (mutations).

    • This produces 2 molecules of DNA.

    • EACH molecule has 1 ORIGINAL strand and 1 NEW strand of DNA!!!

  • DNA replication BIG IDEA

    Base Pairing ensures that when

    DNA is replicated, you get

    2 identical copies of DNA

    each with ONE NEW STRAND and

    ONE ORIGINAL STRAND!