BIOL1951 #4 - DNA Replication

Nucleotide

A five-carbon sugar (pentose), nitrogenous ring (base), and phosphate group

Purines

nitrogenous double rings (A & G)

Pyrimidines

nitrogenous single rings (T, U, & C)

Equation for DNA Base Composition

%A + %G = %T + %C

Nucleic Acids

Polymer of nucleotides linked by phosphodiester bonds

What are the Major/Minor Grooves determined by?

The double helix places the backbone chains at unequal distances across from each other

Major Groove

Provides a wide, accessible location of binding

Antiparallel

The DNA double helix runs side-by-side, but in opposite chemical directions

Components of a Chromosome

1. Nucleotides

2. Single strand

3. Double helix

4. DNA + Histones = Nucleosomes

5. Chromosome

DNA Replication Theories

1. Semiconservative Mechanism

2. Conservative Mechanism

3. Dispersive Mechanism

Semi-conservative Mechanism

two parental strands separate ➡ each new DNA double helix contains one parent strand and one daughter strand

Conservative Mechanism

two parental strands stay intact ➡ totally new (daughter) double helix

Dispersive Mechanism

 two parental strands break into fragments ➡ each new DNA double helix contains alternating patchwork of parental and daughter segments

What did the Meselson & Stahl Experiment prove?

DNA replicated using the semi-conservative method

Polymerase Chain Reaction (PCR)

In-vitro form of DNA Replication

Origin of Replication

• Site within a chromosome that serves as a starting point for DNA replication

  • Prokaryotes: 1 origin of replication

  • Eukaryotes: multiple origins of replication

Replication Bubble

Open, unwound region of DNA replication

Replication Fork

Y-shaped region where the DNA double helix unwinds

Topoisomerase

Enzyme that prevents supercoiling ('“knotting”) in DNA

Helicase

• Enzyme that “unzips” the DNA, breaking hydrogen bonds

  • Lagging strand

Primase

• Enzyme that synthesizes the needed primer

  • Lagging strand

Primers

Small nucleic acids that bind to DNA, providing a 3’ (-OH) that DNA Polymerase can bind to

Single-strand Binding Proteins

Coat the DNA strands, preventing the reformation of the double helix during replication

DNA Polymerase needs _ replicate DNA…

1. A template

2. A primer (3’ hydroxyl)

DNA Ligase

Forms a covalent bond between the first and second Okazaki fragments, forming a third Okazaki fragment

Leading Strand

synthesized as one continuous molecule, ALWAYS synthesized towards the replication fork

Lagging Strand

synthesized as a series of small fragments, ALWAYS synthesized away from the replication fork

Telomerase

Enzyme that synthesizes a repeating sequence to the ends of a strand, preventing chromosome shortening  ➡ Polymerase with a BUILT-IN template

Telomerase Process

1. Binds to a DNA repeat sequence

2. Synthesizes a 6-nucleotide repeat sequence

3. Repeats another sequence

4. Primase makes an RNA primer (matching the 3' end)

5. DNA Polymerase synthesizes the complementary strand

6. Primer is eventually removed

DNA Mutation

A mutation that alters the DNA sequence of a genome, which can affect traits

Point Mutation Types

1. Silent

2. Missense

3. Nonsense

4. Frameshift

Silent Mutation

Does NOT alter the amino acid sequence, even though the nucleotide sequence has changed

Missense Mutation

Changes a single amino acid in a polypeptide sequence

Nonsense Mutation

Changes a codon into a termination codon

Frameshift

Involves the addition/deletion of nucleotides NOT a multiple of three

Germ-Line Mutation

Reproductive cell mutation that affects the entire organism

Somatic Mutation

Body cell mutation that affects a specific patch of cells