The Molecular Basis of Inheritance

Chapter 16AP Biology

Search For The Genetic Material

• Once chromosomes were known to carry genes, the question became which of two organic compounds make up chromosomes.

• Was DNA or protein the genetic material?• In 1952, Alfred Hershey and Martha Chase

showed that DNA must be the genetic material.

• Phage - virus that attacks bacteria and reprograms host to produce more viruses.

Hershey & Chase Experiment

Viruses Infecting Bacteria

Questions About DNA Structure

• James Watson and Francis Crick were the first to solve the puzzle of DNA structure.

• Maurice Wilkins and Rosalind Franklin contributed to their success.

• X-ray crystallography produced a rough blueprint of the molecule.

Rosalind Franklin’s Photo of DNA

Purines – Pure As Gold (2 gold wedding rings)

Watson and Crick’s Model

• Determined three major features of DNA– DNA is a double helix (with Franklin’s image).– The nitrogenous bases are A (adenine), T

(thymine), C (cytosine), and G (guanine). In DNA, A only pairs with T and C only pairs with G.

– The strands are antiparallel. They run in opposite, upside-down directions.

How does DNA Replicate?

How does DNA Replicate?

•Meselson – Stahl, late 1950’s•Grew bacteria on two isotopes of N.•Started on 15N, switched to 14N.•Looked at weight of DNA after one, then 2 rounds of replication.

DNA Replication

• DNA replication is semiconservative.– This means at the end of replication, each of the

daughter molecules has one old strand, derived from the parent strand of DNA, and one strand that is newly synthesized.

Replication - Preview

• DNA splits by breaking the H-bonds between the backbones.

• Then DNA builds the missing backbone using the bases on the old backbone as a template.

How does DNA replicate?

Priming

•DNA pol III cannot initiate DNA synthesis.•Nucleotides can be added only to an existing chain called a Primer.

Primer

• Make of RNA.• 10 nucleotides long.• Added to DNA by an enzyme called Primase.• DNA is then added to the RNA primer.

Priming

• A primer is needed for each DNA elongation site.

Anti-Parallel Alignment

Problem of Antiparallel DNA

• The two DNA strands run antiparallel to each other.

• DNA can only elongate in the 5’--> 3’ direction.

Leading Strand

• Continuous replication toward the replication fork in the 5’--> 3’ direction.

Lagging Strand

• Discontinuous synthesis away from the replication fork.

• Replicated in short segments as more template becomes opened up.

Okazaki Fragments

• Short segments (100-200 bases) that are made on the lagging strand.

• All Okazaki fragments must be primed.• RNA primer is removed after DNA is added.

DNA Replication – Major Points

• The replication of DNA begins at sites called the origins of replication.

• Initiation proteins bind to the origin of replication and separate the two strands, forming a replication bubble. DNA replication proceeds in both directions along the DNA strand.

• A group of enzymes called DNA polymerases catalyze the elongation of new DNA.

• DNA polymerase adds nucleotides working in the 5’ to 3’ direction.

DNA Replication

• The strand running in the 5’ to 3’ direction is called the leading strand. Replication occurs continuously along this strand.

• The other strand running in the 3’ to 5’ direction is called the lagging strand. It is synthesized in separate fragments called Okazaki fragments, which are then sealed together by DNA ligase, forming a continuous DNA strand.

DNA replication animation

• http://highered.mheducation.com/sites/0072943696/student_view0/chapter3/animation__dna_replication__quiz_1_.html

Summary of DNA ReplicationOverview

Origin of replication

Leading strand

Leading strand

Lagging strand

Lagging strandOverall directions

of replication

Leading strand

Lagging strand

Helicase

Parental DNA

DNA pol III

Primer Primase

DNA ligase

DNA pol III

DNA pol I

Single-strand binding protein

5

3

5

5

5

5

3

3

3

313 2

4

Enzymes

• DNA pol I - replaces RNA primers with DNA nucleotides.

• DNA pol III – adds nucleotides in the 5’ -> 3’ direction

• DNA Ligase - joins all DNA fragments together.

Other Proteins in Replication

• Topoisomerase – relieves strain ahead of replication forks.

• Helicase - unwinds the DNA double helix.

• Single-Strand Binding Proteins - help hold the DNA strands apart.

DNA animation DNAi

• https://www.youtube.com/watch?v=I9ArIJWYZHI

Energy for Replication

• From the triphosphate monomers.• Loses two phosphates as each monomer is

added.

DNA Replication Error Rate

• 1 in 1 billion base pairs.• About 3 mistakes in our DNA each time it’s

replicated.

Proofreading and Repairing DNA

• During DNA replication, DNA polymerases proofread each nucleotide against its template as soon as it is added to the growing chain.

• An incorrectly paired nucleotide is removed and the correct one inserted.

• This action is similar to hitting the “delete” key and then entering the correct letter.

Mismatch Repair

• Mismatched nucleotides sometimes evade proofreading by DNA polymerases.

• Special repair enzymes fix incorrectly paired nucleotides.

• If one of these enzymes was structured incorrectly (from a mutation), what would be the result?

Nucleotide Repair

Damage in skin cells due to ultraviolet radiation.

Repair prevents the onset of skin cancer.

Nucleotide Excision Repair

• Incorrectly paired or altered nucleotides can also arise after replication.

• DNA bases can undergo spontaneous chemical changes under normal conditions, but certain environmental conditions can also contribute to change.

• Nucleases which are DNA cutting enzymes, cut out the damaged part and replace it with the correct nucleotides.

Telomeres

• Eukaryotic chromosomes have special nucleotide sequences at their ends.

• These are multiple repetitions (100-1000) of one short nucleotide sequence (TTAGGG in humans).

• These sequences are called telomeres.• They do not prevent shortening of the DNA

molecule in replication, they just postpone the erosion of genes near the ends of the molecule.

Shortening of the Ends of Linear DNA Molecules

With each round of replication the DNA molecule becomes shorter and shorter.

This may limit the number of times a cell can divide.

Telomerase

• What prevents the loss of genes is gametes?• Germ cells (which give rise to sex cells)

produce an enzyme called telomerase.• Telomerase catalyzes the lengthening of

telomeres in the germ cells.• Zygote receives chromosomes with the

maximum length of telomeres.• Telomerase is not active in body cells.

Telomerase in Cancer Cells

• Cancer cells produce the enzyme telomerase.• This prevents the erosion of the ends of

chromosomes and allows for unlimited cell division.

Nucleosomes

Chromatin Packing

Effects of DNA Packing

• As DNA becomes more highly packed, it becomes less accessible for transcription.

• This is one way gene expression is controlled in cells.– DNA methylation– DNA acetylation

• Evidence of effect: Barr bodies (inactivated X chromosome in females) is highly packed.