Molecular Basis of Inheritance Notes

Molecular Basis of Inheritance

DNA was experimentally identified as the genetic material.
Griffith's experiment with Streptococcus pneumoniae demonstrated transformation.
Lethal bacteria could kill mice.
Non-lethal bacteria became lethal when mixed with the remains of killed lethal bacteria, indicating a transfer of genetic material.
This process was termed 'transformation'.

Studies using bacteriophages (viruses that infect bacteria) further supported DNA as the genetic material.
Hershey & Chase used radioactive protein and radioactive DNA to track viral components during infection.
They found that the virus's DNA, not the protein, infected the bacteria.

Chargaff analyzed DNA composition from various organisms.
DNA comprises four nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C).
The percentages of these nucleotides varied among different organisms.
Chargaff's rule: In any organism, the percentage of adenine equals the percentage of thymine, and the percentage of guanine equals the percentage of cytosine.

DNA is a nucleotide polymer consisting of:
- A nitrogenous base (A, T, G, or C).
- A pentose sugar called deoxyribose.
- A phosphate group.
Watson & Crick determined the structure of DNA using data from other scientists.
Wilkins & Rosalind Franklin used X-ray crystallography to analyze DNA.
Chargaff's rule (A=T and G=C) was crucial.
Rosalind Franklin's X-ray crystallography revealed DNA's double helix structure and that nitrogenous bases point inward.

The Watson & Crick DNA model is antiparallel: DNA strands are oriented in opposite directions.
DNA structure suggests a replication mechanism where each strand serves as a template (semiconservative replication).
Strands are held together by hydrogen bonds:
- A and T bases form 2 hydrogen bonds.
- G and C bases form 3 hydrogen bonds.

Replication starts at the origin of replication.
Proteins attach to DNA and separate the two strands, forming a replication bubble.
Replication forks are located at each end of the replication bubble where parental DNA is unwound.
Helicases unwind and separate the DNA at the replication fork.
Single-strand binding proteins keep the separated DNA strands from re-annealing.
Topoisomerase relieves torsional strain caused by unwinding by breaking, swiveling, and rejoining DNA strands.

Unwound DNA strands are ready for copying.
DNA polymerase can only add nucleotides to an existing strand that is base-paired with another strand.
Primase synthesizes a short RNA primer that base pairs with the single DNA strand, which DNA polymerase can then extend.

DNA polymerase III adds nucleotides in the 5' to 3' direction easily, creating the leading strand.
On the lagging strand, DNA polymerase III synthesizes DNA in small segments called Okazaki fragments because it can only add nucleotides in the 5' to 3' direction.
Each Okazaki fragment requires an RNA primer.
DNA ligase is needed to link all fragments together from both the leading and lagging strands.
DNA polymerase I replaces the RNA primer with DNA on the lagging strand.

DNA polymerases have proofreading functions to reduce errors.
Mismatched base pairs can occur, but repair enzymes can fix these.
DNA repair is crucial; humans have at least 170 different DNA repair enzymes.
Nucleotide excision repair involves:
- Nuclease.
- DNA polymerase.
- DNA ligase.

Eukaryotic DNA is linear and has ends.
Telomeres are simple repeat nucleotide sequences (TTAGGG) at the ends of DNA, which shorten after each replication round.
Specific proteins bind to telomere sequences and help prevent shortening.
Telomerase is an enzyme that lengthens telomeres in some cells.

In the cell, DNA is combined with a large amount of protein, forming chromatin.
Chromatin can exist as a 10 nm fiber, but much of it is packaged into a 30 nm fiber.
In interphase, chromatin is either:
- Euchromatin: more open and relaxed.
- Heterochromatin: more condensed and coiled.
Chromatin is first packaged with histones (small basic proteins) into nucleosomes (beads on a string).
Chromatin is compacted into 10 nm fibers, then 30 nm fibers.
30 nm fibers condense into 300 nm fibers, which further condense into chromosomes during mitosis.