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Unit 2: Genetics and Evolution
Chapter 2: Molecular Basis of Inheritance
Search for Genetic Material
Griffith's Experiment:
Showed a "transforming principle" could change bacteria.
Avery, MacLeod, McCarty:
Identified DNA as the transforming principle.
Hershey-Chase Experiment:
Confirmed DNA is the genetic material using bacteriophages.
DNA Structure
Watson & Crick Model:
Double helix with two anti-parallel strands.
Backbone:
Sugar-phosphate.
Base Pairing:
Adenine (A) with Thymine (T) (2 H-bonds)
Guanine (G) with Cytosine (C) (3 H-bonds)
Chargaff’s Rule:
A=T and G=C.
DNA Packaging
Nucleosome:
DNA wrapped around a core of 8 histone proteins.
Chromatin:
A chain of nucleosomes ("beads-on-string").
Chromosomes:
Highly condensed chromatin.
Central Dogma
Describes the flow of genetic information:
DNA → RNA → Protein
Replication:
DNA → DNA
Transcription:
DNA → RNA
Translation:
RNA → Protein
Reverse Transcription:
RNA → DNA (in retroviruses).
DNA Replication
Semi-conservative:
Each new DNA has one old and one new strand.
Proven by:
Meselson and Stahl experiment.
Key Enzymes:
Helicase:
Unwinds DNA.
DNA Polymerase:
Synthesizes new DNA.
Ligase:
Joins DNA fragments.
Transcription
Definition:
Synthesis of RNA from a DNA template.
In Eukaryotes:
Occurs in the nucleus.
Produces
hnRNA
(pre-mRNA).
Post-transcriptional Processing:
Splicing:
Removes introns, joins exons.
Capping:
Adds a cap to the 5' end.
Tailing:
Adds a poly-A tail to the 3' end.
Genetic Code
Triplet Code:
3 bases = 1 codon.
Degenerate:
Multiple codons for one amino acid.
Unambiguous:
One codon for one amino acid.
Universal:
Same code in most organisms.
Start Codon:
AUG (Methionine).
Stop Codons:
UAA, UAG, UGA.
Translation
Definition:
Synthesis of protein from an mRNA template.
Occurs on:
Ribosomes.
Steps:
Initiation:
Ribosome binds to mRNA.
Elongation:
Polypeptide chain grows.
Termination:
Stop codon is reached.
Gene Expression: Lac Operon
An
inducible operon
in
E. coli
for lactose metabolism.
Lactose Absent:
Repressor binds to operator, operon is OFF.
Lactose Present:
Lactose (inducer) binds to repressor, operon is ON.
Human Genome Project (HGP)
Goal:
To sequence the entire human genome.
Key Findings:
~3 billion base pairs.
~20,000-25,000 genes.
Applications:
Disease diagnosis, gene therapy, forensic science.
DNA Fingerprinting
Principle:
Based on Variable Number Tandem Repeats (VNTRs).
Technique:
Involves restriction digestion, gel electrophoresis, and hybridization with a VNTR probe.
Applications:
Forensic science (crime investigation).
Paternity testing.
Conservation biology.