BO101 Molecular Genetics Lecture 1 Nucleic Acids and Inheritance (PDF)

Nucleic Acids and Inheritance BO101 - Molecular Genetics - Lecture 1 Dr Andrew Flaus, Biochemistry DNA is the genetic material DNA is the genetic material Campbell g 16.2 fi Grif th experiment, 1928 ๏ Streptococcus bacteria ‣ Non-pathogenic R cells...

Nucleic Acids and Inheritance BO101 - Molecular Genetics - Lecture 1 Dr Andrew Flaus, Biochemistry DNA is the genetic material DNA is the genetic material Campbell g 16.2 fi Grif th experiment, 1928 ๏ Streptococcus bacteria ‣ Non-pathogenic R cells ‣ Pathogenic S cells ๏ A chemical in heat-killed S cells can transform R cells to make them pathogenic ๏ “Transforming principle” ‣ Here “principle” = fundamental source of something Campbell g 16.2 fi fi Avery experiment, 1944 ๏ Grif th experiment, 1928 ‣ A chemical in heat-killed S cells can transform R cells to make them pathogenic ๏ Avery experiment, 1944 ‣ Avery, McLeod and McCarty ‣ DNA from heat-killed S cells is required to transform R cells Only DNA, not RNA or protein ‣ So DNA is the genetic material Campbell g 16.2 fi fi  Hershey and Chase, 1952 Proteins contain S Label viral proteins with 35S DNA contain P Label viral DNA with 32P Campbell g 16.4 fi Hershey and Chase, 1952 ๏ Used virus infecting bacteria ‣ Bacteriophage = phage 1. Label viral proteins with radioactivity 2. Label viral DNA with radioactivity ๏ What did virus transfer into bacterial cell? ‣ Shake virus and bacteria to separate ‣ Observe bacteria in pellet 1. No protein radioactivity is transferred 2. Lots of DNA radioactivity is transferred Campbell g 16.4 fi DNA structural properties Properties of DNA bases ๏ Nucleotide monomers ‣ Phosphate ‣ Sugar ‣ Nitrogenous base ๏ Nitrogenous base types ‣ Purines: Adenine, Guanine Double ring ‣ Pyrimidines: Cytosine, Thymine Single ring Campbell g 16.5 fi Double helical DNA structure ๏ DNA model based on ‣ X-ray diffraction (Franklin) ‣ Chargaff’s rules ‣ Known chemistry ๏ DNA double helix 1. Sugar-phosphate backbone 2. Antiparallel strands 3. Bases in centre Campbell g 16.7a,b fi Backbone and antiparallel strands 1. Sugar-phosphate backbone ‣ Sugar with phosphate linker ‣ Links via 5’ and 3’ carbons 5’ and 3’ names of sugar ring carbon Dehydration reaction to link 2. Antiparallel strands ‣ Directionality of biology All polymerases move 5’→3’ ‣ All sequences are written and shown from 5’ start to 3’ end Campbell g 16.7b fi All sequences are written 5’ to 3’ Campbell g 16.7b fi Bases in centre 1. Sugar-phosphate backbone 2. Antiparallel strands 3. Bases in centre ‣ Hydrogen bonds between bases A pairs with T G pairs with C ‣ Base pairs have same width Purine base + pyrimidine base Campbell gs 16.8, 16.9 fi Principles of DNA replication ๏ Replication ‣ Making a copy of DNA replication ‣ Required at every cell division Campbell gs 16.10 fi Principles of DNA replication ๏ Replication ‣ Making a copy of DNA ๏ Each strand is a template ‣ Strands separate ‣ Each strand is template for synthesis of new daughter ‣ ‘Semiconservative model’ Each strand conserves its chemical integrity but strands are separated Campbell gs 16.10, 16.11 fi Replication of genomes from origins Typical bacteria: One origin Human cell: Multiple origins Campbell g 16.13 fi Initiation of replication A. Helicase enzyme separates DNA strands ‣ Progressive opening is called ‘replication fork’ B. Primase makes primer ‣ Short, RNA primers ‣ ALL synthesis is 5’ to 3’ ‣ Additional players Single-strand binding proteins Topoisomerases Campbell g 16.14 fi Leading and lagging strands ๏ Replication in single stranded bubble ‣ Bidirectional forks ๏ DNA polymerases ‣ Needs RNA primer to initiate ‣ ALL synthesis is 5’ to 3’ ๏ New strand synthesis ‣ Leading strand continuous ‣ Lagging strand in pieces Campbell g 16.16a fi  Campbell Bio ix 16.09 fl Synthesis of LEADING strand C. DNA polymerase (pol III) ‣ Extends 3’ end of RNA primer ALL synthesis is 5’ to 3’ ‣ Continuous synthesis Initiates on RNA primer In direction of fork Campbell g 16.16b fi  Campbell Bio ix 16.09 fl Synthesis of LAGGING strand D. Discontinuous synthesis 1. Primase makes RNA primers 2. DNA pol III synthesises in gaps Okazaki fragments 3. DNA pol I replaces primers 4. DNA ligase joins fragments Campbell g 16.18 fi  Campbell Bio ix 16.09 fl Review of synthesis at replication fork A B E D C Campbell g 16.18 fi  Campbell Bio ix 16.09 fl Replication enzyme toolbox ๏ Helicase ‣ Separates strands at fork ๏ Primase ‣ Synthesises RNA primers ๏ DNA polymerase III ‣ Leading strand, Okazaki fragments ๏ DNA polymerase I ‣ Replaces RNA primers with DNA ๏ DNA ligase ‣ Joins DNA fragments Campbell table 16.1  Summary of lecture ๏ DNA as genetic material ‣ Grif th, Avery, Hershey-Chase ๏ DNA structural properties ‣ Antiparallel double helix ‣ Sugar-phosphate backbone ‣ Nitrogenous base H bonding ๏ Replication ‣ Origins and replication fork ‣ Leading and lagging strands fi Learning outcomes for lecture ๏ On successful completion of this lecture, you will be able to: ‣ Justify that DNA is the genetic material ‣ Illustrate the key features of DNA ‣ Explain how the mechanism of copying DNA strands leads to semi- conservative replication ‣ Describe how packaging of DNA in eukaryotes as chromatin results in metaphase chromosomes observed in microscopes