DNA Structure & Replication Chapter 13 DNA Structure & Replication

The History of DNA Fredrick Griffith (1928) working with Streptococcus pneumoniae conducted transformation experiments of virulent & nonvirulent bacterial strains Oswald Avery (1949) showed that DNA, not protein or RNA, was responsible for the transformation seen in Griffith’s experiments Hershey & Chase (1952) used bacteriophages (viruses) to show that DNA, not protein, was the cell’s hereditary material Rosalind Franklin (1951) used x-rays to photograph DNA crystals Erwin Chargaff (1950) determined that the amount of A=T and amount of C=G in DNA; called Chargaff’s Rule Watson & Crick (1953) discovered double helix shape of DNA (A pairs with T; C pairs with G) & built the 1st model

Griffith’s Experiments In 1928, Fredrick Griffith was studying the bacteria (S. pneumoniae) that cause pneumonia. - Smooth strain (virulent)  Mouse dies - Rough strain (non-virulent)  Mouse lives - Heat-killed smooth strain  Mouse lives - Heath-killed smooth + rough strains  Mouse dies

Griffith’s Experiments Heat-killed, disease-causing bacteria (smooth colonies) Harmless bacteria (rough colonies) Control (no growth) Harmless bacteria (rough colonies) Heat-killed, disease-causing bacteria (smooth colonies) Disease-causing bacteria (smooth colonies) Dies of pneumonia Dies of pneumonia Lives Lives Live, disease-causing bacteria (smooth colonies)

Griffith’s Experiments Griffith called this process transformation: one type of bacteria turned into another ex. Rough strain turned into smooth strain when the two were mixed Through additional experiments, Oswald Avery concluded that DNA, not RNA or protein, is responsible for transformation in bacteria Only bacteria with undestroyed DNA could transform other bacteria Destroyed RNA and protein didn’t make a difference

Hershey-Chase Experiments Hershey and Chase confirmed that DNA, not protein, is the hereditary material by conducting experiments with bacteriophages

Watson & Crick Watson and Crick created a model of DNA by using Rosalind Franklin’s DNA diffraction x-rays Watson and Crick are associated with the discovery of DNA as a double-helix (two strands wound around each other like a spiral staircase)

James Watson (1928-?) 89-years old Francis Crick (1916-2004) Rosalind Franklin (1920-1958)

DNA Structure

DNA Structure A DNA nucleotide is made of three components: 5-carbon deoxyribose sugar Phosphate group One of four nitrogenous bases: adenine (A), guanine (G), cytosine (C), or thymine (T).

DNA Structure The two categories of nitrogenous bases are purines and pyrimidines Purines: Double-ring structure - Adenine (A) and Guanine (G) Pyrimidines: Single-ring structure - Cytosine (C) and Thymine (T)

DNA Structure Bonds Hold DNA Together Nucleotides along each DNA strand are linked by covalent bonds Complementary nitrogenous bases are bonded by hydrogen bonds Hydrogen bonding between the complementary base pairs, G-C and A-T, holds the two strands of a DNA molecule together

DNA Structure: Antiparallel Strands One strand of DNA goes from 5’ to 3’ The other strand is opposite in direction, going 3’ to 5’ This will be important to remember when it comes to DNA replication

Chargaff’s Rule Base-pairing rules of DNA are known as Chargaff’s Rule Adenine (A) always pairs with thymine (T) Guanine (G) always pairs with cytosine (C) Therefore, in a DNA strand, the % of adenine = % of thymine; % of cytosine = % of guanine Example: If a particular DNA molecule is 30% adenine, then it is 30% thymine, which equals 60%. This means that the remaining 40% of the molecule is 20% cytosine and 20% guanine.

DNA Replication Steps of DNA Replication DNA has to be copied before a cell divides DNA is copied during the S or synthesis phase of interphase New cells will need identical DNA strands Steps of DNA Replication Replication begins with the separation of the DNA strands by helicases To begin process, RNA primase must first add a small primer (sequence of RNA) Then, DNA polymerases form new strands by adding complementary nucleotides to each of the original strands.

DNA Replication Begins at origin of replication DNA strands are opened (“unzipped”) by helicases Two strands open, forming replication forks (Y-shaped region) New strands grow at the forks DNA polymerase adds complementary nucleotides to new strands Replication Fork Parental DNA Molecule 3’ 5’

DNA Replication

https://www.youtube.com/watch?v=Cw8GAPuhAk4 DNA Replication DNA polymerase can only add nucleotides to the 3’ end of the DNA This causes the NEW strand to be built in a 5’ to 3’ direction Leading strand (built toward replication fork) completed in one piece Lagging strand (built moving away from the replication fork) is made in sections called Okazaki fragments RNA primers are removed and replaced with DNA sequences DNA ligase joins segments together

DNA Replication

Errors in DNA Replication Changes in DNA are called mutations DNA proofreading and repair prevent many replication errors Unrepaired mutations that affect genes that control cell division can cause diseases such as cancer

DNA Nucleotide Mutations: Overview

DNA Nucleotide Mutations: Substitution C base was changed….C was substituted with a T Results in a change (if mRNA codon is different) in a single amino acid Sickle-Cell Anemia; Tay-Sachs

DNA Nucleotide Mutations: Insertion Example: Fragile-X Syndrome Inserted an additional base…C Causes a frameshift mutation…pushes each base to the right one place Now, the remainder of the gene’s codons are all changed, and so are the amino acids

DNA Nucleotide Mutations: Deletion Example: Niemann-Pick Disease Deleted a base…A Causes a frameshift mutation…pushes each base to the left one place Now, the remainder of the gene’s codons are all changed, and so are the amino acids