DNA: History, Structure and Replication

DNA: History, Structure and Replication

COPY What does DNA stand for? Deoxyribonucleic Acid ALL LIVING THINGS ON EARTH, SHARE THE GENETIC CODE FOUND IN DNA

DNA History

How did scientists know that DNA carries the information for life? • Many thought that proteins instead of DNA were inherited from parent to offspring. • Scientists (Griffith, Hershey, and Chase) showed that DNA actually carries the directions on how to make proteins. COPY

COPY Who discovered that DNA is the blueprint for life? • In 1952 Alfred Hershey and Martha Chase performed an experiment using radioactively labeled viruses that infect bacteria. • Because viruses are protein and DNA only, they figured out that viral DNA (not viral protein) could force the bacteria to make new viruses. • This was evidence that DNA can determine cell activity.

COPY Who discovered the double helix structure? • In 1953, Watson and Crick proposed that DNA is made of two chains of nucleotides held together by nitrogenous bases and twisted together. • They called it a double helix. • They used Rosalind Franklin’s X-ray crystallography work to figure this out.

DNA Structure

DNA is a Polymer • A polymer is a chemical structure made of repeating units called NUCLEOTIDE • DNA nucleotides always have: • phosphate group • deoxyribose sugar (5-Carbon Sugar) • nitrogenous base. DNA nucleotide

Four DNA Nitrogenous Bases • A nitrogenous bases in DNA are referred as the GENETIC CODE. • ALL traits such as hitch hiker thumb’s, widow’s peak, attached earlobes, etc are part of the genetic code. Guanine (G) Adenine (A) • In DNA, there are four possible nitrogenous bases: • Adenine (A) • Guanine (G) • Cytosine (C) • Thymine (T) Thymine (T) Cytosine (C)

Double Helix Structure

Phosphate Group Nitrogenous base (A, G, C, or T) O O=P-O O 5 CH2 O N Sugar (deoxyribose) C1 C4 C3 C2 DNA Nucleotide – Draw This!

“Rungs of ladder” Nitrogenous Base (A,T,G or C) “Legs of ladder” Phosphate & Sugar Backbone DNA Double Helix – Draw This!

COPY 3 O P P 5 C O G 3 2 4 1 5 O P P T A 3 O O 5 P P DNA Double Helix-DRAW THIS Covalent Bond Hydrogen Bond Nucleotide

C T A G COPY Chargaff’s Rule • Adeninemust pair with Thymine • Guanine must pair with Cytosine • Their amounts in a given DNA molecule will be about the same.

H-bonds G C A T BASE-PAIRINGS

Complementary DNA pairing C-A-C-T-G-G-T G-T-G-A-C-C-A Now you… A-C-A-G-T-T-G

COPY Before Replication Begins… • Chromosomes must be “unwound” back to chromatin through the removing of histone proteins.

COPY Enzymes for DNA Replication • Helicase (“Unzipper”) • DNA polymerase • Ligase (“Glue”)

COPY Helicase (“Unzipper”) • Breaks hydrogen bonds between nitrogen bases of nucleotides • Opens double helix starting at origin of replication Helicase

COPY DNA polymerase • Enzyme that makes the new DNA strand. Leading strand A T T A C A - 3’ T A A T G T - 5’ G C T A 5’ – A A A T T C G T A T 3’ – T T T A A G C A T A Lagging strand C G G C T A A T G T - 5’

DNA replication in 5’ to 3’ direction Original strand DNA polymerase New strand Growth DNA polymerase Growth Replication fork Replication fork New strand Original strand

DNA replication in 5’ to 3’ direction

COPY Leading vs. Lagging strands • Leading strand: made continuously • Lagging strand: made in fragments (called Okazaki fragments)

COPY Ligase (“Glue”) • Reforms HYDROGEN bonds between parts of the nucleotides and between 2 nucleotides.

DNA Synthesis Overview

COPY THIS IMAGE  IN YOUR NOTEBOOK IN THE NEXT RIGHT BLANK PAGE AND TITLE IT: “CELL CYCLE”

DNA: History, Structure and Replication