DNA: History, Structure and Replication

What does DNA stand for?

Deoxyribonucleic Acid

ALL LIVING THINGS ON EARTH, SHARE THE GENETIC CODE FOUND IN DNA

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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.

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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.

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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.

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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.

Adenine (A)

Guanine (G)

Thymine (T)Cytosine (C)

• In DNA, there are four possible nitrogenous bases:– Adenine (A) – Guanine (G)– Cytosine (C)– Thymine (T)

Double Helix Structure

DNA Nucleotide – Draw This!

OO=P-O O

Phosphate Group

N

Nitrogenous base (A, G, C, or T)

CH2

O

C1C4

C3 C2

5

Sugar(deoxyribose)

DNA Double Helix – Draw This!

NitrogenousBase (A,T,G or C)

“Rungs of ladder”

“Legs of ladder”

Phosphate &Sugar Backbone

DNA Double Helix-DRAW THIS

P

P

P

O

O

P

P

PO

O

O

1

2 3

4

5

5

3

5

3

G C

T A

Hydrogen BondCovalent Bond

Nucleotide

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Chargaff’s Rule

• Adenine must pair with Thymine

• Guanine must pair with Cytosine

• Their amounts in a given DNA molecule will be about the same.

G CT A

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BASE-PAIRINGS

CG

H-bonds

T A

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

Before Replication Begins…

• Chromosomes must be “unwound” back to chromatin through the removing of histone proteins.

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Enzymes for DNA Replication

• Helicase (“Unzipper”)• DNA polymerase• Ligase (“Glue”)

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Helicase (“Unzipper”)

• Breaks hydrogen bonds between nitrogen bases of nucleotides

• Opens double helix starting at origin of replication

Helicase

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DNA polymerase

5’ – A A A T T C G T

3’ – T T T A A G C A

AT

CG A T T A C A - 3’

T A A T G T - 5’

A

TT

A C

GG

C T A A T G T - 5’

Leading strand

Lagging strand

• Enzyme that makes the new DNA strand.

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DNA replication in 5’ to 3’ direction

Growth

Growth

Replication fork

DNA polymerase

New strand

Original strand DNA

polymerase

Replication fork

Original strand

New strand

DNA replication in 5’ to 3’ direction

Leading vs. Lagging strands

• Leading strand: made continuously• Lagging strand: made in fragments (called

Okazaki fragments)

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Ligase (“Glue”)

• Reforms HYDROGEN bonds between parts of the nucleotides and between 2 nucleotides.

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DNA Synthesis Overview

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