Download 1. DNA is the Genetic Material 1 Chapter 10A: DNA Structure & Replication

Chapter 10A:
DNA Structure & Replication
1. DNA is the Genetic Material
2. DNA Structure
3. DNA Replication
1. DNA is the Genetic Material
The Genetic Material is contained
in Chromosomes
• chromosomes are made of
both protein (histones) & DNA
1
Is the Genetic Material
Protein or DNA?
Until the early 1950’s no one knew for sure,
but it was generally thought that protein was
the genetic material. Why?
• protein is made of 20 different amino acids
• DNA is made of only 4 different nucleotides
• protein could theoretically store more info
• a “20 letter alphabet” vs a “4 letter alphabet”
• it was assumed that life was so complex, therefore a “bigger
alphabet” was necessary to somehow encode it!
Viral DNA, not Protein, enters Host
• famous “Hershey & Chase” experiment from 1952
2. DNA Structure
2
The Structure of DNA was
Determined in the early 1950’s
Using the technique of x-ray crystallography,
Rosalind Franklin, James Watson & Francis Crick
figured out the structure of DNA
• Watson & Crick have historically
been given the most credit
DNA: a Polymer of 4 Nucleotides
pyrimidines
purines
A
T
C
G
DNA is a Double-stranded Helix
G
5’
C
T
O
A
A
–O
T
P
O
G
C
A
C
C
O
–O
G
P
O
3’
OH
OH
H2C
O
O
O
O
G
–O
T
A
O
A
–O
A
T
A
O
C
G
O
O
CH2
P
O
O
O
C
O
CH2
O
O
T
A
3’
O
T
O
C
O–
P
O
A
CH2
OH
G
O
O–
P
G
O
O
P
O
H2C
O–
P
O
O
H2C
T
CH2
O
H2C
T
A
T
O
HO
P
O
O–
T
5’
• each strand has a “sugar-phosphate backbone”
• the 2 strands are anti-parallel and interact via base pairs
3
DNA “Base-Pairing”
The base pairs in DNA are held together by
hydrogen bonds (H-bonds):
• same phenomenon that makes water “sticky”
• attraction between partial “+” and partial “-” charges
Why do base pairs form only between A/T, C/G?
1) the size of the bases
• each base pair must must consist of a purine (“big”)
and a pyrimidine (“small”)
• the width of each base pairs must be the same
2) the position of groups involved in H-bonding
• partial negative must interact with partial positive
The DNA “Sequence”
The DNA sequence is the linear order of
nucleotides in a DNA strand:
• each DNA strand in the double helix has its
own sequence
• the sequences in each strand are considered
as complementary to each other
• they differ, but “fit just right” with each other
• ea strand will “fit” with only 1 complementary strand
e.g.
5’
3’
A–C–A–C–A–C–A–C–A–C
T–G–T–G–T–G–T–G–T–G
3’
5’
3. DNA Replication
4
How is DNA Replicated?
Every time a cell reproduces (i.e., divides) it
must replicate its chromosomes (DNA).
The process of DNA replication depends on
the rules of “base pairing”:
• A-T or T-A; C-G or G-C
• the sequence of one strand dictates the
sequence of the other
• each strand of the double helix can serve as a
template to make a complementary strand
The separation of each original DNA strand followed
by the production of a new complementary strand
on each original template strand…
A
T
A
T
A
T
A
T
A
T
C
G
C
G
C
G
C
G
C
G
G
C
G
C
G
C
G
C
A
T
A
T
A
T
A
T
T
A
T
A
T
A
T
A
C
A
Nucleotides
Both parental strands serve
as templates
Parental molecule
of DNA
Two identical daughter
molecules of DNA
…results in 2 double stranded DNA molecules
identical to the original molecule!
• note that ea original strand ends up in a different molecule
Each exposed
template strand
guides the
synthesis of a new
complementary
sequence.
G
C
A
T
G
C
G
C
A
T
T
A
A
C
G
T
C
G
G
C
C
G
G
C
A
A
T
A
A
C
G
T
T
G
T
C
G
C
T
A
A
A
A
G
T
T
T
A
A
C
T
T
5
DNA Replication at multiple sites
Parental strand
Origin of replication
Daughter strand
Bubble
Two daughter DNA molecules
How is DNA Replication Accomplished?
3′
5′
DNA polymerase
molecule
5′
3′
Daughter strand
synthesized
continuously
Parental DNA
3′
5′
Daughter
strand
synthesized
in pieces
Through a
collection
of specific
enzymes
including:
DNA polymerase
• synthesizes new
complementary
DNA (always 5’ > 3’)
5′
3′
DNA helicase
• unwinds orig. DNA
DNA ligase
DNA ligase
• connects DNA pieces
Overall direction of replication
Key Terms for Chapter 10A
• chromosome, nucleotide
• pyrimidine, purine
• double helix, anti-parallel
• sugar-phosphate backbone
• base-pair, complementary
• DNA replication, template
• DNA polymerase, ligase, helicase
Relevant Review Questions:
2, 6, 8
6