Transcript DNA & DNA Replication

DNA & DNA Replication
History

DNA
Comprised of genes
 In non-dividing cell nucleus
as chromatin

 Protein/DNA

complex
Chromosomes form during
cell division
 Duplicate
to yield a full set in
daughter cell
From Chapter 3

Nucleic acids are polymers
Monomers are called nucleotides
 Nucleotides = base + sugar + phosphate
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 Base
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=
adenine, guanine
thymine, cytosine, uracil
 Sugar
= deoxyribose or ribose
 Phosphate, a single phosphate in DNA
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Sugar is linked to the phosphate
DNA is a Double Helix
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Nucleotides

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A, G, T, C
Sugar and phosphate
form the backbone
Bases lie between the
backbone
Held together by
H-bonds between the
bases
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A-T – 2 H bonds
G-C – 3 H bonds
H - Bonds
 Base-pairing
rules
AT only (AU
if DNA-RNA
hybrid)
GC only
Double Helix
Nucleotides as Language

We must start to think of the nucleotides –
A, G, C and T as part of a special
language – the language of genes that we
will see translated to the language of
amino acids in proteins
Genes as Information Transfer
A gene is the sequence of nucleotides
within a portion of DNA that codes for a
peptide or a functional RNA
 Sum of all genes = genome

DNA Replication
Semiconservative
 Daughter DNA is a double helix with 1
parent strand and 1 new strand
 Found that 1 strand serves as the template
3’
for new strand

5’
Parental DNA Molecule
Replication
Fork
3’
5’
DNA Template

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Each strand of the parent DNA is used as a
template to make the new daughter strand
DNA replication makes 2 new complete double
helices each with 1 old and 1 new strand
How is DNA Synthesized?
 DNA
is synthesized by
 Simple addition of nucleotides
along one strand (1/2 the double
helix)
Mistakes during Replication

Base pairing rules must be maintained
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Mistake = genome mutation, may have
consequence on daughter cells
Only correct pairings fit
 If wrong nucleotide is included

Special enzyme has “proofreading” and
can remove incorrect nucleotide
 Another enzyme then adds correct base
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Proofreading
Other Necessary Proteins
 Helicase
opens the double helix
and helps it uncoil
 Specialized binding proteins
keep strands separated
DNA Repair
For the rare mutations occurring during
replication that isn’t caught by DNA
proofreading enzyme
 If no repair
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In germ (sex) cells  inherited diseases
 In somatic (regular) cells  cancer
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Effect of Mutation
Repair
Mechanisms
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Different enzymes
correct different
mistakes
Other enzymes make
the proper strand
piece
Still another enzyme
will join new the new
piece in its proper spot
on the strand