AP Biology

Molecular Genetics (from DNA to Trait)

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Double helix structure of DNA

“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” Watson & Crick

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Directionality of DNA You need to

number the carbons! it matters!

OH

CH2O

4

5

3 2

1

PO4

N base

ribose

nucleotide

This will beIMPORTANT!!

AP Biology

Anti-parallel strands Nucleotides in DNA

backbone are bonded from phosphate to sugar between 3 & 5 carbons DNA molecule has

“direction” complementary strand runs

in opposite direction

3

5

5

3

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Base pairing in DNA Purines

adenine (A) guanine (G)

Pyrimidines thymine (T) cytosine (C)

Pairing A : T

2 bonds C : G

3 bonds

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Copying DNA Replication of DNA

base pairing allows each strand to serve as a template for a new strand

new strand is 1/2 parent template & 1/2 new DNA

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Replication: 1st step Unwind DNA

helicase enzyme unwinds part of DNA helix stabilized by single-stranded binding proteins

single-stranded binding proteins replication fork

helicase

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DNAPolymerase III

Replication: 2nd step

But…We’re missing

something!What?

Where’s theENERGY

for the bonding!

Build daughter DNA strand add new

complementary bases DNA polymerase III

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Adding bases can only add

nucleotides to 3 end of a growing DNA strand need a “starter”

nucleotide to bond to

strand only grows 53

DNAPolymerase III

DNAPolymerase III

DNAPolymerase III

DNAPolymerase III

energy

energy

energy

Replication energy

3

3

5B.Y.O. ENERGY!

The energy rulesthe process

5

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Editing & proofreading DNA 1000 bases/second =

lots of typos! DNA polymerase I

proofreads & corrects typos

repairs mismatched bases removes abnormal bases

repairs damage throughout life

reduces error rate from 1 in 10,000 to 1 in 100 million bases

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Fast & accurate! It takes E. coli <1 hour to copy

5 million base pairs in its single chromosome divide to form 2 identical daughter cells

Human cell copies its 6 billion bases & divide into daughter cells in only few hours remarkably accurate only ~1 error per 100 million bases ~30 errors per cell cycle

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The “Central Dogma” Flow of genetic information in a cell

How do we move information from DNA to proteins?

transcriptiontranslation

replication

protein

RNA

DNA

trait

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RNA

ribose sugar N-bases

uracil instead of thymine U : A C : G

single stranded

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There are Different RNAs with Distinct Functions

AP Biology 2007-2008

Transcription makes an RNA copy of DNA

fromDNA nucleic acid language

toRNA nucleic acid language

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Transcription

RNA polymerase acts here

The enzyme RNA polymerase opens the DNA strands and synthesizes an RNA complementary to only one of the DNA strands.

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Transcription Making mRNA

transcribed DNA strand = template strand untranscribed DNA strand = coding strand

same sequence as RNA synthesis of complementary RNA strand

transcription bubble enzyme

RNA polymerase

template strand

rewinding

mRNA RNA polymerase

unwinding

coding strand

DNAC C C

C

C

C

C

C

C CC

G

G G G

G G

G G

G

G

GAA

AA A

A

A

AA

A A

A

AT

T T

T

T

T

T

T

T T

T

T

U U

5

35

3

3

5build RNA 53

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Eukaryotic Genes are Segmented link

Genes are made of parts represented in the mRNA (exons) and parts that are transcribed but not present in the mRNA (introns).

Introns are removed from the primary transcript and exons are spliced together to make mRNA.

In some genes more than 90% of the pre-mRNA is destroyed, never to appear in the mRNA.

AP Biology 2007-2008

Translation in Eukaryotes

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Triplet Code Codons

blocks of 3 nucleotides decoded into the sequence of amino acids

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DNA code mRNA (copied code) gives amino acid sequence tRNA gets amino acids ribosomes assembles polypeptide chains make up proteins…

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The Cryptogram of LIFE

These are the words of the genetic language.

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tRNA An adaptor Couples

codons and anticodons

Amino Acids in proper order as per DNA

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E P A

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Can you tell the story?

Review

DNA

pre-mRNA

ribosome

tRNA

aminoacids

polypeptide

mature mRNA

5' cap

polyA tail

large ribosomal subunit

small ribosomal subunit

aminoacyl tRNAsynthetase

E P A

5'

3'

RNA polymerase

exon introntRNA