Today…

Today…Genome 351, 8 April 2013, Lecture 3

• The information in DNA is converted to protein through an RNA intermediate (transcription)

• The information in the RNA intermediate is converted into protein (translation)

• DNA & RNA use a triplet code for amino acids

• A gene is a segment of DNA that specifies a protein

• Promoters: start sites of transcription

How does info in DNA flow to protein?The genetic material: DNA

-Four subunits (bases A, C, G, T)

But virtually all cellular functions are mediated by proteins-Twenty subunits (amino acids)

CA T

G

The RNA Tie Club

-Proposed that a transient ribonucleic acid (RNA) intermediate is involved in the conversion of info from DNA to protein

The “Central Dogma”

DNA Protein

DNA Bases includeA,C,G,T

DNA is double-

stranded

RNA

5’

3’

sugar-phosphate backbone

deoxyribose sugar in

DNA

ribose sugar in RNA

RNA Bases includeA,C,G,U

RNA is (mostly) single-stranded

(G:C & A:U)

The “Central Dogma”

-Information only flows one way

-It’s universal (works the same way in prokaryotes & eukaryotes)

DNA mRNA Proteintranscription translation

Transcription: copy gene into mRNA to make a specific protein

promoter

mRNA

terminator

genegene

gene

117.3117.1116.9116.7

GASZ CFTR CORTBP2

position in human sequence (millions of bases)

This region from chromsome 7 contains the CFTR (cystic fibrosis gene):

Mendel’s units of information (genes) are particular sequences along the chromosomes.


mRNA

mRNA

promotergene

genegene

5’ 3’3’ 5’

5’ 3’3’ 5’

A

T

GAC

GCC

G

A G C

T C G

promoter

5’ 3’3’ 5’

T

A

coding or sense strand(same sequence as mRNA)

template strand (complementary to mRNA)

mRNA

AUT

ACC

G



I. initiation

II. elongation

III. termination

Transcription in vivo

gene

nascent RNA transcriptsDNA

RNA polymerases

Which way (right or left) are RNA polymerases moving?

Where (right or left) is the promoter?

Which strand (top or bottom) is the template?

Transcription

Translation

RNA carries the information from

DNA in the nucleus to the

cytoplasm

DNA

RNA

Protein

The Morse Code

… --- … = SOS

Morse code keyLetters: Numbers:

Translating the nucleic acid code (4 different bases) to a protein code (20 aa’s)…

Possible coding systems:

1 base per amino acid

2 bases per amino acid

3 bases per amino acid

How is information coded in DNA?

20 different amino acids and 64 possible combinations of three

bases (64 “codons”)AlanineArginineAspartic acidAspargineCysteineGlutamic acid GlutamineGlycineHistidineIsoleucineLeucineLysineMethioninePhenylalanineProlineSerineThreonineTryptophanTyrosineValineStop

AlaArgAspAsnCysGlTGlnGlyHisIleLeTLysMetPheProSerThrTrpTyrVal

ARDNCEQGHILKMFPSTWYV*

GCA, GCC, GCG, GCTAGA, AGG, CGA, CGC, CGG, CGTAAC, AATGAC, GATTGC, TGTGAA, GAGCAA, CAGGGA, GGC, GGG, GGTCAC, CATATA, ATG, ATTTTA, TTG,CTA, CTC, CTG, CTTAAA, AAGATGTTC, TTTCCA, CCC, CCG, CCTAGC, AGT, TCA, TCC, TCG, TCTACA,ACC, ACG, ACTTGGTAC, TATGTA, GTC, GTG, GTTTAA, TAG, TGA

There is redundancy (more than one codon) for some amino acids, but each codon specifies only one amino acid

The Genetic Code

Table 10.5

= stop

= stop

Another type of genetic code table

Met PheThrValSerThr

AUGACUU U U UA AAA

AAC CC CG5’ 3’

mRNA

protein

The triplet code3 bases = 1 amino acidMore than 1 triplet can code for the same amino acid Translation: read the

information in RNA to order the amino acids in a protein

Translation: converting the nucleic acid code to protein

Punctuation:

Met PheThrValSerThr

AUGACUU U U UA AAA

AAC CC CG

NH3+ COO-

5’ 3’mRNA

protein

start:AUG = methionine, the first amino acid in (almost) all proteins

stop:UAA, UAG, and UGA.

STOP

Translation: converting the nucleic acid code to protein

Translation of the mRNA requires an RNA adaptor called transfer RNA

(tRNA)

Each codon has a specific tRNA with a complementary anticodon, linked to a specific amino acid.

anticodon

tRNAs ferry amino acids to the mRNA during translation

aminoacyl-tRNAsynthetase

UAC

Met

UAC

Met3’

5’ AUG 3’Recognizes AUG codon in mRNA

• A large complex of ribosomal RNAs (rRNAs) & proteins make up a ribosome

• Two subunits that join during protein synthesis

• rRNAs Provide structural support and serve as catalysts (ribozymes)

The ribosome: mediates translation

1,900 base rRNA~33 proteins

5,080 bases of rRNA (2-3 different rRNAs~49 proteins

ribosome

UAC

Met

...

ribosome + met-tRNA locates the 1st AUG (from 5’ end) & sets the reading frame for codon-anticodon base-pairing

Translation

5’ 3’

mRNA

…AUAUGACUUCAGUAACCAUCUAACA…

ribosome

UAC

Met

... UGA

Thr

...

ribosome + met-tRNA locates the 1st AUG (from 5’ end) & sets the reading frame for codon-anticodon base-pairing

Translation

5’ 3’

mRNA

…AUAUGACUUCAGUAACCAUCUAACA…

UAC

Met

the ribosome breaks the Met-tRNA bond; Met is joined to the second amino acid… the Met-tRNA is released

UGA

Thr

...

…then ribosome moves over by 1 codon in the 3’ direction

ribosome

5’ 3’…AUAUGACUUCAGUAACCAUCUAACA…

mRNA

Translation

Met

UGA

Thr

...

and the next tRNA can bind, and the process repeats

AGU...

Ser


mRNA

Translation

Met

UGA

Thr

AGU...

Ser


mRNA

Translation

UAG...

MetThr SerVal ThrPhe

STOP

When the ribosome reaches the Stop codon… termination


Translation

MetThr SerVal ThrPheNH3

+

COO-

The finished peptide!


Transcription and translation occur in separate compartments in eukaryotes…

Transcription

Translation

transcription and translation take place in the same compartment

…but bacteria lack a nucleus

Where are the 5’ and 3’ ends of the mRNA?

DNAmRNAribosome

Questions

A B

Which strand on the DNA sequence is the coding (sense) strand? How can you tell?

Questions

On the DNA sequence, circle the nucleotides that correspond to the start codon.

Questions

How many amino acids are encoded by this gene?

Questions

Do you expect the start and stop codons of gene 2 to be represented in the DNA sequence that is shown?

Questions

The form of mRNA

An mRNA starts out with non-coding sequence at the beginning, followed by a start codon, the coding sequence, a stop codon and more non-coding sequence

The non-coding portion is often referred to as the ‘untranslated region’ or UTR.

5’ 3’

Non-coding

Non-coding

Coding sequence that gets translated into protein

Translation start Translation stop

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