1. Transcription

Gene Expression

The expression of a gene into a

protein occurs by:

1) Transcription of a gene into RNA

• produces an RNA copy of the coding region of a gene

• the RNA transcript may be the actual gene product (rRNA, tRNA) or

be translated into a polypeptide gene product (mRNA)

2) Translation of mRNA transcript into polypeptide

• accomplished by ribosomes with the help of tRNA

DNA(gene)

3

5

5

3

5 3

NH2 Methionine

TRANSLATION(by ribosomes)

Polypeptide

mRNA

TRANSCRIPTION

Arginine Tyrosine Leucine

Functional Protein

RNA polymerase attaches

nonspecifically to DNA

And travels down its

length until it recognizes

a promoter sequence.

Upon recognition of the

promoter, RNA polymerase

unzips the DNA molecule

beginning at the promoter.

Initiation of transcription

Attachment of RNA polymerase

Promoter

RNA polymerase

Unzipping of DNA, movement of RNA polymerase

Template

DNA strand

Terminator

3DNA

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3

5

1

2

Initiation of Transcription

• RNA polymerase binds to the promoter sequence of gene

1) Initiation

• promoter serves to target and orient RNA polymerase

• once “docked” at promoter, RNA polymerase unzips DNA

Elongation of the RNA Transcript

• only 1 DNA strand is used as a template, the other strand

is referred to as the “coding strand”

• ribonucleotides (rNTPs) are added to the 3’ end of growing RNA

strand, thus elongation is in a 5’ 3’ direction

• RNA polymerase synthesizes a complementary RNA strand

After rNTPs align with

their DNA complements,

RNA polymerase links

them together, synthesizing

RNA. The triphosphate

ribonucleotides also provide

the energy required for

RNA synthesis. No primer

is needed.

Elongation of the RNA transcript

Growing

RNA molecule

Template

DNA

strand

3

5

3

2) Elongation

In self-termination, the transcription of DNA

terminator sequences cause the RNA to fold,

loosening the grip of RNA polymerase on the DNA.

In enzyme-dependent termination, a termination

enzyme pushes between RNA polymerase and

the DNA, releasing the polymerase.

Termination of transcription

RNA transcript released

RNA polymerase

released

Rho termination

protein

3

5

4 5

Rho protein moves

along RNA

3

C-G rich

stem-loop

Termination of Transcription

3) Termination

• triggered by stem/loop structure in RNA or termination

factors such as the Rho protein

Various Roles of RNA Transcripts

1) messenger RNA (mRNA)

• RNA copy of a gene that encodes a polypeptide

2) ribosomal RNA (rRNA)

• RNA that is a structural component of ribosomes

3) transfer RNA (tRNA)

• delivery of “correct”

amino acids to

ribosomes during

translation

For some genes, the end-product

is the RNA itself (rRNA, tRNA)

2. Translation

The Genetic Code

If the DNA sequence is:

CATGCCTGGGCAATAG

The mRNA copy is:

CAUGCCUGGGCAAUAG

The polypeptide is:

*Met-Pro-Gly-Gln

all proteins begin w/Met which is quickly removed

(transcription)

(translation)

(a) Tobacco plant expressing a firefly gene gene

(b) Pig expressing a jellyfish

The Genetic Code is UniversalGenes from one species can be expressed in another!

Acceptorstem

Hydrogenbonds

3

5

tRNA icon

3

5

Anticodon

Anticodon

tRNA Structure & Function

amino

acid

Overview of Translation

The building of a polypeptide, 1 amino acid at a time, by

ribosomes using info in mRNA:

• ribosomes bind directly to mRNA, “read” codon by codon

• translation also involves tRNAs, each of which is attached to

1 of the 20 amino acids (AAs)

• ribosomes match the right tRNA (via anticodon) with the right

codon in the mRNA, then add its AA to the growing protein

• ribosomes always start at AUG (methionine)

InitiatortRNA

mRNA Start codon

Anticodon

Smallribosomal

subunit

P35

A

1

tRNAfMet

2

P A P A

3

E

Largeribosomal

subunit

Initiation complex

Initiation of Translation

1. Specific sequences in the rRNA of the small ribosomal

subunit align (base pair) with complementary sequences

near the start codon (AUG) in the mRNA

2. Anticodon of initiator tRNA carrying MET base pairs with

start codon

3. Joins with the large ribosomal subunit to complete the

initiation complex with tRNAMET in P-site, A-site empty

5

E

Peptide bond

4

5

P A3

5

E

P A3

6

5

E P A3

Movement ofribosome

5

E

Two more cycles

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8

P A3

5

E

P A3

5

E

P A3

Movement of ribosome

Growingpolypeptide

Elongation of

Translation

Translation

terminates

when a

stop codon

is reached

5. ribosome catalyzes peptide bond formation between amino acids

attached to each tRNA

Elongation & Termination of Translation

6. ribosome shifts 3 nucleotides (1 codon) on mRNA (in a 5’ to 3’ direction)

8. A “stop” codon triggers termination of the process

ELONGATION

TERMINATION

4. tRNA with anticodon complementary to codon in A site joins the

initiation complex

7. new tRNA with complementary anticodon enters the A site and the

process continues until…

3. Mutations

Mutations

A mutation is any change in DNA sequence:

• change of one nucleotide to another

• insertion or deletion of nucleotides or DNA fragments

• inversion or recombination of DNA fragments

What causes mutations?

• errors in DNA replication or DNA repair

• chemical mutagenesis

• high energy electromagnetic radiation

• UV light, X-rays, gamma rays

Nuclease

DNA polymerase

DNA ligase

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5

5

5

5

3

3

3

3

3

3

3

3

DNA Repair

When DNA is damaged it is essential that the

DNA is repaired so it can be replicated and

expressed properly.

• special enzymes recognize and remove the

damaged portion of DNA

• a DNA polymerase will fill in the gap

• DNA ligase will then connect the newly

made DNA to the adjacent strand

Sometimes mistakes (mutations)

are introduced by DNA repair

Effects of

Mutations

*insertions & deletions

can cause “frame shifts”

Types of Mutations

Silent mutations:

• have no effect on amino acid specification

Missense mutations:

• result in the change of a single amino acid

Nonsense mutations:

• convert a codon specifying an amino acid to a stop codon

• results in premature termination of a polypeptide

Insertion/deletion mutations:

• cause a shift in the reading frame of the gene

• all codons downstream of insertion/deletion will be incorrect