An Analogy of Gene Expression

Transcribe (copy) a set of ingredients

from the cookbook to make a recipe

Translate the ingredients into a

dish

The Key Players….

• DNA

• RNA

• Ribosomes

• Amino acids

RNA- ribonucleic acid

Structural Differences with DNA: 1) Ribose sugar instead of Deoxyribose 2) Single Stranded instead of double 3) Uracil instead of Thymine (GCAU)

Uracil

Thymine

Functional Differences:

1) Deliver instructions/blueprints from DNA to ribosomes for protein-building

3 different types of RNA

1.Messenger RNA (mRNA) - made from a DNA template. The sequence of

bases in mRNA determines the sequence of amino acids in a protein. It carries the protein-building instructions out of the nucleus to the ribosomes

- acts as a template for protein construction2.Transfer RNA (tRNA) - translates mRNA code by attaching to the

specific amino acid that is indicated in the code and bringing the amino acid to the ribosome to help build the required protein.

3.Ribosomal RNA (rRNA) -structural component of a ribosome, involved in

synthesis of ribosomes

mRNA

• protein synthesis begins in the nucleus with the production of mRNA (Transcription)

• mRNA code is translated into polypeptide (Translation)

• DNA RNA protein transcription translation

known as the CENTRAL DOGMA of MOLECULAR GENETICS

• process of converting DNA to messenger RNA

• mRNA is made inside the nucleus and has the job of carrying the instructions from the DNA out into the cytoplasm. DNA never leaves the nucleus!

• divided into three sequential processes: initiation, elongation and termination (Fig.2 page 243)

How is mRNA made?

•RNA polymerase binds to the DNA molecule in a region upstream of the gene to be transcribed – called promoter region

•This region is rich in A’s and T’s

INITIATION

• To build the chain of mRNA ( from the 5’ 3’ end), RNA polymerase adds free-floating nucleotides that are complimentary to the strand of DNA that is being transcribed (template strand)

ELONGATION

Template strand

Coding strand

• RNA polymerase recognizes the end of the gene when it reaches the terminator sequence

• The newly synthesized mRNA detaches from the template DNA, and DNA zips back up.

• mRNA takes the coded message to the ribosomes in the cytoplasm or attached to the ER

TERMINATION

Posttranscriptional ModificationsBefore exiting the nucleus, the mRNA strand

needs to be modified:

• Capping - A 5’ cap is added– made of 7-methyl guanosine – protects the mRNA from digestion by enzymes & helps in the initiation of translation.

• Tailing - A poly-A tail (string of ~ 200 adenines) is added to the 3’ end, protects the strand from degradation

• Removal of introns and joining of exons Introns (noncoding regions) must be removed by splicesomes and the remaining exons (coding regions) must be joined

• mRNA leaves nucleus, spliced-out introns stay and get degraded

- mRNA transcript is now ready to leave the nucleus

How can we have over 120 000 proteins, and only about 25 000 genes?

• What would be the mRNA strand for the following DNA sequence?

DNA: 3’ T G G C A T G 5’mRNA: 5’ A C C G U A C 3’

• So far:

The order of bases in the DNA specifies the

order of bases in the mRNA

Now,

The order of bases in the mRNA specifies

the order of amino acids in the protein

• A sequence of 3 mRNA bases is called a codon (codes for an amino acid)

21

methionine glycine serine isoleucine glycine alanine stopcodon

proteinprotein

A U G G G C U C C A U C G G C G C A U A AmRNAmRNA

startcodon

Primary structure of a proteinPrimary structure of a protein

aa1 aa2 aa3 aa4 aa5 aa6

peptide bonds

codon 2 codon 3 codon 4 codon 5 codon 6 codon 7codon 1

41 = 4 amino acids42 = 16 amino acids

43 = 64 amino acids

Genetic Code

U C A G

U UUU =phe

UUC =phe

UUA = leu

UUG =leu

UCU = ser

UCC = ser

UCA -= ser

UCG = ser

UAU = tyr

UAC = tyr

UAA = stop

UAG = stop

UGU = cys

UGC = cys

UGA = stopUGG = trp

U

C

A

G

C CUU = leu

CUC = leu

CUA = leu

CUG = leu

CCU = pro

CCC = pro

CCA = pro

CCG = pro

CAU = his

CAC = his

CAA = gln

CAG = gln

CGU = arg

CGC = arg

CGA = arg

CGG = arg

U

C

A

G

A AUU = ile

AUC = ile

AUA = ile

AUG= met & start

ACU = thr

ACC = thr

ACA = thr

ACG = thr

AAU = asn

AAC = asn

AAA = lys

AAG = lys

AGU = ser

AGC = ser

AGA = arg

AGG = arg

U

C

A

G

G GUU = val

GUC = val

GUA = val

GUG = val

GCU = ala

GCC = ala

GCA = ala

GCG = ala

GAU = asp

GAC = asp

GAA = glu

GAG = glu

GGU = gly

GGC = gly

GGA = gly

GGG = gly

U

C

A

G

Now that you have the mRNA (carrying the code), thecode needs to be translated (into a protein). Thisprocess is called TRANSLATION

• Uses the mRNA’s code to build proteins• Occurs in the cytoplasm on ribosomes

• Need tRNA to transport amino acids to the ribosome• tRNA have anticodons that are complimentary to the mRNA

codons• Bonds to corresponding mRNA sites on ribosome• **Note – the amino acid picked up matches the mRNA codon,

not the anticodon

A closer look at ribosomes:

26

amino acidamino acidattachment siteattachment site

U A C

anticodonanticodon

amino acidamino acidmethionine

• Initiation – ribosome binds to mRNA. Start codon read (AUG). Special initiator tRNA brings in first amino acid- enters P-site

• Elongation – another tRNA molecule with anticodon complementary to next mRNA codon enters A-site. Peptide bonds forms between adjacent amino acids. Initiator tRNA exits ribosome. Ribosome moves along the mRNA – one codon. tRNA that was previously in the A-site moves into the P-site (carrying it’s growing peptide chain), opening up the A-site for another tRNA. Process continues until stop codon is reached

• Termination - stop codon reached. No tRNA for these codons. Release factor protein binds to stop codons causing polypeptide to be released from the ribosome. Ribosome disassembles.

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mRNAmRNA

A U G C U A C U U C G

2-tRNA

G

aa2

A U

A

1-tRNA

U A C

aa1

anticodon

hydrogenbonds codon

30

mRNAmRNA

A U G C U A C U U C G

1-tRNA 2-tRNA

U A C G

aa1 aa2

A UA

anticodon

hydrogenbonds codon

peptide bond

3-tRNA

G A A

aa3

ElongationElongation

31

mRNAmRNA

A U G C U A C U U C G

1-tRNA

2-tRNA

U A C

G

aa1

aa2

A UA

peptide bond

3-tRNA

G A A

aa3

Ribosomes move over one codon

(leaves)

32

mRNAmRNA

A U G C U A C U U C G

2-tRNA

G

aa1aa2

A U

A

peptide bonds

3-tRNA

G A A

aa3

4-tRNA

G C U

aa4

A C U

(leaves)

Ribosomes move over one codon

33

mRNAmRNA

G C U A C U U C G

aa1aa2

A

peptide bonds

3-tRNA

G A A

aa3

4-tRNA

G C U

aa4

A C U

U G A

5-tRNA

aa5

Ribosomes move over one codon

34

mRNAmRNA

A C A U G U

aa1

aa2

U

primaryprimarystructurestructureof a proteinof a protein

aa3

200-tRNA

aa4

U A G

aa5

C U

aa200

aa199

terminatorterminator or stopor stop codoncodon

TerminationTermination

http://www.cytochemistry.net/Cell-biology/cit1.htm

• Protein synthesis animation:• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?

it=swf::535::535::/sites/dl/free/0072437316/120077/micro06.swf::Protein%20Synthesis