1. Important Features• a. DNA contains genetic template" for

proteins.

• b. DNA is found in the nucleus

• c. Protein synthesis occurs in the cytoplasm - ribosome.

• d. "Genetic information" must be transferred to the cytoplasm where proteins are synthesized.

2. Processes of Protein Synthesis

• a. Transcription - genetic template for a protein is copied and carried out to the cytoplasm

• b. Translation - template serves as a series of codes for the amino acid sequence of the protein

Cells Use RNA to Make Protein• The RNA Players – mRNA, rRNA, tRNA

• During polypeptide synthesis, ribosomal RNA (rRNA) is the site of polypeptide assembly.– Transfer RNA (tRNA) transports and positions

amino acids.– Messenger RNA (mRNA) directs which amino

acids are assembled into polypeptides.

• Central Dogma– DNA RNA Protein

Central Dogma of Gene Expression

Gene Expression

• Transcription – in the nucleus (if you have one)– DNA sequence is transcribed into RNA sequence

• initiated when RNA polymerase binds to promoter binding site

– moves along DNA strand and adds corresponding complementary RNA nucleotide

» disengages at stop signal

3. Steps of Transcription a. DNA unwinds

b. One side of DNA "codes for a protein"

c. Genetic code of DNA is a triplet code of 3 nucleotides or bases

d. Each triplet is specific for the coding of a single amino acid

e. Sequence of triplet codes on DNA will specify the amino acid sequence on the protein

f. Major step is the synthesis of the coded "messenger" molecule – mRNA

g. mRNA is "transcribed" from DNA by complementary base pairing (mRNA has no thymine, which is replaced by uracil)

h. mRNA passes out to cytoplasm to the ribosome

Transcription

• RNA polymerase– only one of two DNA strands (template) is

transcribed– non-transcribed strand is termed coding

strand - same as RNA (except T’s are U’s)– In both bacteria and eukaryotes, the

polymerase adds ribonucleotides to the growing 3’ end of an RNA chain.

• synthesis proceeds in 5’3’ direction

Transcription Bubble

Transcription

• Promoter– Transcription starts at RNA polymerase

binding sites called promoters on DNA template strand.

• Initiation– Other eukaryotic factors bind, assembling a

transcription complex.• RNA polymerase begins to unwind DNA helix.

Transcription

• Elongation– Transcription bubble moves down DNA at

constant rate leaving growing RNA strands protruding from the bubble.

• Termination– Stop sequences at the end of the gene cause

phosphodiester bond formation to cease, transcription bubble to dissociate, and RNA polymerase to release DNA.

Transcription

• Eukaryotic transcription differs from prokaryotic transcription:– three RNA polymerase enzymes– initiation complex forms at promoter– RNAs are modified after transcription

Spliced Gene Transcripts• DNA sequence specifying a protein is broken

into segments (exons) scattered among longer noncoding segments (introns).

• Initially, primary RNA transcript is produced for the entire gene.– Small nuclear ribonuclearproteins (snRNPs)

associate with proteins to form spliceosomes.• Excise introns and splice exons to form mature mRNA.

RNA Splicing

• During RNA processing, intron sequences are cut out of primary transcript before it is used in polypeptide synthesis.

• remaining exon sequences are spliced together to form final processed mRNA

Eukaryotic Genes are Fragmented

Now TRANSLATION!!!!

4. Translation overviewa. mRNA attaches to the ribosome

b. tRNA's attach to free amino acids in the cytoplasmic "pool" of amino acids

c. tRNA carries its specific amino acid to the ribosome

Translation overview (cont.)

d. tRNA "delivers" its amino acid based on complementary pairing of a triplet code (anticodon) with the triplet code (codon) of the mRNA.

e.Enzyme "hooks" the amino acid to the last one in the chain forming a peptide bond.

f. Protein chain continues to grow as each tRNA brings in its amino acid and adds it to the chain. - This is translation!!

Gene Expression• Translation – in the cytoplasm at ribosome

– nucleotide sequence of mRNA transcript is translated into amino acid sequence in the polypeptide

• rRNA recognizes and binds to start sequence– moves three nucleotides at a time

» disengages at stop signal

• Gene expression - collective of transcription and translation

Translation

• Begins when initial portion of mRNA molecule binds to rRNA in a ribosome– tRNA molecule with complimentary anticodon

binds to exposed codon on mRNA• some tRNA molecules recognize more than one

codon

Translation

• Start and stop signals– start signal coded by AUG codon– stop signal coded by one of three nonsense

codons: UAA - UAG - UGA

• Initiation– Polypeptide synthesis begins with the

formation of an initiation complex.• initiation factors

Translation

• Translocation– ribosome moves nucleotides along mRNA

molecule

Translation• Termination

– Nonsense codons are recognized by release factors that release the newly made polypeptide from the ribosome.

Genetic Code

• Genetic code consists of a series of information blocks called codons.– reading frame (triplet)

• each codes for one amino acid– genetic code is nearly universal

» mitochondria» chloroplasts

The Genetic Code1.A triplet code comprised of three nucleotide bases in

a sequence.

2.How many triplet codes?20 common amino acids in a protein

4 diff. bases on DNA A,T,C, & G

| | | |

4 diff. bases on RNA U,A,G, & C

4 things put together in combinations of 3 = 43= 64Therefore - 64 different DNA triplet codes or RNA codons

The 64 triplet codes• 60 code for amino acids

• 4 act as "stop" and "start codes

• Degenerate Code- more than one triplet code for some amino acids e.g.,

All code for the amino acid glycine

GGG

GGU

GGC

GGA

Differences Between Prokaryotic and

Eukaryotic Gene Expression• Most eukaryotic genes possess introns.

• Individual bacterial mRNA molecules often contain transcripts of several genes.

• Eukaryotic mRNA molecules must be completely formed and must pass across the nuclear membrane before translation.

• In prokaryotes, translation begins at the AUG codon preceded by a special nucleotide sequence.

Differences Between Prokaryotic and

Eukaryotic Gene Expression• Eukaryotic mRNA molecules have introns cut

out and exons joined together before translation.

• Eukaryotic ribosomes are larger than prokaryotic ribosomes.

Challenge Question

GGTCAGTATTATTACGCGACGGGCAATCCGCACGTGTCATCAATAGTGCCTTCGTCTGAGATC

DNA Template:

mRNA:

Polypeptide:

5’ (?) 3’mRNA (?)

3’ 5’