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TRANSCRIPTION AND TRANSLATION

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Central Dogma of Gene Expression

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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.

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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 protein4 diff. bases on DNA A,T,C, & G

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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

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Genetic Code

• Genetic code consists of a series of information blocks called codons.

– Triplet Codon and non overlapping each codes for one amino acid

genetic code is nearly universal UGA….tryptophan in mamalian mitochondria AGA….terminal code in mamalian mitochondria AGA…serin in drosophila mitochondria

Degeneracy of Genetic code

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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.,

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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

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Transcription

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

antisense strand) is transcribed– non-transcribed strand is termed coding

strand or sense 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

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U C G UU C A A AmRNA

A

GC TTCA AAT

GC AAT

TG Ttemplate Strand

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A view of transcription

Fig. 14.12 Brum

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Transcription Bubble

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3. Steps of Transcription - overviewa. 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

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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.

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Termination of Transcription

• Simple Termination• GC rich Reigon• Hairpin Shape (Palindromic Sequences)• Nusa• Tau• Rho Termination Factor

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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

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RNA roles

• Messenger RNA (mRNA)– Encodes protein sequences

• Transfer RNA (tRNA)– Adaptor between mRNA molecules and

amino-acids (protein building blocks)• Ribosomal RNA (rRNA)

– Part of the ribosome, a machine for translating mRNA to proteins

• ...

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Transcription

• Eukaryotic transcription differs from prokaryotic transcription:

– three RNA polymerase enzymes– initiation complex forms at promoter– RNAs are modified after transcription

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Transfer RNA

Anticodon:• matches a codon (triplet of mRNA nucleotides)

Attachment site:• matches a specific amino-acid

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Transcription and translation in eukaryotic cells are separated in space and time. Extensive processing of primary RNA transcripts in eukaryotic cells.

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RNA Editing

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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.

Lariat forms, excising introns and splicing exons to form mature mRNA.

alternative splicing

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RNA Splicing

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

– remaining sequences are not translated remaining exon sequences are spliced

together to form final processed mRNA

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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

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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

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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

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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.

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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.

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Translation

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Translation

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Translation

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Translation

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Protein Structure

• Proteins are poly-peptides• A typical proteins consists of 70-3000 amino-

acids• Proteins fold into secondary structure:

– helices & sheets• These components then fold onto each other

to form tertiary structure• This structure is (mostly) determined by the

sequence of amino-acids that make up the protein

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Protein Structure

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Evolution

• Related organisms have similar DNA– Similarity in sequences of proteins– Similarity in organization of genes along

the chromosomes• Evolution plays a major role in biology

– Many mechanisms are shared across a wide range of organisms

– During the course of evolution existing components are adapted for new functions

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Evolution

Evolution of new organisms is driven by• Diversity

– Different individuals carry different variants of the same basic blue print

• Mutations– The DNA sequence can be changed due

to single base changes, deletion/insertion of DNA segments, etc.

• Selection bias