Protein Synthesis-Transcription

Why are proteins so important?

• Nearly every function of a living thing is carried out by proteins…

-DNA replication-Structural proteins

(skin, muscles, etc.)-Transport proteins (hemoglobin, etc.)-Fight infection (antibodies)-Enzymes (digest food, copy DNA, etc.)

STEP 1: TRANSCRIPTION:·  DNA has the information (“blueprints”) to

make proteins, BUT…it can’t leave the nucleus (too big!)

·   So, it needs a MESSENGER to carry the blueprints from the nucleus to the protein-making factories…the RIBOSOMES!

·  the “messenger” is “messenger RNA” (or mRNA)…..RNA is different from DNA?

·  RNA (ribonucleic acid) is different from DNA in 3 ways:

1)  RNA is single stranded

2) The sugar in RNA is ribose (instead of deoxyribose)

3) Instead of thymine (T), RNA contains uracil (U)

(base pairing rules still apply: C with G; A with U)

• Enzymes “unzip” the DNA• RNA nucleotides link to the DNA

bases, forming a mRNA strand

·  For each gene, only 1 of the 2 strands is transcribed (the Antisense

strand); the antisense strand acts as a template!

·  the mRNA therefore, has the sequence of the SENSE strand of DNA (the coding strand)

Anti-sense(Template)

SenseStrand

·  Transcription of mRNA from template DNA is catalyzed by RNA polymerases which separate the 2 DNA strands and link RNA nucleotides as they base-pair along the DNA template

·  RNA nucleotides are added only to the 3’ end of growing RNA strand thus

• mRNA is synthesized in the 5’ 3’ direction

3 Stages of Transcription

1. Initiation:RNA polymerases bind to DNA at regions called PROMOTERS --(specific sequence of DNA-

TATA Box)(eukaryotes). --This binding site is where transcription

begins (initiation site)---

What does this provide?

Direction of transcription“downstream”

start TAC on DNA downstream from TATA box

2. Elongation·  During transcription elongation, mRNA

grows about 30-60 nucleotides per second

• as the mRNA strand elongates, it peels away from the DNA template

• 2 strands of DNA double helix are reunited (bonds reformed)

3. Termination --In prokaryotes, Transcription proceeds until RNA polymerase

reaches a termination site on the DNA

 In eukaryotes, the mechanism for cleaving the pre-mRNA from DNA is more complicated & not completely understood

--Transcription ends when RNA polymerase “falls off” the DNA

Protein Synthesis in Prokaryotes vs. Eukaryotes

• In bacteria, transcription and translation happen in the same location and often simultaneously!

• In eukaryotic cells, the nuclear envelope separates transcription from translation…this provides time for RNA processing (this is an extra step between transcription and translation)– does not occur in prokaryotes

Post-transcription modification of mRNA in

eukaryotes:1) Alteration of mRNA ends:

*the 5’ end is “capped” with a modified form of guanine (G) (5’ CAP)

-protects the mRNA from hydrolytic enzymes-serves as an “attach here” signal for

small ribosomal subunits

Post-transcription modification of mRNA in

eukaryotes:*at the 3’ end, an enzyme adds a

poly-A tail (30-200 adenine nucleotides)

-inhibits degradation of the mRNA

-may facilitate the export of mRNA from the nucleus to the cytoplasm

Post-transcription modification of mRNA in

eukaryotes:2) RNA Splicing

*INTRONS = noncoding segments of DNA are cut out of the mRNA

*EXONS = coding regions of DNA; exons are eventually expressed—the remaining exons are spliced together

• For ALL life!– strongest support for a

common origin for all life

• Code has duplicates– several codons for

each amino acid– mutation insurance!

Start codon AUG methionine

Stop codons UGA, UAA, UAG

The mRNA code (for nDNA)