DNA – RNA- Protein Synthesis

1. 5 carbon sugar- deoxyribose

2. Phosphate group

3. Nitrogen bases- A T G C

(adenine, thymine, guanine, cytosine)

DNA ( Deoxyribonucleic Acid)

Similar to DNA, except…Sugar is riboseSingle strandedUracil (U) instead of Thymine (T)

RNA (Ribonucleic Acid)

Messenger RNA (mRNA) – carries copies of instructions for making proteins

Ribosomal RNA (rRNA) – makes up ribosomes

Transfer RNA (tRNA) – transfers specific amino acids to the ribosome according to instructions.

Three types of RNA

•Carries DNA message from nucleus to cytoplasm

•mRNA reads the message in “triplets” called codons

•64 different codons code for 20 different amino acids

•1 start codon- AUG •3 stop codons- UUA, UAG, UGA

Messenger RNA

*Responsible for Transcription- copying the DNA code in the nucleus

•Code- universal to all life= evidence for common ancestry

•Redundancy or “wobble”- codons for same amino acid can differ in 3rd base •( a relaxing of the base-pairing rule in the tRNA)

mRNA

*Promotor- initiates transcription- DNA polymerase attaches

•Terminator- sequence that signals the end of transcriptor

•The direction of transcription is known as “downstream” and “upstream”

•Promotor sequence- upstream from terminator

•The stretch of DNA that is transcribed is the transcription unit

Transcription

1. Elongation2. Termination3. Initiation

3 stages of Transcription

•Start Codon- AUG- methionine•Once polymerase is attached to the promoter DNA, the 2 strands unwind, and the enzyme starts transcribing the template strand

•promotor- start point- •extends several dozen nucleotide pairs upstream from start point

Initiation

•RNA polymerase continues to move along DNA and untwist the helix, exposing 10-20 DNA base pairs.

•Adds nucleotides to 3’ end of the growing RNA molecule

•DNA re-forms and new RNA molecule pulls away from its DNA template

•About 60 nucleotides/second

Elongation

Transcription ends after RNA polymerase transcribes the terminator (stop codon)

Continues past terminator signal

(AAU AAA)

Cuts loose

Stop codons= UAA, UAG, UGA

Termination

Initiator Elongation Termination

PromotorStart CodonRNA Polymerase

UnzipsCopies Zips back up

Continues past terminator signal(AAUAAA)Cuts loose

Modification of RNA after Transcription

1.

1. Modified guanine attaches to 5’ end of pre- mRNA nucleotide and a string of adenine nucleotides (poly(A)tail) added to 3’ end.2. 5’(5 prime)= phosphate end attached to 5 carbon sugar 3’(3 prime)= OH ( hydroxyl end)( 3 carbon)3. The 5’ cap serves as a recognition site for ribosome.4. Both 5’ cap and poly(A)tail protect end of mRNA from hydrolytic enzymes5. Poly(A)tail also facilitates transport of mRNA from nucleus to cytoplasm6. Cap & tail are attached to non-translated leader & trailer segments

In other words, during transcription in Pre mRNA,* the 5’ end gets capped off w/ a form of guanine nucleotide. The 5’ cap:a. protects mRNA from degradingb. after mRNA reaches cytoplasm it acts as a “Attach Here” sign.

The 3’ end is modified before leaving nucleus.A. an enzyme makes a Poly(A)tail-50 to 250 adenine nucleotides. This prevents degradation and helps ribosomes attach to it. B. It also helps mRNA export from the nucleus.

The sequence of DNA nucleotides that codes for a eukaryotic polypeptide is NOT

continuous!There are non-coding segments that lie

between coding segments called intervening segments.

( Introns)

*Introns- long segments of non-coding base sequences

•Exons- coding regions- expressed in protein synthesis.

•The leader & trailer RNA are not translated

•Introns are removed & are exons joined before the mRNA leaves nucleus—this is called RNA splicing. Signals for splicing are called sn(RNP’s) or “snurps”

Split Genes & RNA Splicing

*removal of portion of RNA

*about 8000 nucleotides & only 1200 needed for avg. sized protein of 400 amino acids

*there are long stretches of non-coding nucleotide regions that are not translated & they are mixed between coding segments.•Introns- non-coding segments•Exons- coding segments•Introns are cut out and exons move to cytoplasm for translation

RNA Splicing

•snRNP’s- (“snurps”)- small nuclear ribonuclear proteins that are signals for RNA splicing.•Spliceosome- responsible for actual splicingof introns & rejoining of exons.•Ribozymes- RNA molecule that functions as an enzyme. Functions as a catalyst.

Cont.

•Carried out by tRNA

•They are specific for the amino acids they carry•Have a specific base triplet (anti-codon)-that is complimentary to the mRNA codon•61 codons for amino acids can be read from mRNA; but only 45 tRNA molecules.•That is where Wobble come into effect•Wobble enables 3rd nucleotide of some tRNA anti-codons to pair w/more than 1 kind of base in a codon.

Translation

•1 tRNA can recognize more than 1 mRNA codon

•A modified base inosine (I) is the 3rd position and can pair w/ U,C,A

•Example: tRNA CCI anti-codon can bind to mRNA GGU, GGC, or GGA

•Aminoacyl-tRNA synthetase- joins amino acid to tRNA

•The phosphate ion is what gives DNA and RNA the acidic property.

Cont.