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RNA PROTEINStranscription translation
in-text, p. 201
DNA
Proteins are coded for by Genes- long stretches of DNA that code for a particular amino acid sequence
Occurs in the nucleus Occurs in the cytoplasm
Same two steps produce ALL proteins:Example: insulin from pancreas
1) DNA is transcribed to form RNA
– Occurs in the nucleus– mRNA moves into cytoplasm
2) RNA is translated to form polypeptide chains, which
fold to form proteins
Steps from DNA to Proteins
Three Classes of RNAs
• Messenger RNA
– Carries protein-building instruction
• Ribosomal RNA
– Major component of ribosomes
• Transfer RNA
– Delivers amino acids to ribosomes
Base Pairing During Transcription
• A new RNA strand can be put together
on a DNA region according to base-
pairing rules (What enzyme?)
• As in DNA: C pairs with G
• Uracil (U) pairs with adenine (A)
Promoter• A base sequence in the DNA that
signals the start of a gene
• For transcription to occur, RNA polymerase must first bind to a promoter
The
Terminator
Transcript Modificationunit of transcription in a DNA strand
exon intron
mature mRNA transcript
poly-A tail
5’
5’ 3’
3’
snipped out
snipped out
exon exonintron
cap
transcription into pre-mRNA
3’ 5’
Code Is Redundant
• Twenty kinds of amino acids are
specified by 61 codons
• Most amino acids can be specified by
more than one codon
• Six codons specify leucine
– UUA, UUG, CUU, CUC, CUA, CUG
tRNA Structure
codon in mRNA
anticodon in tRNA
amino acid OH
tRNA molecule’s attachment site for amino acid
Initiation
• Initiator tRNA binds to small ribosomal subunit
• Small subunit/tRNA complex attaches to mRNA and moves along it to an AUG “start” codon
• Large ribosomal subunit joins complex
Binding Sites on Large Subunit
binding site for mRNA
P (first binding site for tRNA)
A (second binding site for tRNA)
Elongation
• mRNA passes through ribosomal subunits
• tRNAs deliver amino acids to the ribosomal binding site in the order specified by the mRNA
• Peptide bonds form between the amino acids and the polypeptide chain grows
Termination
• A stop codon in the mRNA moves onto the ribosomal binding site
• No tRNA has a corresponding anticodon
• Proteins called release factors bind to the ribosome
• mRNA and polypeptide are released
Polysome
• A cluster of many ribosomes translating one mRNA transcript
• Transcript threads through the multiple ribosomes like the thread of bead necklace
• Allows rapid synthesis of proteins
What Happens to the New Polypeptides?
• Some just enter the cytoplasm
• Many enter the endoplasmic reticulum and move through the cytomembrane system where they are modified
Effect of Base-Pair Substitution
original base triplet in a DNA strand
As DNA is replicated, proofreadingenzymes detect the mistake andmake a substitution for it:
a base substitution within the triplet (red)
One DNA molecule carries the original, unmutated sequence
The other DNAmolecule carries a gene mutation
POSSIBLE OUTCOMES:
OR
Frameshift Mutations
• Insertion
– Extra base added into gene region
• Deletion
– Base removed from gene region
• Both shift the reading frame
• Result in many wrong amino acids