Transcript You Light Up My Life

From DNA
to Proteins
Occurs in the nucleus
DNA
transcription
Proteins are coded for by
Genes- long stretches of
DNA that code for a
particular amino acid
sequence
Occurs in the cytoplasm
RNA
translation
PROTEINS
in-text, p. 201
Steps from DNA to Proteins
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
Three Classes of RNAs
• Messenger RNA
– Carries protein-building instruction
• Ribosomal RNA
– Major component of ribosomes
• Transfer RNA
– Delivers amino acids to ribosomes
A Nucleotide Subunit of RNA
uracil (base)
phosphate
group
sugar (ribose)
Base Pairing During
Transcription
• A new RNA strand can be put together
on a DNA region according to basepairing 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
Terminator
Gene Transcription
mRNA
transcript
Where is the promoter?
Adding Nucleotides
5’
growing RNA transcript
3’
5’
3’
direction of transcription
Transcript Modification
unit of transcription in a DNA strand
3’
exon
intron
exon
transcription
intron
5’
exon
into pre-mRNA
poly-A
tail
3’
cap
5’
snipped
out
snipped
out
5’
3’
mature mRNA transcript
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
tRNA molecule’s
attachment site
for amino acid
OH
Ribosomes
tunnel
small ribosomal subunit large ribosomal subunit
intact ribosome
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
Elongation
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
Transcription
mRNA
Mature mRNA
transcripts
Translation
rRNA
ribosomal
subunits
tRNA
mature
tRNA
Gene Mutations
Base-Pair Substitutions
Insertions
Deletions
Effect of Base-Pair
Substitution
original
base triplet
in a DNA
strand
a base
substitution
within the
triplet (red)
As DNA is replicated, proofreading
enzymes detect the mistake and
make a substitution for it:
POSSIBLE OUTCOMES:
OR
One DNA molecule
carries the original,
unmutated sequence
The other DNA
molecule carries
a gene mutation
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
Frameshift Mutation
mRNA
PARENTAL DNA
amino acid sequence
ARGININE
GLYCINE
TYROSINE
TRYPTOPHAN
ASPARAGINE
ARGININE
GLYCINE
LEUCINE
LEUCINE
GLUTAMATE
altered mRNA
BASE INSERTION
altered amino acid sequence