From DNA to Proteins
Chapter 13
Central Dogma
• DNA
• RNA
• Protein
Steps from DNA to Proteins
Same two steps produce all proteins:
1) DNA is transcribed to form RNA
– Occurs in the nucleus
– RNA moves into cytoplasm
2) RNA is translated to form polypeptide
chains, which fold to form proteins
Three Classes of RNAs
• Messenger RNA (mRNA)
– Carries protein-building instruction
• Ribosomal RNA (rRNA)
– Major component of ribosomes
• Transfer RNA (tRNA)
– Delivers amino acids to ribosomes
DNA vs RNA
• 1) Sugar (ribose) differs
– DNA sugar is deoxyribose (missing O)
– RNA sugar is ribose
• 2) Possible bases differ
– DNA uses bases A, C, G, T
– RNA uses bases A, C, G, U
• 3) DNA stable as double-stranded structure
RNA stable as single-stranded structure
A Nucleotide Subunit of RNA
phosphate
group
base
(uracil)
sugar (ribose)
Base Pairing during
Transcription
• A new RNA strand can be put together
on a DNA region according to basepairing rules
• As in DNA, C-G, but A-T does not exist
• Uracil (U) pairs with adenine (A)
Transcription & DNA
Replication
• Like DNA replication
– Nucleotides added to growing RNA strand
in the 5’ to 3’ direction
• Unlike DNA replication
– Small in length compared to DNA
– RNA polymerase makes RNA from DNA
– Product is a single strand of RNA
Base Pairing Compared
DNA
base pairing
during
transcription
RNA
DNA
base pairing
during DNA
replication
DNA
Promoter - place where RNA
polymerase binds
• 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
promoter region
Gene Transcription
newly forming
RNA transcript
DNA template
winding up
DNA template at selected
transcription site
DNA template
unwinding
Adding Nucleotides
direction of transcription
3´
5´
3´
5´
growing RNA transcript
Transcript Modification
unit of transcription in a DNA strand
exon
intron
exon
intron
exon
transcription into pre-mRNA
poly-A tail
cap
snipped out
snipped out
mature mRNA transcript
Genetic Code
• Set of 64 base triplets
DNA
• Codons
mRNA
– Nucleotide bases read
in blocks of three
mRNA
codons
• 61 specify amino acids
• 3 stop translation
amino
acids
threonine
proline
glutamate
glutamate
lysine
Code Is Redundant
• Twenty (20) kinds of amino acids are
specified by 61 codons
• Most amino acids can be specified by
more than one codon
• i.e. six codons specify leucine
– UUA, UUG, CUU, CUC, CUA, CUG
Near-Universal
Genetic Code
AUG
AAA
CGA
UGA
tRNA Structure - brings 20 different amino
acids to ribosome
codon in mRNA
anticodon in tRNA
amino
acid
There are at
least 20
different
populations of
tRNAs inside
the cytoplasm
of the cell.
Ribosomes - structures where
proteins are made
funnel
small ribosomal
subunit
+ large ribosomal
subunit
intact ribosome
Three Stages of Translation
Initiation
Elongation
Termination
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 moves
into the area where
the chain is being
built. It is the signal to
release the mRNA
transcript from the
ribosome.
The new polypeptide
chain is released
from the ribosome. It
is free to join the pool
of proteins in the
cytoplasm or to enter
rough ER of the
endomembrane
system.
The two ribosomal
subunits now
separate also.
Polysome - a lot of ribosomes
• A cluster of many ribosomes translating
one mRNA transcript
• Transcript threads through the multiple
ribosomes like the thread of bead
necklace
• Why? - Allows rapid synthesis of
proteins
What Happens to the
New Polypeptides (proteins)?
• Some just enter the cytoplasm
• Many enter the endoplasmic reticulum
and move through the cytomembrane
system where they are modified
Overview
transcription
Pre mRNA
transcript
processing
Assembly of RNA on unwound regions of
DNA molecule
mRNA
rRNA
tRNA
protein
subunits
mature mRNA
transcripts
translation
At an intact
ribosome,
synthesis of a
polypeptide
chain at the
binding sites for
mRNA and tRNAs
ribosomal
subunits
mature
tRNA
Convergence
of RNAs
cytoplasmic
pools of
amino acids,
ribosomal
subunits and
tRNAs
final protein
For use in cell or for transport
Gene Mutations
Base-Pair Substitutions (small)
Insertions (small or large)
Deletions (small or large)
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
Mutations in Genes - example
of substitution & deletion
part of DNA template
mRNA transcribed from DNA
THREONINE
PROLINE
GLUTAMATE
GLUTAMATE
LYSINE
resulting amino acid sequence
base substitution in DNA
altered mRNA
THREONINE
PROLINE
VALINE
GLUTAMATE
LYSINE
altered amino acid sequence
deletion in DNA
altered mRNA
THREONINE
PROLINE
GLYCINE
ARGININE
altered amino acid sequence
Transposons - naughty things!
• DNA segments that move
spontaneously about the genome
• When they insert into a gene region,
they usually inactivate that gene
Mutation Rates
• Each gene has a characteristic mutation
rate
• Average rate for eukaryotes is between
10-4 and 10-6 per gene per generation
• Only mutations that arise in germ cells
can be passed on to next generation
Causes of Mutations
• Exposure to harmful radiation and
chemicals in the environment can cause
DNA mutations
• Genetic defects to genes that repair
mutations leave mutations behind in
large numbers.