Download 5b . Students know how to apply base-pairing rules to explain... semiconservative replication and transcription of information from DNA into mRNA.

5b. Students know how to apply base-pairing rules to explain precise copying of DNA during
semiconservative replication and transcription of information from DNA into mRNA.
4a Students know the general pathway by which ribosomes synthesize proteins, using tRNAs to
translate genetic information in mRNA.
4b Students know how to apply the genetic code rules to predict the sequence of amino acids from
a sequence of codons in RNA
•DNA double helix unwinds
•DNA now single-stranded
•New DNA strand forms using complementary base
pairing (A-T, C-G)
•Used to prepare DNA for cell division
•Whole genome copied/replicated

Flow of genetic information in a cell
 How do we move information from DNA to proteins?
DNA
replication
RNA
protein
DNA gets
all the glory,
but proteins do
all the work!
trait
a
nucleus
DNA
transcription
a
a
Cytoplasm a
(ribosomes, rough aER)
mRNA
translation
a
a
a
a
a
protein
a
a
a
a
a
a
ribosome
trait
DNA
Transcription
RNA
Translation
Protein

From DNA nucleic acid language
to RNA nucleic acid language
 RNA forms
base pairs
with DNA
 C-G
 A-U

mRNA- type of RNA
that encodes
information for the
synthesis of
proteins and carries
it to a ribosome
from the nucleus

Making mRNA
 transcribed DNA strand = template strand
 untranscribed DNA strand = coding strand
▪ same sequence as RNA
 synthesis of complementary RNA strand =mRNA
 enzyme
▪ RNA polymerase
coding strand
5′
DNA
C
G
3′
build RNA 5′→3′
A
G
T
A T C
T A
rewinding
mRNA
5′
G
C
A
G C
A
T
C
G
T
T
A
3′
G C A U C G U
C
G T A G C A
T
RNA polymerase
T
A
A
C
T
A G
C T
G
A
T
unwinding
3′
5′
template strand

RNA
polymeraseProduces
mRNA by
adding
together the
chain of RNA
nucleotides

Promoter region
 binding site before beginning of gene
 TATA box binding site
 binding site for RNA polymerase
& transcription
factors

Enhancer region
 binding site far
upstream of gene
▪ turns transcription
on HIGH

Initiation complex
 transcription factors bind to promoter region
▪ group of proteins which bind to DNA
▪ hormones?
▪ turn on or off transcription
 trigger the binding of RNA polymerase to DNA

Match RNA bases to DNA
bases on one of the DNA
strands
G
G
U
C
A
A G
C
A
U
A
G
U
C
G
A
U
A
C
5'
RNA
A C C polymerase
A
G
U
3'
T G G T A C A G C T A G T C A T C G T A C C G T
U
C

Eukaryotic genes are not continuous
 exons = the real gene
▪ expressed / coding DNA
 introns = removed, “junk” DNA
▪ inbetween sequence
introns
come out!
intron = noncoding (inbetween) sequence
eukaryotic DNA
exon = coding (expressed) sequence

Post-transcriptional processing
 eukaryotic mRNA needs work after transcription
 primary transcript = pre-mRNA
 mRNA splicing
▪ edit out introns
 make mature mRNA transcript
intron = noncoding (inbetween) sequence
~10,000 bases
eukaryotic DNA
exon = coding (expressed) sequence
primary mRNA
transcript
mature mRNA
transcript
pre-mRNA
~1,000 bases
spliced mRNA

Need to protect mRNA on its trip from nucleus
to cytoplasm
 enzymes in cytoplasm attack mRNA
▪ protect the ends of the molecule
▪ add 5′ GTP cap
▪ add poly-A tail
▪ longer tail, mRNA lasts longer: produces more protein
3'
mRNA
5'
PP
G P
A
Now we have mature mRNA transcribed from
the cell’s DNA. It is leaving the nucleus
through a nuclear pore. Once in the
cytoplasm, it finds a ribosome so that
translation can begin.
We know how mRNA is made, but how do we
“read” the code?

From nucleic acid language
to amino acid language


Second stage of protein production
mRNA is on a ribosome



Every 3 DNA bases pairs with 3 mRNA bases
Every group of 3 mRNA bases encodes a
single amino acid
Codon- coding triplet of mRNA bases
DNA
TACGCACATTTACGTACGCGG
codon
mRNAAUGCGUGUAAAUGCAUGCGCC
?
protein
MetArgValAsnAlaCysAla



Second stage of protein production
mRNA is on a ribosome
tRNA brings amino acids to the ribosome


Code for ALL life!
 strongest support for a
common origin for all life
Code is redundant
 several codons for each
amino acid
 3rd base “wobble”
Why is the
wobble good?

Start codon



AUG
methionine
Stop codons

UGA, UAA, UAG
DNA
3′
5′
5′
3′
TACGCACATTTACGTACGCGG
mRNAAUGCGUGUAAAUGCAUGCGCC
3′
tRNA
amino
acid
UAC
Met
codon
5′
GCA
CAU
Arg
Val
anti-codon

“Clover leaf” structure
 anticodon on “clover leaf” end
 amino acid attached on 3′ end

Facilitate coupling of
tRNA anticodon to
mRNA codon

Structure
 ribosomal RNA (rRNA) & proteins
 2 subunits
▪ large
▪ small
E P A

A site (aminoacyl-tRNA site)
 holds tRNA carrying next amino acid to be
added to chain

P site (peptidyl-tRNA site)
 holds tRNA carrying growing polypeptide
chain

Met
E site (exit site)
 emptytRNA
leaves ribosome
from exit site
U A C
A U G
5'
E
P
A
3'



Initiation
 mRNA, ribosome subunits, initiator
tRNA come together
Elongation
 adding amino acids based on codons
Termination
 STOP codon = Release factor
3
2
Val
Leu
Met
Met
Met Leu
Met
Leu
1
Ala
Leu
release
factor
Ser
Trp
tRNA
UAC
5'
C UG A A U
mRNA A U G
3'
E
P
A
5'
UA C G A C
A UG C U GA AU
5'
3'
U A C GA C
A U G C U G AA U
3'
5'
UAC G AC
AA U
A U G C UG
3'
A CC
U GG UA A
3'
Destinations:

Signal peptide
 address label





start of a secretory pathway


secretion
nucleus
mitochondria
chloroplasts
cell membrane
cytoplasm
etc…
RNA polymerase
DNA
Can you tell the
story?
amino
acids
exon
intron
pre-mRNA
tRNA
5' GTP cap
mature mRNA
poly-A tail
large ribosomal subunit
aminoacyltRNA
synthetase
3'
polypeptide
5'
small ribosomal subunit
tRNA
E P A
ribosome
enhancer
1000+b
3'
20-30b
RNA
TATA
polymerase
DNA
promoter
translation
start
TAC
translation
stop
exons
transcriptional unit (gene)
UTR
UTR
introns
transcription
start
transcription
stop
5'
pre-mRNA
5'
GTP mature mRNA
5'
DNA
ACT
3'
3'
AAAAAAAA
Bacterial chromosome
Transcription
mRNA
Psssst…
no nucleus!
Cell
membrane
Cell wall

Prokaryotes

Eukaryotes
 DNA in cytoplasm
 DNA in nucleus
 circular chromosome
 linear chromosomes
 naked DNA
 DNA wound on
 no introns
histone proteins
 introns vs. exons
intron = noncoding (inbetween) sequence
eukaryotic
DNA
exon = coding (expressed) sequence
introns
come out!

Transcription & translation are simultaneous in
bacteria
 DNA is in
cytoplasm
 no mRNA
editing
 ribosomes
read mRNA
as it is being
transcribed

Differences between prokaryotes &
eukaryotes
 time & physical separation between processes
▪ takes eukaryote ~1 hour
from DNA to protein
 no RNA processing

Point mutations
 single base change
▪ silent mutation
▪ no amino acid change
▪ redundancy in code
▪ missense
▪ change amino acid
▪ nonsense
▪ change to stop codon
When do mutations
affect the next
generation?
What kind of mutation?
Missense!

Frameshift
 shift in the reading
frame
▪ changes everything
“downstream”
 insertions
▪ adding base(s)
 deletions
▪ losing base(s)
Where would this mutation
cause the most change:
beginning or end of gene?
Transcription
 Process by which
genetic information
encoded in DNA is
copied onto
messenger RNA
Translation
 Process by which
information encoded
in mRNA is used to
assemble a protein at
a ribosome

Occurs in the nucleus

Occurs on a Ribosome

DNA

mRNA
mRNA
protein