Transcription and Translation
Transcription and Translation
1. What is transcription? Explain what
happens during transcription.
2. What is translation? Explain what
happens during translation.
3. Explain how transcription and translation
are related to DNA replication.
4. If you begin with a parent DNA strand of
A A T G C A G T, what will the
complementary mRNA strand be? (Think
about it before you answer.)
Compare DNA and RNA
DNADeoxyribonucleic Acid
1. Bases-cytosine,
guanine, adenine and
thymine
2. Double stranded
3. Function-store genetic
information
4. Deoxyribose sugar
RNARibonucleic Acid
1. Base-Cytosine,
guanine, adenine and
uracil
2. Single stranded
3. Functions- See next
slide
4. Ribose sugar
RNA Functions
Functionsa. rRNA-ribosomal RNA (makes up about 60% of
ribosomal structure
b. mRNA-messenger RNA (record information from
DNA and carry it to ribosomes)
c. tRNA-transfer RNA (delivers amino acids to
proteins at the ribosome to extend the chains)
RNA
nucleotide
DNA
nucleotide
deoxyribose
Comparison of RNA and DNA
sugars
Deoxyribose
Ribose
Compared
structures of
DNA and RNA
RNA single strand with hairpin loop
Although it looks like a double helix, it is one strand
wrapped around itself. It is similar to a twisted bobby pin.
Reading Quiz
1. Transcription is the process of making:
a. RNA
b. tRNA
c. mRNA d. rRNA
2. An intron is found in:
a. DNA
b. RNA
c. mRNA d. tRNA
3. The enzyme used in transcription is:
a. RNA primase
b. RNA polymerase
c. DNA polymerase
d. a and b are both correct
4. How many bases make a codon?
a. 1
b. 2
c. 3
d. 4
5. Translation occurs in the:
a. nucleus b. cytoplasm
c. both
Transcription
Transcription (Initiation)
• RNA polymerase binds to the promoter site
and begins base pairing the transcription unit
(area of DNA that will be transcribed
• The area where transcription factors and RNA
polymerase are bound to the promoter is called
the Transcription initiation complex
• Promoter =region of DNA where RNA
polymerase attaches and initiates transcription
– -Determines which strand of DNA will serve as the
template
Transcription (Initiation)
TATA box -promoter DNA sequence
-the actual sequence is 5'-TATAAA-3'
-RNA polymerase binding site
Transcription (RNA strand
elongation)
A. RNA polymerase moves along DNA
template
B. It unwinds 10-20 DNA bases at a time
C. RNA polymerase adds nucleotides in the
5’→3’ direction
D. As RNA polymerase moves along, the
DNA double helix reforms
E. The new section of RNA ‘peels away’ as
the double helix reforms
Transcription (termination)
A. Transcription stops when RNA
polymerase reaches a section of DNA
called the terminator
B. Terminator sequence = AAUAAA
C. Next, the RNA strand is released and
RNA polymerase dissociates from the
DNA
D. The RNA strand will go through more
processing
Sense vs. Antisense DNA strands
A. The DNA double helix has two strands
B. Only one of them is transcribed
C. The transcribed strand is the antisense
strand
D. The non transcribed strand is the sense
strand
E. mRNA is complementary to the
anitsense strand
Sense vs. Antisense DNA strands
F. The 5’ end of the RNA nucleotides are
added to the 3’ end of the growing chain
G. RNA nucleotides are linked together in
the same fashion as DNA molecules
Because mRNA becomes degraded by
other enzymes, it must go through mRNA
processing.
RNA Processing
RNA splicing (in eukaryotes)
Introns-non-coding sections of nucleic acid found
between coding regions
Exons-coding regions of nucleic acids
(eventually these are expressed as amino
acids)
RNA splicing (in eukaryotes)
Alternative Splicing
Exit Slip
• Where exactly in the cell do the following
processes take place?
– Transcription
– Translation
• Distinguish between introns and exons.
• Why is it important to modify the mRNA
after it is produced by transcription in
eukaryotic cells?
Translation
Translation
A. Translation-forming of a polypeptide
-uses mRNA as a template for a.a.
sequence
-4 steps (initiation, elongation,
translocation and termination)
-begins after mRNA enters cytoplasm
-uses tRNA (the interpreter of mRNA)
Translation
B. Ribosomes
-made of proteins and rRNA
-each has a large and small subunit
-each has three binding sites for tRNA on
its surface
-each has one binding site for mRNA
-facilitates codon and anticodon bonding
-components of ribosomes are made in
the nucleus and exported to the cytoplasm
where they join to form one functional unit
#8. Translation
B. Ribosomes (continued)
-the three tRNA binding sites are:
1. A site=holds tRNA that is carrying
the next amino acid to be added
2. P site= holds tRNA that is carrying
the growing polypeptide chain
3. E site= where discharged tRNAs
leave the ribosome
Ribosomal structure
Large subunit
Exit
site
Peptidyl-tRNA binding site
E
5’
Aminoacyl-tRNA binding site
P A
3’
mRNA
Small subunit
Translation
C. The genetic code
– Four RNA nucleotides are arranged 20
different ways to make 20 different amino
acids
– Nucleotide bases exist in triplets
– Triplets of bases are the smallest units that
can code for an a.a.
– 3 bases = 1 codon = 1 a.a.
– There are 64 possible codes (64=43)
Translation
C. The genetic code
– Most of the 20 a.a. have between 2 and 4
possible codes
– The mRNA base triplets are codons
– In translation the codons are decoded into
amino acids that make a polypeptide chain
– It takes 300 nucleotides to code for a
polypeptide made of 100 amino acids (Why?)
Translation
C. The genetic code (continued)
– 61 of 64 codons code for a.a.
– Codon AUG has two functions
-codes for amino acid methionine (Met)
-functions as a start codon
– mRNA codon AUG starts translation
– The three ‘unaccounted for’ codons act as
stop codons (end translation)
Translation
D. How it works
DNA (antisense)
ACCAAACCG
mRNA (transcription)
UGGUUUGGC
polypeptide (translation)
Trp - Phe - Gly-
Translation
E. More on tRNA
– tRNA is transcribed in the nucleus and must enter the
cytoplasm
– tRNA molecules are used repeatedly
– Each tRNA molecule links to a particular mRNA
codon with a particular amino acid
– When tRNA arrives at the ribosome it has a specific
amino acid on one end and an anticodon on the other
– Anticodons (tRNA) bond to codons (mRNA)
p. 304 (red book)
Where the a.a. attaches
=
Although we
draw tRNA in
a clover
shape it’s
true 3-D
conformation
is L-shaped.
Hydrogen bonds
Anticodon
tRNA diagrams
Translation (Initiation)
A. Initiation
1. Brings together mRNA, tRNA (w/ 1st
a.a.) and ribosomal subunits
2. Small ribosomal subunit binds to mRNA
and an initiator tRNA
-start codon= AUG
-start anticodon-UAC
-small ribosomal subunit attaches to 5’
end of mRNA
#9. Translation (Initiation)
B. Initiation
2. (continued)
-downstream from the 5’ end is the
start codon AUG (mRNA)
-the anticodon UAC carries the a.a.
Methionine
3.After the union of mRNA, tRNA and
small subunit, the large ribosomal subunit
attaches
4. Initiation is complete
Translation (Initiation)
B. Initiation
5. The intitiator tRNA and a.a. will sit in the
P site of the large ribosomal subunit
6. The A site will remain vacant and ready
for the aminoacyl-tRNA
Translation (Initiation)
Translation (Elongation)
A. Amino acids are added one by one to the first
amino acid (remember, the goal is to make a
polypeptide)
B. Step 1- Codon recognition
a. mRNA codon in the A site forms hydrogen
bonds with the tRNA anitcodon
C. Step 2- Peptide bond formation
a. The ribosome catalyzes the formation of the
peptide bonds between the amino acids (the
one already in place and the one being
added)
b. The polypeptide extending from the P site
moves to the A site to attach to the new a.a.
Translation (Translocation)
A. The tRNA w/ the polypeptide chain in the
A site is translocated to the P site
B. tRNA at the P site moves to the E site
and leaves the ribosome
C. The ribosome moves down the mRNA in
the 5’→3’ direction
Translation (Termination)
A. Happens at the stop codon
B. Stop codons are UAA, UAG and UGA
-they do not code for a.a.
C. The polypeptide is freed from the ribosome and
the rest of the translation assembly comes
apart
D. Animation (you move it)
E. Animation (you watch it)
F. Animation (McGraw-Hill)
Gene expression
A. Jacob and Monad (1961)
-studied control of protein synthesis in E.
coli and lactose digesting enzymes
-found that E. coli do not produce lactose
digesting enzymes when grown in a
medium without lactose
-when bacteria were placed in a lactose
environment, enzymes were found within
minutes
Gene expression
B. Genes can be switched on or off as
necessary
-a gene that is ‘on’ will be transcribed
-in E.coli, the enzyme lactase will be
produced if the gene is ‘on’
-if the gene is ‘off’ mRNA will not be
created and translation can not occur
Gene expression
C. The operon model
-proposed by Jacob and Monad
-explains how genes switch on and off
-operon=promoter, operator and structural
genes
-lac operon is found in E.coli
Gene expression
D. The lac operon
Gene expression
D. The lac operon (no lactose)
-lactose is absent, repressor is active,
operon is off, no mRNA is produced, RNA
polyermase cannot bind because it is
blocked by the repressor that has bound to
the operator
Gene expression
D. The lac operon (lactose is present)
-lactose is present, repressor is inactive, operon
is on, mRNA is transcribed, RNA polymerase
binds to operator
-an isomer of lactose binds to the repressor and
changes its shape
-this prevents it from binding to the operator
-lactase is produced
#11. Compare Transcription in
Eukaryotes and Prokaryotes
Link