Transcript Transcription and Translation - University of British Columbia

Unit 4
Biological Information Flow
Information Flow
REPLICATION - The Players
•
•
•
•
•
Leading strand DNA
Lagging strand DNA
DNA Polymerase III
Helicase
RNA Primase and
RNA Primers
• Okazaki Fragments
• Ligase
REPLICATION (DNA Synthesis)
• DNA Helicase partially unwinds the
double helix at an area known as the
replication fork.
• As the two DNA strands separate and the
bases are exposed, the enzyme DNA
polymerase III moves into position at the
point where synthesis will begin.
• Primase synthesizes RNA primers (why
are there more on lagging strand?)
REPLICATION cont.
• Leading strand is synthesized
contiguously but lagging strand is
synthesized discontiguously….WHY?
• After DNA is synthesized, another DNA
Polymerase removes RNA primers and
replaces them with DNA
• The gaps in sugar-phosphate backbone
are then sealed with ligase resulting in 2
exact copies of DNA! 
Replication cont.
TRANSCRIPTION =
making mRNA from DNA
DNA = STORING GENETIC INFORMATION
RNA = three different functions:
• Ribosomal RNAs – (rRNA) makes us the
ribosome. Each ribosome consists of about 60%
ribosomal RNA and 40% protein.
• Messenger RNAs – (mRNA) "record" information
from DNA in the cell nucleus and carry it to the
ribosomes.
• Transfer RNAs- (tRNA) delivers amino acids one
by one to protein chains growing at ribosomes
TRANSCRIPTION cont.
•
•
•
Not all DNA is transcribed, only
transcription units (genes).
Each transcription unit encodes an
RNA transcript or family of transcripts.
These transcripts are
–
–
–
processed
transported to the cytoplasm
translated (mRNA)
Note: mRNA is made in the NUCLEUS!
Transcription Unit
(Left to right, 3'' to 5'' on copied [template] strand)
Transcription
1. Carried out by an enzyme complex known as RNA
polymerase.
2. Faithfully copies one of two DNA strands
3. RNA polymerase binds to a special region (DNA
sequence) called the promoter.
4. RNA polymerase unwinds about 1 turn of DNA (about
10 base pairs) to produce a single stranded template.
(hydrogen bonds of the DNA are broken by the
unwinding process)
5. The first two nucleotides are hydrogen-bonded to DNA
and are joined 5' to 3'
6. Continue to add new nucleotides to the 3' end.
Transcription cont.
7. As synthesis proceeds, the RNA polymerase moves in
the 3' to 5' direction on the copied strand.
8. When a specific terminator sequence is reached,
transcription stops.
9. Enzymes and the single stranded RNA product are then
released.
10. The orientation of the RNA is determined by the
promoter, that is, by which strand contain the sequence
that is recognized by RNA polymerase. This can be
oriented in either direction.
NOTE: DNA is always read 3' to 5' so that the new RNA
is made in the 5' to 3' direction.
TRANSCRIPTION
Many RNA polymerase molecules may
process the same transcription unit at
the same time.
See animation of transcription
http://vcell.ndsu.nodak.edu/anima
tions/transcription/movie.htm
TRANSLATION
- Involves the transfer of biological
information from the 4 nucleotide
code of nucleic acids (A,C,G,T) into
the 20 amino acid code of proteins
- Each 3 nucleotide coding unit that
specifies an amino acid is called a
codon. A codon is a group of three
consecutive nucleotides required to
specify a single amino acid
TRANSLATION
(3 easy steps)
1. INITIATION
- an mRNA, a ribosome, and the first tRNA
molecule come together.
- The tRNA containing MET will recognize the
start signal and bind the “P” site of the
ribosome complex.
TRANSLATION cont.
2. ELONGATION
- The incoming tRNA binds the A site (the only
tRNA able to attach is complementary to the
codon of the A site on the mRNA.)
- A peptide bond forms bewteen amino acids
in A and P sites. (growing peptide now at A
site)
- The ribosome complex moves so the growing
peptide is now in P site, tRNA from Met is in the
Exit and released, a new tRNA containing
another amino acid binds the A site.
TRANSLATION cont.
3. TERMINATION
- A "stop" codon (UAA, UGA, or UAG) signals
the end of the process.
- An enzyme called the releasing factor binds
the P site freeing the PROTEIN
- The mRNA molecule is released from the
ribosome complex which then falls apart
- All mRNA messages are eventually degraded
when the protein no longer needs to be made.
Translation
Animation:
http://www-class.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a3.html
Key Points to remember
• Transcription: DNA to RNA
– DNA is read 3’ to 5’ while RNA is made 5’ to 3’
• Translation: making a polypeptide chain from
mRNA
– mRNA is read 5’ to 3’.
• Complementary base pairing. A with T, C with
G. In RNA the T is replaced by Uracil (U).
Practice Problem:
• Look at the following DNA template
strand. Figure out the complementary
mRNA sequence, the tRNA
anticodons, and the appropriate
amino acid sequence
3’- A G C T T A C C G T G G - 5’