Transcript Slide 1 - SCHOOLinSITES

Transcription and Translation
Topic 3.5
Assessment Statements
3.5.1 Compare the structure of RNA and
DNA
3.5.2 Outline DNA transcription in terms of
the formation of an RNA strand
complementary to the DNA strand by
RNA polymerase
3.5.3 Describe the genetic code in terms of
codons composed of triplets of bases
3.5.4 Explain the process of translation,
leading to polypeptide formation
3.5.5 Discuss the relationship between one
gene and one polypeptide
Control center?
• DNA controls proteins
produced in cell
• Enzymes control
biochemistry of cell
• Protein synthesis occurs
b/c of transcription and
translation
• Both processes produce or
require RNA
Structure comparison of DNA &
RNA
DNA
5-carbon sugar
deoxyribose
Contains 4
bases: C, G, A, T
RNA
5-carbon sugar
ribose
Contains 4
bases: C, G, A, U
(uracil)
Double stranded Single stranded
Transcription
• Produces RNA molecules
• A section of DNA that
codes for polypeptides are
called genes (inside
nucleus)
• Messenger RNA (mRNA)
carries message of DNA to
the cytoplasm where
protein synthesis takes
place
Process of transcription
1.
2.
3.
4.
5.
•
•
RNA polymerase unzips DNA at one
gene
RNA polymerase creates a
complimentary strand of RNA
nucleotides to one DNA strand
Base pairing the same, except uracil
binds to adenine
mRNA detaches from the single-strand
DNA template and floats free in the
nucleoplasm
Floats through nuclear pores and then to
cytoplasm
Information used to make one
polypeptide (amino acids covalently
bonded together in a specific
sequence)
Written in triplets (every 3 bases=one
amino acid) and is called a codon
Types of RNA
• mRNA – messenger RNA,
complementary copy of a DNA gene
and codes for a single polypeptide
• rRNA – ribosomal RNA, each
ribosome is composed of rRNA and
ribosomal protein
• tRNA – transfer RNA, each type of
tRNA transfers 1 of the 20 amino acids
to the ribosome for polypeptide
formation
Process of translation
1.
2.
3.
4.
5.
6.
7.
8.
9.
mRNA locates a ribosome
Aligns so that the first two codon triplets are within
boundaries of the ribosome
A specific tRNA floats in, its tRNA anticodon
complementary to the first codon triplet of the mRNA
molecule, with an amino acid attached
While the first tRNA sits in the ribosome, a second tRNA
molecule brings a second amino acid
An enzyme catalyzes a condensation reaction
between the two amino acids and the resulting
covalent bond between them is called a peptide bond
Bond is broken between the first tRNA molecule and
the amino acid that it transferred to the chain
First tRNA floats away to “reload”
Process continues until last codon triplet which does
not code for an amino acid signals “stop”
Entire polypeptide breaks away from the final tRNA
molecule and becomes a free floating polypeptide
One gene/one polypeptide
hypothesis
• Early work led to hypothesis that
every gene produced one
enzyme
• Later discovered that many
proteins are composed of more
than one polypeptide
• Latest, one gene may lead to a
single mRNA molecule, but the
mRNA molecule may then be
modified
Transcription
Topic 7.3
Assessment Statements
7.3.1 State that transcription is carried
out in a 5’ to 3’ direction
7.3.2 Distinguish between the sense and
antisense strands of DNA
7.3.3 Explain the process of transcription
in prokaryotes, including the role of
the promoter region, RNA
polymerase, nucleoside
triphosphates and the terminator
7.3.4 State that eukaryotic RNA needs
the removal of introns to form mature
mRNA
Transcription carried out in a 5’ to
3’ direction
• RNA polymerase combines
with region of DNA called
a promoter (not
transcribed)
• RNA polymerase unzips
DNA
• RNA polymerase initiates
synthesis of an RNA
molecule in a 5’ to
3’direction
Sense and antisense strands
• Sense strand – coding
strand (same sequence as
RNA strand)
• Antisense strand –
template strand (copied
during transcription)
The terminator
• Sequence of nucleotides
that causes the RNA
polymerase to detach
from the DNA
• NTPs pair with antisense
strand and polymerization
of the mRNA occurs
• Portion of transcription
known as elongation
Post-transcription processing
• Within eukaryotic DNA proteincoding regions there are noncoding regions
• Exons – coding regions
• Introns – non-coding regions
• Introns have to be removed to
make a functional mRNA strand
• Prokaryotic mRNA does not
require processing because no
introns are present
Translation
Topic 7.4
Assessment Statements
7.4.1 Explain that each tRNA molecule is recognized by
a tRNA-activating enzyme that binds a specific
amino acid to the tRNA, using ATP for energy
7.4.2 Outline the structure of ribosomes, including
protein and RNA composition, large and small
subunits, three tRNA binding sites and mRNA binding
sites
7.4.3 State that translation consists of initiation,
elongation, translocation, and termination
7.4.4 State that translation occurs in a 5’ to 3’ direction
7.4.5 Draw and label the structure of a peptide bond
between two amino acids
7.4.6 Explain the process of translation, including
ribosomes, polysomes, start codons, and stop
codons
7.4.7 State that free ribosomes synthesize proteins for
use primarily within the cell, and that bound
ribosomes synthesize proteins primarily for secretion
or for lysosomes
Translation
• Process that changes the
language of DNA to the
language of protein
• Occurs at the ribosome
Ribosomes
• Each consists of a large
subunit and a small subunit
composed of rRNA proteins
and many distinct proteins
• rRNA proteins are small and
are near core
• 2/3 of ribosome mass is rRNA
• Made in nucleolus
• Prokaryotic ribosomes smaller
than eukaryotic ribosomes
• Decoding of mRNA occurs in
space between subunits
Binding sites between subunits
Site
Function
A
site
Holds the tRNA carrying the
next amino acid to be
added to the polypeptide
chain
P
site
Holds the tRNA carrying the
growing polypeptide chain
E
site
Site from which tRNA that has
lost its amino acid is
discharged
• tRNA moves sequentially
through the binding sites:
from the A site, to the P
site, to the E site
• Growing polypeptide
chain exits the ribosome
through a tunnel in the
large subunit core
RNA → protein
1.
2.
3.
4.
Initiation
Elongation
Translocation
Termination
Background information
• What are codons?
• 3 nucleotides that together carry
genetic code from DNA to ribosomes
via mRNA
• Start codon (AUG) is on the 5’ end of
all mRNAs
• Each codon, other than the 3 stop
codons, attaches to a particular tRNA
• 3’ end of tRNA has base sequence
CCA which is site of amino acid
attachment
• One loop of tRNA contains an
exposed anticodon which pairs with a
specific codon of mRNA
• Each of the 20 amino acids
binds to the appropriate tRNA
due to the action of a
particular enzyme
• How many enzymes?
• Active site of each enzyme
allows a fit only between a
specific amino acid and the
specific tRNA
• Requires energy which is
supplied by ATP
• Ready for delivery!!
Initiation
1.
2.
3.
4.
5.
Activated amino acid (methionine
attached to tRNA with the
anticodon UAC) combines with
mRNA strand and small ribosomal
subunit
Small subunit moves down the
mRNA until it contacts the start
codon AUG
Contact starts translation
Hydrogen bonds form between the
initiator tRNA and the start codon
Large ribosomal subunit combines to
form translation initiation complex
and uses energy from guanosine
triphosphate (GTP)
Elongation
1. tRNAs bring amino acids to the
mRNA-ribosomal complex in the
order specified by codons
2. Proteins called elongation factors
assist in binding the tRNAs to the
exposed mRNA codons at the A
site
3. Initiator tRNA then moves to the P
site
4. Ribosome catalyzes formation of
peptide bonds between adjacent
amino acids brought to the
polypeptide assembling area
Translocation
1. Occurs during elongation
2. Transferring of
polypeptide chain to the
new tRNA that moves into
the now exposed A site
3. Now empty tRNA is
transferred to the E site
where it is released
Termination
1. Begins when one of the
three stop codons
appears at the open A
site
2. Protein fills the A site
3. Frees the polypeptide
from the ribosome
4. Ribosome separates from
the mRNA and splits into
two subunits
Newly synthesized proteins
• If produced by free
ribosomes:
– Used within cell
• If produced by bound
ribosomes:
– Secreted from the cell
– Used in lysosomes