Transcript No Slide Title

Bio 1010
Dr. Bonnie A. Bain
DNA Structure and Function
Part 2
Chemical Languages:
The best way to understand the next section is
to think of there being a DNA language, an
RNA language, and a polypeptide language
DNA Language:
DNA has 4 kinds of monomers (nucleotides):
A,T, C, G
Monomers are the letters in this language
The language of DNA is written as a linear
sequence of nucleotides:
AAACCGGCAAAA
DNA Language:
Gene:
A specific sequence of bases, each with a
beginning and an end
A typical gene can consist of thousands of
nucleotides
One DNA molecule can contain thousands of
genes
RNA Language:
RNA has 4 kinds of monomers (nucleotides):
A,U, C, G
The language of RNA is written as a linear
sequence of nucleotides:
UUUGGCCGUUUU
Transcription
When the language of DNA is transcribed into
the language of RNA
This is how a mRNA molecule is built:
DNA sequence is
AAACCGGCAAAA
mRNA sequence is UUUGGCCGUUUU
Figure 10.10
Transcription
The nucleotide bases of the RNA molecule are
complementary to the ones on the DNA strand
The DNA molecule was used as a template to
make mRNA
Translation
Conversion of the nucleic acid language to the
polypeptide language
The monomers or letters of the polypeptide
language are the 20 different amino acids
Translation
The sequence of nucleotides in a mRNA
molecule dictates the sequence of amino acids
in the polypeptide
mRNA
UUU GGC CGU
Polypeptide
AA1
AA2
AA3
UUU
AA4
Figure 10.10
The Triplet Code
Every 3 bases codes for one amino acid
Each triplet in the mRNA is called a Codon
mRNA
UUU GGC CGU UUU
Polypeptide
AA1
AA2
AA3 AA4
Why a triplet code?
This is the only way to get enough “words” to
specify all of the required amino acids
4 bases (in mRNA): A, G, C, U
20 different amino acids
43 = 64 different possible codons
With 64 possible code words,
can have redundant coding for each amino acid
Example:
Glutamic Acid codons: GAA, GAG
Glycine codons: GGU, GGC, GGA, GGG
REDUNDANCY in the code, but not
AMBIGUITY
Also, can have START and STOP codons
START codon
AUG
Note: also codes for an amino acid (Met)
STOP codons
UAA
UAG
UGA
First Base
Second Base
Third Base
In a codon, first base is the first letter,
second base is the second letter,
third base is the third letter
AUG
Question:
What are these codons or triplets?
Answer:
The Genetic Code
It is shared by all organisms, although there is
a tiny bit of variation
Figure 10.11
Figure 10.00d
Tobacco plant + a Firefly gene
Should say most, not all share the same genetic
code—there are a few variations
Transcription (more details)
A mRNA molecule is transcribed from the DNA
molecule in the nucleus of the cell
Enzyme required:
RNA Polymerase
Figure 10.13a
Transcription (con't)
Similar to DNA Replication, but not quite the
same
1. DNA molecule is split apart by RNA
Polymerase
2. One of the DNA strands serves as a
template
Transcription (con't)
3. Initiation of transcription
4. Elongation of mRNA strand
5. Termination of transcription
Figure 10.13b
3. Initiation of Transcription
Promoter region:
A specific site on the DNA where the RNA
Polymerase attaches
Located at the beginning of a gene
Dictates which DNA strand is to be transcribed
The RNA Polymerase “knows” that it starts
transcription at the promoter
Figure 10.13b
3. Initiation of Transcription
First, the RNA Polymerase binds to promoter
Second, RNA synthesis begins
During transcription, an entire gene is
transcribed into a mRNA molecule
Figure 10.13a
Transcription (con't)
4. Elongation of mRNA strand
During this phase, the mRNA grows longer
Behind it, the DNA strands come back together
Figure 10.13b
Transcription (con't)
5. Termination of transcription
Eventually the RNA Polymerase reaches the
Terminator region of the gene (end of the gene)
The terminator region says, “Stop transcribing”
At this point, the RNA Polymerase detaches and
the mRNA leaves the nucleus
Prokaryotic cells:
Lack a nucleus, after mRNA is transcribed, it
immediately takes part in translation
Eukaryotic cells
Have a nucleus, after mRNA is transcribed, it
has to be processed before leaving the nucleus
Processing of Eukaryotic RNA
1. Addition of extra nucleotides
Cap and Tail are added to the RNA strand
Protect the mRNA from cellular enzymes
Help ribosomes recognize the mRNA
Figure 10.14
Processing of Eukaryotic RNA
2. RNA Splicing
Introns are removed from the mRNA
Introns are non-coding stretches of the
mRNA
Intron functions: ?? (still unknown)
Figure 10.14
2. RNA Splicing (con't)
Exons:
The coding regions of the mRNA
After the Introns are removed, only the Exons
are left
2. RNA Splicing (con't)
In humans, the Exons in the mRNA can be
selectively removed, making different
polypeptide “recipes”
Allows 25,000 genes (in humans) to make lots
more than 25,000 different polypeptides
EXAM GOES TO HERE
Note: this lecture continues on Part 4—this file is
too big!!
Figure 10.10
Translation (p. 183-186)
(translate the mRNA code into a polypeptide)
The Players
mRNA
tRNA
Ribosomes (rRNA + protein)
The Process
Initiation
Elongation
Termination
Figure 10.17
mRNA
Carries the information from the DNA
to the Ribosome
Figure 10.15
tRNA
Transfer RNA or tRNA
The “Interpreter”
Converts the 3-letter codon to an amino acid
At the ribosome (protein factory), the tRNA
molecules match the amino acids with the
appropriate codons in the mRNA
Transfer RNA or tRNA
Source of amino acids:
Cytoplasm of the cell
tRNA tasks:
1. Pick up the appropriate amino acids
2. Bring them to the ribosome and place them on
the right spot in the mRNA
tRNA Structure
Single strand of RNA
consists of approx. 80 nucleotides
The single strand is twisted and folded into a
particular shape
Figure 10.15
tRNA Structure
Has two distinct ends:
Anticodon:
A triplet (3 bases) which will pair with a
specific codon in the mRNA
Amino Acid attachment site
A triplet which pairs with a specific amino acid
20 different tRNA molecules
One for each of the 20 amino acids
Figure 10.16
Ribosomes:
Protein Synthesis occurs here
Ribosomes
Site where polypeptides are made
(protein factory)
Ribosome structure
Small subunit
has binding site for mRNA
Large subunit
2 binding sites for tRNA
Large subunit
2 binding sites for tRNA:
P site
Holds the tRNA carrying the growing
polypeptide chain
A site
Holds a tRNA carrying the next amino
acid to be added to the growing
polypeptide
Figure 10.16a
Figure 10.16
Translation
(translate the mRNA code into a polypeptide)
The Players
*mRNA
*tRNA
*Ribosomes (rRNA + protein)
The Process
Initiation
Elongation
Termination
Initiation of Translation
1. An mRNA molecule binds to a small ribosomal
subunit
A special “initiator” tRNA then binds to the Start
codon on the mRNA
The “initiator” tRNA carries the amino acid
methionine on its amino acid attachment site
Translation
Initiation
Elongation
Termination
Translation
Initiation
Elongation
Termination
Translation
Initiation
Elongation
Termination