Chapter 7
Gene Expression and Control
Part 2
Transcription: DNA to RNA
 The same base-pairing rules that govern DNA
replication also govern transcription with one
exception: In RNA, A pairs with U.
Transcription: DNA to RNA
 During transcription, a strand of DNA (the antisense
strand) acts as a template upon which a strand of RNA is
assembled, using RNA nucleotides.
 Transcription is similar to DNA replication in that a DNA
strand acts as a template for assembling another strand,
in this case, of RNA.
 However, a difference is that only part of the DNA strand
(a gene) is used as a template, not the entire DNA strand.
 Also, the enzyme, RNA polymerase, builds the growing
RNA strand (whereas, in DNA replication, DNA
polymerase builds the growing DNA strands).
The Process of Transcription
Transcription occurs in the nucleus of cells.
1. RNA polymerase and several regulatory proteins
attach to a specific binding site in the DNA, called a
promoter, at a transcription site close to a gene.
2. The RNA polymerase moves along the DNA, over the
gene. As it moves, the RNA polymerase unwinds the
double helix so that it can “read” the base
sequence of the antisense (noncoding) strand of
DNA.
3. The RNA polymerase joins free RNA nucleotides into
a chain in the order dictated by the DNA sequence
4. When the RNA polymerase reaches the end of the
gene, the DNA and the new mRNA strand are
released.
The process of Transcription
 The new mRNA strand is complementary in base
sequence to the DNA strand from which it was
transcribed.
 It is essentially an RNA copy of the gene (DNA).
 Typically, many RNA polymerases transcribe a
gene at the same time, so many new mRNA
strands can be produced at the same time very
quickly.
Transcription: Homework 3
 What is the anti-sense strand?
 What enzyme is responsible for transcribing DNA
into RNA?
 Where does transcription happen?
 What is the promoter?
 Transcribe the following DNA sequence:
 TACAAATTTCGCATC
RNA Players in Translation:
mRNA
 mRNA is a disposable copy of a gene.
 Its job is to carry DNA’s protein-building message to the
other two types of RNA for translation.
 This protein building message consists of a linear sequence
of of genetic “words” spelled with the genetic alphabet of
A, G, C, and U.
 Each of these genetic “words” contains three letters and is
called a codon.
 Each codon codes for a particular amino acid.
 Since there are 4 possible bases and three possible
positions in a codon that each base could hold, there is a
total of 64 (or 43) possible codons.
 These 64 codons constitute the genetic code.
The Genetic Code
The Genetic Code
 Since one codon follows another in the mRNA,
the order of codons in the mRNA determines the
order of amino acids in the polypeptide that will
be formed from it.
 Thus the base sequence of a gene (DNA) is
transcribed into the base sequence of an mRNA,
which is in turn translated into an amino acid
sequence in the polypeptide.
The Genetic Code
 There are only twenty amino acids found in
proteins.
 So why are there 64 codons?
 Many amino acids are specified by more than
one codon. For example, GAA and GAG both
code for glutamic acid.
 Because of this, the genetic code is said to be a
degenerate code.
The Genetic Code
 In addition, some codons signal the beginning and end of a
protein-coding sequence.
 For example, in most species, the codon AUG in the mRNA
signals for translation to start.
 AUG also codes for the amino acid methionine. Therefore,
methionine is always the first amino acid to added to a new
polypeptide in most species.
 In addition, the codons UAA, UAG, and UGA do not code for any
amino acids at all. Therefore, their presence stops translation
and so they are called stop codons.
 These codons signal the end of a protein-coding sequence in an
mRNA molecule.
RNA Players in Translation:
rRNA
 Ribosomes consist of one large subunit and one
small subunit., both of which are composed of
structural proteins and rRNA, ribosomal RNA.
 In prokaryotes, the small subunit is a 30s and the
large subunit is a 50s. Together these two form the
70s prokaryotic ribosome.
 In eukaryotes, the small subunit is a 40s and the
large subunit is a 60s. Together these two form the
80s eukaryotic ribosome.
 These two ribosomal subunits come together to
form an intact ribosome on an mRNA molecule
during translation.
RNA Players in Translation:
rRNA
RNA Players in Translation:
tRNA
 Transfer RNA’s (tRNA) deliver amino acids amino
acids to ribosomes in the order specified by the
mRNA.
 Each tRNA has two attachment sites.
 One is the anticodon loop, where a triplet of
nucleotides that base pair with the mRNA codons
are located.
 The second is where the amino acid binds, the
amino acid specified by the codon.
 tRNA’s with different anticodons carry different
amino acids.
 The tRNA carrying its appropariate amino acid is
called a “loaded” tRNA.
Translation: RNA to Protein
 Translation occurs in the cytoplasm of the cell
upon ribosomes.
 It has three stages:
 Initiation
 Elongation
 Termination
Translation: RNA to Protein
 Initiation: The small ribosomal subunit binds to an
mRNA molecule. The anticodon of a special
tRNA called an initiator base pairs with the first
AUG codon of the mRNA. Then a large
ribosomal subunit joins the small subunit.
met
TAC
AUG
Translation: RNA to Protein
 Elongation: The ribosome assembles a polypepide
chain as it moves along the mRNA. The initiator tRNA
carries the amino acid methionine so the first amino
acid of the new polypeptide chain is methionine. A
second tRNA brings in a second amino acid as its
anticodon base pairs with the second codon in the
mRNA. The ribosome then joins the two amino acids
together by way of a peptide bond. This process
continues as new amino acids are added to the
growing polypeptide chain.
Translation: RNA to Protein
 Termination: When the ribosome reaches a stop
codon in the mRNA, the mRNA and polypeptide
detach from the ribosome and the ribosomal
subunits detach from one another. Translation is
now complete. The new polypeptide chain will
join other proteins in the cytoplasm or it will enter
the rough ER.
Translation: RNA to Protein
 In cells that make a lot of protein, many
ribosomes may be translating the same mRNA.
 These ribosomes are referred to as polysomes.
Translation: RNA to Protein
 Not only do cells make many polypeptides from one mRNA,
but they also make many copies of the same mRNA. Why?
 Since RNA is not a very stable molecule, it lasts only minutes
before being disassembled by cytoplasmic enzymes.
 This fast turnover allows cells to adjust their protein synthesis
rapidly in response to changing needs in a changing
environment.
 This also explains why cells make many copies of the same
mRNA molecule.
Translation: Homework 4
 What is a codon?
 How many codons are there in the genetic
code?
 Why is the genetic code called a degenerate
code?
 What is the start codon and what does it do?
 What are the stop codons and what do they do?
Translation: Homework 5
 Briefly describe the structure of a ribosome.
 What is the function of:
 mRNA?
 tRNA?
 rRNA?
 List the three steps of translation and briefly
describe what happens in each.
 Translate the mRNA sequence that you
produced from the DNA sequence that was
given in Homework 3.