Albia Dugger • Miami Dade College
Chapter 9
9.2 DNA, RNA, and Gene Expression
• Transcription converts information in a gene to RNA
DNA → transcription → mRNA
• Translation converts information in an mRNA to protein
The Nature of Genetic Information
• Each DNA strand consists of a chain of four kinds of
nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C)
• The sequence of the bases in the strand is the genetic code
Converting a Gene to an RNA
• Transcription
• Enzymes use the nucleotide sequence of a gene to synthesize a complementary strand of RNA
• DNA is transcribed to RNA
• Most RNA is single stranded
• RNA uses uracil in place of thymine
A DNA Nucleotide
base
(guanine)
3 phosphate groups
A DNA nucleotide: guanine (G), or
deoxyguanosine triphosphate
sugar
An RNA Nucleotide
base
(guanine)
3 phosphate groups
An RNA nucleotide: guanine (G), or
guanosine triphosphate
Figure 9-3 p151
DNA
deoxyribonucleic acid
RNA
ribonucleic acid
adenine A adenine A
sugar– phosphate
backbone
guanine G guanine G
cytosine C cytosine C
thymine T uracil U
DNA has one function: It permanently stores a cell’s genetic
information, which is passed to offspring.
RNAs have various functions. Some serve as disposable copies of DNA’s genetic
RNA in Protein Synthesis
• Messenger RNA (mRNA)
• Contains information transcribed from DNA
• Ribosomal RNA (rRNA)
• Main component of ribosomes, where polypeptide chains are built
• Transfer RNA (tRNA)
Converting mRNA to Protein
• Translation
• The information carried by mRNA is decoded into a
sequence of amino acids, resulting in a polypeptide chain that folds into a protein
• mRNA is translated to protein
Gene Expression
• A cell’s DNA sequence (genes) contains all the information needed to make the molecules of life
• Gene expression
• A multistep process including transcription and translation, by which genetic information encoded by a gene is
Take-Home Message
: What is the nature of
genetic information carried by DNA?
• Genetic information occurs in DNA sequences (genes) that encode instructions for building RNA or protein products
• A cell transcribes the nucleotide sequence of a gene into RNA
• Although RNA is structurally similar to a single strand of DNA, the two types of molecules differ functionally
• A messenger RNA (mRNA) carries a protein-building code in its nucleotide sequence; rRNAs and tRNAs interact to
9.3 Transcription: DNA to RNA
• RNA polymerase assembles RNA by linking RNA nucleotides into a chain, in the order dictated by the base sequence of a gene
DNA Replication and Transcription
• DNA replication and transcription both synthesize new molecules by base-pairing
• In transcription, a strand of mRNA is assembled on a DNA template using RNA nucleotides
• Uracil (U) nucleotides pair with A nucleotides
The Process of Transcription
• RNA polymerase and regulatory proteins attach to a
promoter (a specific binding site in DNA close to the start of a gene)
RNA polymerase binds to a promoter in the DNA. The binding positions the polymerase near a gene. In most cases, the base sequence of the gene occurs on only one of the two DNA strands. Only the DNA strand complementary to the gene sequence will be translated into RNA.
promoter sequence in DNA
gene region RNA
polymerase
1
Stepped Art
2 The polymerase begins to move along the DNA and unwind it. As it does, it links RNA nucleotides into a strand of RNA in the order specified by the base sequence of the DNA. The DNA winds up again after the polymerase passes. The structure of the “opened” DNA at the transcription site is called a transcription bubble, after its appearance.
DNA unwinding DNA winding up
RNA
3
Zooming in on the gene region, we can see that RNA polymerase covalently bonds successive nucleotides into an RNA strand. The base sequence of the new RNA strand is complementary to the base sequence of its DNA template strand, so it is an RNA copy of the gene.
direction of transcription
Figure 9-5 p153
Take-Home Message:
How is RNA assembled?
• In transcription, RNA polymerase uses the nucleotide
sequence of a gene region in a chromosome as a template to assemble a strand of RNA
9.4 RNA and the Genetic Code
• Base triplets in an mRNA encode a protein-building message
mRNA – The Messenger
• mRNA carries protein-building information to ribosomes and tRNA for translation
• Codon
• A sequence of three mRNA nucleotides that codes for a specific amino acid
Genetic Code
• Genetic code
• Consists of 64 mRNA codons (triplets)
• Twenty kinds of amino acids are found in proteins
• Some amino acids can be coded by more than one codon
• Some codons signal the start or end of a gene
• AUG (methionine) is a start codon
From DNA to RNA to Amino Acids
a gene
region in DNA
transcription
codon codon codon
rRNA and tRNA – The Translators
• tRNAs deliver amino acids to ribosomes
• tRNA has an anticodon complementary to an mRNA codon, and a binding site for the amino acid specified by that codon
tRNA Structure
anticodon
Take-Home Message
: What roles do mRNA,
tRNA, and rRNA play during translation?
• mRNA carries protein-building information; the bases in mRNA are “read” in sets of three during protein synthesis; most base triplets (codons) code for amino acids; the genetic code consists of all sixty-four codons
• Ribosomes, which consist of two subunits of rRNA and proteins, assemble amino acids into polypeptide chains
9.5 Translation: RNA to Protein
• Translation converts genetic information carried by an mRNA into a new polypeptide chain
Translation
• Translation occurs in the cytoplasm of cells
• Translation occurs in three stages:
• Initiation
• Elongation
Initiation
• An initiation complex is formed
• A small ribosomal subunit binds to mRNA
• The anticodon of initiator tRNA base-pairs with the start codon (AUG) of mRNA
Elongation
• The ribosome assembles a polypeptide chain as it moves along the mRNA
• Initiator tRNA carries methionine, the first amino acid of the chain
Termination
• When the ribosome encounters a stop codon, polypeptide synthesis ends
• Release factors bind to the ribosome
1
Ribosome subunits and an initiator tRNA converge on an mRNA. A second tRNA binds to the second codon. first amino acid
of polypeptide start codon (AUG) initiator tRNA Stepped Art A peptide bond forms between the first two amino acids.
peptide bond
2
The first tRNA is released and the ribosome moves to the next codon. A third tRNA binds to the third codon.
3
A peptide bond forms between the second and third amino acids.
4
The second tRNA is released and the ribosome moves to the next codon. A fourth tRNA binds the fourth codon.
5 A peptide bond forms between the third and fourth amino acids.
The process repeats until the ribosome encounters a stop codon in the mRNA.
6
Figure 9-12a p157
Transcription polysomes
ribosome subunits
tRNA
Convergence of RNAs
mRNA
Polysomes
• Many ribosomes may simultaneously translate the same mRNA, forming polysomes
Take-Home Message:
How is mRNA translated into protein?
• Translation converts protein-building information carried by mRNA into a polypeptide
• During initiation, an mRNA, an initiator tRNA, and two ribosome subunits join
• During elongation, amino acids are delivered to the complex by tRNAs in the order dictated by successive mRNA codons; the ribosome joins each to the end of the polypeptide chain
9.6 Mutated Genes
and Their Protein Products
• If the nucleotide sequence of a gene changes, it may result in an altered gene product, with harmful effects
• Mutations
Mutations and Proteins
• A mutation that changes a UCU codon to UCC is “silent” – it has no effect on the gene’s product because both codons specify the same amino acid
Common Mutations
• Base-pair-substitution
• May result in a premature stop codon or a different amino acid in a protein product
• Example: sickle-cell anemia
• Deletion or insertion
• Can cause the reading frame of mRNA codons to shift, changing the genetic message
Hemoglobin and Anemia
• Hemoglobin is a protein that binds oxygen in the lungs and carries it to cells throughout the body
• The hemoglobin molecule consists of four polypeptides (globins) folded around iron-containing hemes – oxygen molecules bind to the iron atoms
Sickle-Cell Anemia
• Sickle-cell anemia is caused by a base-pair substitution which produces a beta globin molecule in which the sixth amino acid is valine instead of glutamic acid (sickle hemoglobin, HbS)
• HbS molecules stick together and form clumps – red blood cells become distorted into a sickle shape, and clog blood vessels, disrupting blood circulation throughout the body
Figure 9-14 p159
sickled cell
glutamic acid valine
Thalassemia and Frameshifts
• Another type of anemia, beta thalassemia, is caused by the deletion of the twentieth base pair in the beta globin gene
• Deletions cause a frameshift, in which the reading frame of the mRNA codons shifts
Thalassemia and Transposable Elements
• Beta thalassemia can also be caused by insertion mutations, which also cause frameshifts
• Insertion mutations are often caused by the activity of
Take-Home Message:
What happens
after a gene becomes mutated?
• Mutations that result in an altered protein can have drastic consequences
• A base-pair substitution may change an amino acid in a
protein, or shorten it by introducing a premature stop codon
• Frameshifts that occur after an insertion or deletion change an mRNA’s codon reading frame, so they garble its
