Download Chapter 10B: Gene Expression

Chapter 10B:
Gene Expression
1. Overview of Gene Expression
2. Transcription
3. The Genetic Code
4. Translation
1. Overview of Gene Expression
Overview of
Gene Expression
DNA (genetic info)
transcription
(in nucleus)
RNA (copy)
translation
(via ribosomes
in cytoplasm)
Protein
Gene Expression
The expression of a gene into an actual
protein occurs by 2 basic processes:
1) Transcription of a gene into RNA
• RNA is a nucleic acid very similar to DNA
(RNA uses “U” instead of “T”)
• this is essentially creating a “photocopy” of the gene
• occurs in the nucleus
2) Translation of the RNA transcript into protein
• accomplished by ribosomes, in the cytoplasm
Comparison of DNA & RNA
RNA
DNA
• sugar = Ribose
• sugar = Deoxyribose
• single-stranded
• double-stranded
• A, C, G & U (uracil)
• A, C, G & T (thymine)
2. Transcription
Transcription
Transcription of gene to make an RNA copy is much
like DNA replication except for the following:
RNA nucleotides
RNA
polymerase
A
T
C C A A
U
T
U
A
G G T
Direction of
transcription
Newly made RNA
C
• produces
complementary RNA
C
G
A
G
C A U C C A
G
T
T
• catalyzed by RNA
polymerase
T
C
• only 1 strand is used
as a template
T
A
A
• involves only 1 gene
Template
Strand of DNA
• RNA is released, DNA
“zips” back up!
*results in a copy of one DNA strand*
The Process of Transcription
Functions of
RNA made by
Transcription
*
1) mRNA*
• copy of gene
2) tRNA
• delivers AAs
to ribosomes
3) rRNA
*provides genetic
info for translation
• part of
ribosomes
3. The Genetic Code
How are Genes related to DNA?
Genes are segments of DNA that code for a
particular protein (or RNA molecule)
• the human genome contains ~3 billion base
pairs (bps) and ~25,000 genes
• almost all genes encode proteins
• when we talk about “genes” we will focus on those
that express proteins
( the “end products” for a small percentage of genes are
special types of RNA molecules)
What does DNA actually code for?
In other words, “How do genes encode
proteins”?
• recall that proteins are linear polymers made of
the 20 different amino acids
***genes need simply to encode the identity of
each amino acid in a given protein***
• i.e., genes must be capable of encoding 20 different
amino acids and their order in a protein
• although DNA contains only 4 “letters” (i.e.,
nucleotides), this is more than sufficient…
The Genetic Code
Each amino acid in a protein is specified by
3 nucleotide sequences called codons
• each of the 20 amino acids is coded for by a
unique set of codons:
e.g.
ATG = methionine (start codon)
GGN = glycine
CAA or CAG = glutamine
• there are 64 possible “codon” triplets (4 x 4 x 4)
• more than enough to encode 20 amino acids and the
signal to “stop” or end the protein (TGA, TAA or TAG)
Table of the Genetic Code
*always starts w/AUG (met)
If the DNA sequence is: CATGCCTGGGCAATAG
The RNA copy is:
CAUGCCUGGGCAAUAG (transcription)
The protein sequence is: *Met-Pro-Gly-Gln-“stop” (translation)
From DNA to RNA to Protein
The Effects of Mutation
MUTATION: any change in DNA sequence
Normal gene
A U G A A G U U U G G C G C A
mRNA
Met
Protein
Lys
Phe
Gly
Ala
Base substitution
A U G A A G U U U A G C G C A
Met
Lys
Phe
Ser
Ala
U Missing
Base deletion
A U G A A G U U G G C G C A U
Met
Lys
Leu
Ala
His
• deletions,
insertions
cause a
change in
reading frame
(frameshift)
Sickle Cell Anemia is due to a
Base Substitution
Normal hemoglobin DNA
C
T
Mutant hemoglobin DNA
T
mRNA
C
A
T
G
U
A
mRNA
G
A
A
Normal hemoglobin
Glu
Sickle-cell hemoglobin
Val
1 nucleotide changes 1 amino acid resulting in a misfolded
hemoglobin protein (clump together in RBCs > sickle shape)
4. Translation
Ribosomes translate mRNA to Protein
• in the cytoplasm
• with the help of tRNAs
tRNA Structure & Function
*
…the amino acid*
attached to the tRNA
is then added to the
growing polypeptide
mRNA codons will
“base pair” with
a complementary
anticodon in a tRNA…
*
Translation (aka “protein synthesis”)
The building of a polypeptide, 1 amino acid at
a time, by ribosomes using info in mRNA:
• ribosomes bind directly to mRNA, “read” codon by
codon
• ribosomes always start at AUG (methionine)
• translation also involves tRNAs, each of which is
attached to 1 of the 20 amino acids (AAs)
• ribosomes match the right tRNA (via the anticodon)
with the right codon in the mRNA, then add its AA to
the growing protein
Translation by multiple Ribosomes
• the same mRNA
can be translated
many, many times
• a given mRNA can
be translated by
many ribosomes
at the same time
mRNA
General Steps of Translation
1) ribosome begins translation at AUG of mRNA
2) ribosome binds 2 tRNA-AAs, 2 codons at a time
• i.e., tRNAs with anti-codons complementary to the mRNA
codons
3) ribosome then catalyzes peptide bond formation
between the amino acids attached to each tRNA
4) ribosome shifts 3 nucleotides (1 codon) on mRNA
and repeats the process
5) this continues until the ribosome reaches a
“stop” codon which causes translation to end
Key Terms for Chapter 10B
• mRNA, tRNA, rRNA
• transcription, RNA polymerase
• codon, anti-codon
• genetic code
• mutation: substitution, deletion, insertion
• reading frame, frameshift
• translation, ribosome
Relevant Review Questions:
1, 3-5, 7, 9