Chapter 10B: Gene Expression 4. Translation 3. The Genetic Code 2. Transcription 1. Overview of Gene Expression
Chapter 10B:Gene Expression
4. Translation
3. The Genetic Code
2. Transcription
1. Overview of Gene Expression
1. Overview of Gene Expression
Overview of Gene Expression
DNA (genetic info)
RNA (copy)
Protein
transcription(in nucleus)
translation(via ribosomesin cytoplasm)
Gene ExpressionThe expression of a gene into an actual protein occurs by 2 basic processes:
1) Transcription of a gene into RNA
• 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
• RNA is a nucleic acid very similar to DNA (RNA uses “U” instead of “T”)
Comparison of DNA & RNA
• sugar = Ribose
• single-stranded
• A, C, G & U (uracil)
RNA DNA• sugar = Deoxyribose
• double-stranded
• A, C, G & T (thymine)
2. Transcription
RNApolymerase
RNA nucleotides
Direction of transcription Template
Strand of DNA
Newly made RNA
TC
A T C C A A TT
GG
CC
AATTGGAT
G
U
C A U C C A AU
TranscriptionTranscription of gene to make an RNA copy is much like DNA replication except for the following:
• only 1 strand is used as a template
• catalyzed by RNApolymerase
• produces complementary RNA
• involves only 1 gene
• RNA is released, DNA “zips” back up!
*results in a copy of one DNA strand*
The Process of Transcription
Functions ofRNA made byTranscription
1) mRNA*• copy of gene
3) rRNA• part of ribosomes
2) tRNA• delivers AAsto ribosomes
*provides geneticinfo for translation
*
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 basepairs (bps) and ~25,000 genes
• almost all genes encode proteins
( the “end products” for a small percentage of genes arespecial types of RNA molecules)
• when we talk about “genes” we will focus on thosethat express proteins
What does DNA actually code for?In other words, “How do genes encode
proteins”?
• recall that proteins are linear polymers made ofthe 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 differentamino acids and their order in a protein
• although DNA contains only 4 “letters” (i.e.,nucleotides), this is more than sufficient…
The Genetic CodeEach amino acid in a protein is specified by3 nucleotide sequences called codons
• 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)
• each of the 20 amino acids is coded for by aunique set of codons:
e.g. ATG = methionine (start codon)
GGN = glycineCAA or CAG = glutamine
Table of the Genetic Code
If the DNA sequence is: CATGCCTGGGCAATAGThe RNA copy is: CAUGCCUGGGCAAUAG (transcription)The protein sequence is: *Met-Pro-Gly-Gln-“stop” (translation)
*always starts w/AUG (met)
From DNA to RNA to Protein
Normal gene
mRNA
Base substitution
Base deletion Missing
Met Lys Phe Gly Ala
Met Lys Phe Ser Ala
Met Lys Leu Ala His
A U G A A G U U U G G C G C A
A U G A A G U U U A G C G C A
A U G A A G U U G G C G C A U
U
Protein
• deletions, insertions cause a change in reading
The Effects of MutationMUTATION: any change in DNA sequence
frame(frameshift)
C T T C A T
Normal hemoglobin
Mutant hemoglobin DNA
G A A G U A
Sickle-cell hemoglobin
Normal hemoglobin DNA
Glu Val
mRNA mRNA
Sickle Cell Anemia is due to aBase Substitution
1 nucleotide changes 1 amino acid resulting in a misfoldedhemoglobin protein (clump together in RBCs > sickle shape)
4. Translation
Ribosomes translate mRNA to Protein
• with the help of tRNAs
• in the cytoplasm
tRNA Structure & Function
mRNA codons will “base pair” with a complementary
anticodon in a tRNA…
…the amino acid*attached to the tRNAis then added to thegrowing polypeptide
*
*
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
• translation also involves tRNAs, each of which isattached 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
• ribosomes always start at AUG (methionine)
Translation by multiple Ribosomes
• the same mRNAcan be translatedmany, many times
• a given mRNA canbe translated bymany ribosomesat the same time
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 formationbetween the amino acids attached to each tRNA
General Steps of Translation
4) ribosome shifts 3 nucleotides (1 codon) on mRNAand repeats the process
5) this continues until the ribosome reaches a“stop” codon which causes translation to end
1) ribosome begins translation at AUG of mRNA
Key Terms for Chapter 10B
• transcription, RNA polymerase
• translation, ribosome
• mRNA, tRNA, rRNA
Relevant Review Questions: 1, 3-5, 7, 9
• codon, anti-codon
• genetic code
• mutation: substitution, deletion, insertion
• reading frame, frameshift