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