Chapter 14 Lecture

Concepts of Genetics Tenth Edition

Translation and Proteins

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Chapter Contents

14.1 Translation of mRNA Depends on Ribosomes and Transfer RNAs

14.2 Translation of mRNA Can Be Divided into Three Steps

14.3 High-Resolution Studies Have Revealed Many Details about the Functional Prokaryotic Ribosome

14.4 Translation Is More Complex in Eukaryotes

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Chapter Contents

14.5 The Initial Insight That Proteins Are Important in Heredity Was Provided by the Study of Inborn Errors of Metabolism

14.6 Studies of Neurospora Led to the One- Gene:One-Enzyme Hypothesis

14.7 Studies of Human Hemoglobin Established That One Gene Encodes One Polypeptide

14.8 The Nucleotide Sequence of a Gene and the Amino Acid Sequence of the Corresponding Protein Exhibit Colinearity

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Chapter Contents

14.9 Variation in Protein Structure Provides the Basis of Biological Diversity

14.10 Posttranslational Modification Alters the Final Protein Product

14.11 Proteins Function in Many Diverse Roles 14.12 Proteins Are Made Up of One or More

Functional Domains

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14.1 Translation of mRNA Depends on Ribosomes and Transfer RNAs

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Section 14.1

• Translation is the biological polymerization of amino acids into polypeptide chains

• This process requires – amino acids – messenger RNA (mRNA) – ribosomes – transfer RNA (tRNA)

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Section 14.1

• tRNAs adapt genetic information present as specific triplet codons in mRNA to their corresponding amino acid – tRNAs have anticodons that complement the

mRNAs – tRNAs carry the corresponding amino acid

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Section 14.1

• Ribosomes consist of ribosomal proteins and ribosomal RNAs (rRNAs)

• They have a large subunit and a small subunit • The rRNAs perform important catalytic functions

associated with translation – They promote the binding of the various molecules

involved in translation and fine-tune the process

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Section 14.1

• The rRNA genes, called rDNA, are part of a moderately repetitive DNA fraction and are present in clusters at various chromosomal sites – Each cluster contains tandem repeats separated by

noncoding spacer DNA

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Section 14.1

• tRNAs are small in size and very stable – Composed of 75–90 nucleotides – Transcribed from DNA and contain

posttranscriptionally modified bases • Modified bases enhance hydrogen bonding

efficiency during translation

• The two-dimensional structure of tRNAs is a cloverleaf

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Section 14.1

• A tRNA has an anticodon that complementarily base-pairs with the codon in the mRNA

• The corresponding amino acid is covalently linked to the CCA sequence at the 3' end of all tRNAs

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Section 14.1

• Before translation can proceed, tRNA molecules must be chemically linked to their respective amino acids – Activation (charging or aminoacylation) done by

aminoacyl tRNA synthetase • There are 20 different synthetases, one for each

amino acid, and they are highly specific since they recognize only one amino acid

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14.2 Translation of mRNA Can Be Divided into Three Steps

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Section 14.2

• Initiation requires – the small and large ribosomal subunits – GTP – charged initiator tRNA – Mg2+ – initiation factors

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Section 14.2

• In bacteria, the AUG start codon is preceded by a Shine-Dalgarno sequence (AGGAGG), which base-pairs with a region of the 16S rRNA of the 30S small subunit, facilitating initiation

• This initiation complex (small ribosomal subunit + initiation factors + mRNA at codon AUG) then combines with the large ribosomal subunit

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Section 14.2

• Elongation requires both ribosomal subunits assembled with the mRNA to form the P (peptidyl) site and A (aminoacyl) site

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Section 14.2

• The charged tRNAs enter the A site, and peptidyl transferase catalyzes peptide bond formation between the amino acid on the tRNA at the A site and the growing peptide chain bound to the tRNA in the P site

• The uncharged tRNA moves to the E (exit) site • The tRNA bound to the peptide chain moves to

the P site • The sequence of elongation and translocation is

repeated over and over

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Section 14.2

• Termination is signaled by a stop codon (UAG, UAA, UGA) in the A site

• GTP-dependent release factors cleave the polypeptide chain from the tRNA and release it from the translation complex

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Section 14.2

• Polysomes (or polyribosomes) are mRNAs with several ribosomes translating at once

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14.3 High-Resolution Studies Have Revealed Many Details about the Functional Prokaryotic Ribosome

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Section 14.3

• The crystal structures of the individual ribosomal subunits and the intact 70S bacterial ribosome have been solved

• The entire translational complex (size and shape) was seen at the atomic level

• These studies provide many details about ribosomal function and the importance of the rRNA component

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14.4 Translation Is More Complex in Eukaryotes

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Section 14.4

• In eukaryotes, – the ribosomes are larger and longer lived than

in bacteria – transcription occurs in the nucleus

• The 5′ end of mRNA is capped with a 7-methylguanosine residue at maturation, which is essential for translation

• A poly-A tail is added at the 3′ end of the mRNA. – translation occurs in the cytoplasm

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Section 14.4

• Many eukaryotic mRNAs contain a purine (A or G) three bases upstream from the AUG initiator codon, which is followed by a G

• This Kozak sequence is considered to increase the efficiency of translation by interacting with the initiator tRNA

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Section 14.4

• Translation in eukaryotes generally requires more factors for initiation, elongation, and termination than translation in bacteria does

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Section 14.4

• Many ribosomes are not free-floating as in bacteria but instead are associated with the endoplasmic reticulum

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14.5 The Initial Insight That Proteins Are Important in Heredity Was Provided by

the Study of Inborn Errors of Metabolism

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Section 14.5

• Alkaptonuria and phenylketonuria result from mutations that lead to metabolic blocks – Hundreds of medical conditions are caused

by errors in metabolism resulting from mutant genes

• Pedigree analysis of these diseases indicated that human diseases can have a genetic basis

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Section 14.5

• Phenylketonuria (PKU) results when phenylalanine is not converted to tyrosine – Phenylalanine hydroxylase is inactive in affected

individuals • Phenylalanine and its derivatives enter the

cerebrospinal fluid with elevated levels, resulting in mental retardation

• Newborns are routinely screened throughout United States, with afflicted babies put on special diets

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14.6 Studies of Neurospora Led to the One-Gene:One-Enzyme Hypothesis

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Section 14.6

• Beadle and Tatum showed that nutritional mutations in the bread mold Neurospora caused the loss of an enzymatic activity that catalyzes an essential reaction in wild-type organisms

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Section 14.6

• Beadle and Tatum had provided sound experimental evidence for the one-gene: one-enzyme hypothesis

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14.7 Studies of Human Hemoglobin Established That One Gene Encodes One Polypeptide

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Section 14.7

• Not all proteins are enzymes, and some proteins have more than one subunit

• Because of this, the one-gene:one-enzyme hypothesis was modified to one-gene: one-protein and then to one-gene: one-polypeptide chain

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Section 14.7

• Sickle-cell anemia is a recessive genetic disease in which afflicted individuals are homozygous for the HbS hemoglobin allele

• Heterozygotes are carriers of the affected gene but are largely unaffected

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Section 14.7

• Fingerprinting demonstrated that the HbS and HbA hemoglobins differ by a single peptide fragment

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Section 14.7

• Hemoglobin genes are developmentally regulated, and different hemoglobins are expressed at different times in human development

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14.8 The Nucleotide Sequence of a Gene and the Amino Acid Sequence of the

Corresponding Protein Exhibit Colinearity

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Section 14.8

• In colinearity, the order of nucleotides in a gene correlates directly with the order of amino acids in the corresponding polypeptide

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14.9 Variation in Protein Structure Provides the Basis of Biological Diversity

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Section 14.9

• Amino acids are assembled on and released from the ribosomes as polypeptides

• Following translation, polypeptides fold up and assume higher order structures and may interact with other polypeptides

• The three-dimensional conformation is essential to a protein's specific function

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Section 14.9

• Amino acids all have – a carboxyl group – an amino group – an R (radical) group bound to a central carbon

atom • The R group of an amino acid confers

specific chemical properties

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Section 14.9

• A peptide bond forms by a dehydration reaction between the carboxyl group of one amino acid and the amino group of another

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Section 14.9

• There are four levels of protein structure: – primary – secondary – tertiary – quaternary

• The three-dimensional conformation of any protein is a product of its primary structure

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14.10 Posttranslational Modification Alters the Final Protein Product

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Section 14.10

• Some proteins may be modified after they have been synthesized. This is called posttranslational modification

• These modifications are crucial to the functional capability of the final protein product

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Section 14.5

• Several examples of posttranscriptional modification are given below: – The N-terminus amino acid is usually removed or

modified – Individual amino acid residues are sometimes

modified – Carbohydrate side chains are sometimes attached – Polypeptide chains may be trimmed – Signal sequences are removed – Polypeptide chains are often complexed with metals

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14.11 Proteins Function in Many Diverse Roles

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Section 14.11

• Proteins play diverse roles in the body • Hemoglobin and myoglobin transport

oxygen, which is essential for cellular metabolism

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Section 14.11

• Collagen and keratin are structural proteins associated with the skin, connective tissue, and hair of organisms

• Actin and myosin are contractile proteins found in abundance in muscle tissue

• Tubulin is the basis of function of microtubules in mitotic and meiotic spindle fibers

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Section 14.11

• Other examples are – the immunoglobulins, which function in the immune

system of vertebrates – transport proteins, involved in movement of

molecules across membranes – some of the hormones and their receptors, which

regulate various types of chemical activity – histones, which bind to DNA in eukaryotic organisms – Transcription factors, which regulate gene

expression

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Section 14.11

• Enzymes, the largest group of proteins, are involved in biological catalysis, a process whereby the energy of activation for a given reaction is lowered

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14.12 Proteins Are Made Up of One or More Functional Domains

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Section 14.12

• Distinct regions made up of specific amino acid sequences are associated with unique functions in proteins

• These sequences constitute protein domains that fold into stable, unique conformations

• Different protein domains impart different functional capabilities

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Section 14.12

• Exons have been proposed to encode protein domains, and exon shuffling may be a kind of evolution to form unique genes in eukaryotes

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