gene activity 1outline function of genes one gene-one enzyme hypothesis genetic code transcription ...

Gene ActivityGene Activity 1

OutlineOutline

Function of GenesFunction of GenesOne Gene-One Enzyme HypothesisOne Gene-One Enzyme Hypothesis

Genetic CodeGenetic CodeTranscriptionTranscription

Processing Messenger RNAProcessing Messenger RNA

TranslationTranslationTransfer RNATransfer RNARibosomal RNARibosomal RNA


Function of GenesFunction of GenesGenes Specify EnzymesGenes Specify Enzymes

Beadle and Tatum:Beadle and Tatum: Experiments on fungus Neurospora crassaExperiments on fungus Neurospora crassa Proposed that each gene specifies the Proposed that each gene specifies the synthesis of one enzymesynthesis of one enzyme One-gene-one-enzyme hypothesisOne-gene-one-enzyme hypothesis

Genes Specify a PolypeptideGenes Specify a PolypeptideA gene is a segment of DNA that specifies the A gene is a segment of DNA that specifies the sequence of amino acids in a polypeptide sequence of amino acids in a polypeptide

Suggests that genetic mutations cause Suggests that genetic mutations cause changes in the primary structure of a proteinchanges in the primary structure of a protein


Types of RNATypes of RNA

RNA is a polymer of RNA nucleotidesRNA is a polymer of RNA nucleotidesRNA Nucleotides are of four types: Uracil, RNA Nucleotides are of four types: Uracil, Adenine, Cytosine, and GuanineAdenine, Cytosine, and Guanine

Uracil (U) replaces thymine (T) of DNAUracil (U) replaces thymine (T) of DNATypes of RNATypes of RNA

Messenger (mRNA) - Takes genetic message Messenger (mRNA) - Takes genetic message from DNA in nucleus to ribosomes in cytoplasmfrom DNA in nucleus to ribosomes in cytoplasm Ribosomal (rRNA) - Makes up ribosomes which Ribosomal (rRNA) - Makes up ribosomes which read the message in mRNAread the message in mRNA Transfer (tRNA) - Transfers appropriate amino Transfer (tRNA) - Transfers appropriate amino acid to ribosome when “instructed”acid to ribosome when “instructed”

4Structure of RNA

5Overview of Gene Expression

Gene ActivityGene Activity 6Steps in Gene Expression:Steps in Gene Expression:TranscriptionTranscription

TranscriptionTranscriptionGene unzips and exposes unpaired basesGene unzips and exposes unpaired basesServes as template for mRNA formationServes as template for mRNA formationLoose RNA nucleotides bind to exposed DNA Loose RNA nucleotides bind to exposed DNA bases using the C=G & A=U rulebases using the C=G & A=U rule

When entire gene is transcribed into mRNA, When entire gene is transcribed into mRNA, result is an mRNA transcript of the generesult is an mRNA transcript of the gene

The base sequence in the mRNA is The base sequence in the mRNA is complementary to the base sequence in DNAcomplementary to the base sequence in DNA

Gene ActivityGene Activity 7Steps in Gene Expression:Steps in Gene Expression:TranslationTranslation

tRNA molecules have two binding sitestRNA molecules have two binding sites One associates with the mRNA transcriptOne associates with the mRNA transcript The other associates with a specific amino acidThe other associates with a specific amino acid Each of the 20 amino acids in proteins associates with one or Each of the 20 amino acids in proteins associates with one or

more of 64 species of tRNAmore of 64 species of tRNATranslationTranslation

An mRNA transcript migrates to rough endoplasmic reticulumAn mRNA transcript migrates to rough endoplasmic reticulum Associates with the rRNA of a ribosomeAssociates with the rRNA of a ribosome The ribosome “reads” the information in the transcriptThe ribosome “reads” the information in the transcript Ribosome directs various species of tRNA to bring in their Ribosome directs various species of tRNA to bring in their

specific amino acid “fares”specific amino acid “fares” tRNA specified is determined by the code being translated in tRNA specified is determined by the code being translated in

the mRNA transcriptthe mRNA transcript


The Genetic CodeThe Genetic Code

The unit of a code consists of codons, each of which is a The unit of a code consists of codons, each of which is a unique arrangement of symbolsunique arrangement of symbols

Each of the 20 amino acids found in proteins is uniquely Each of the 20 amino acids found in proteins is uniquely specified by one or more codons specified by one or more codons The symbols used by the genetic code are the mRNA basesThe symbols used by the genetic code are the mRNA bases Function as “letters” of the genetic alphabetFunction as “letters” of the genetic alphabet Genetic alphabet has only four “letters” (U, A, C, G)Genetic alphabet has only four “letters” (U, A, C, G)

Codons in the genetic code are all three bases (symbols) longCodons in the genetic code are all three bases (symbols) long Function as “words” of genetic informationFunction as “words” of genetic information Permutations:Permutations:

There are 64 possible arrangements of four symbols taken three There are 64 possible arrangements of four symbols taken three at a timeat a time

Often referred to as tripletsOften referred to as triplets Genetic language only has 64 “words”Genetic language only has 64 “words”


The Genetic CodeThe Genetic CodeProperties of the genetic code:Properties of the genetic code:

UniversalUniversal With few exceptions, all organisms use the code the With few exceptions, all organisms use the code the

same waysame way Encode the same 20 amino acids with the same 64 Encode the same 20 amino acids with the same 64

tripletstriplets Degenerate (redundant)Degenerate (redundant) There are 64 codons available for 20 amino acidsThere are 64 codons available for 20 amino acids Most amino acids encoded by two or more codonsMost amino acids encoded by two or more codons

Unambiguous (codons are exclusive)Unambiguous (codons are exclusive) None of the codons code for two or more amino acidsNone of the codons code for two or more amino acids Each codon specifies only one of the 20 amino acidsEach codon specifies only one of the 20 amino acids

Contains start and stop signalsContains start and stop signals Punctuation codonsPunctuation codons Like the capital letter we use to signify the beginning of Like the capital letter we use to signify the beginning of

a sentence, and the period to signify the enda sentence, and the period to signify the end

10The Genetic Code (mRNA)


Transcription of mRNATranscription of mRNAA single chromosomes consists of one very long molecule A single chromosomes consists of one very long molecule

encoding hundreds or thousands of genesencoding hundreds or thousands of genesThe genetic information in a gene describes the amino acid The genetic information in a gene describes the amino acid

sequence of a proteinsequence of a protein The information is in the base sequence of one side (the “sense” The information is in the base sequence of one side (the “sense”

strand) of the DNA moleculestrand) of the DNA molecule The gene is the functional equivalent of a “sentence”The gene is the functional equivalent of a “sentence”

The segment of DNA corresponding to a gene is unzipped to The segment of DNA corresponding to a gene is unzipped to expose the bases of the sense strandexpose the bases of the sense strand The genetic information in the gene is transcribed (rewritten) into The genetic information in the gene is transcribed (rewritten) into

an mRNA moleculean mRNA molecule The exposed bases in the DNA determine the sequence in which The exposed bases in the DNA determine the sequence in which

the RNA bases will be connected togetherthe RNA bases will be connected together RNA polymerase connects the loose RNA nucleotides togetherRNA polymerase connects the loose RNA nucleotides together

The completed transcript contains the information from the The completed transcript contains the information from the gene, but in a mirror image, or complementary formgene, but in a mirror image, or complementary form

12Transcription

13RNA Polymerase


Processing Messenger RNAProcessing Messenger RNAPrimary mRNA transcript is modified before it leaves Primary mRNA transcript is modified before it leaves

the eukaryotic nucleusthe eukaryotic nucleus RNA splicing:RNA splicing: Primary transcript consists of:Primary transcript consists of:

Some segments that will not be expressed (introns)Some segments that will not be expressed (introns) Segments that will be expressed (exons)Segments that will be expressed (exons)

Performed by spliceosome complexes in nucleoplasmPerformed by spliceosome complexes in nucleoplasm Introns are excisedIntrons are excised Remaining exons are spliced back togetherRemaining exons are spliced back together

Modifications to ends of primary transcript:Modifications to ends of primary transcript: Cap of modified guanine on 5Cap of modified guanine on 5′′ end end Poly-A tail of 150+ adenines on 3Poly-A tail of 150+ adenines on 3′′ end end

Result is mature mRNA transcriptResult is mature mRNA transcript

15mRNA Processing in Eukaryotes


Functions of IntronsFunctions of IntronsAs organismal complexity increases;As organismal complexity increases;

Number of protein-coding genes does not keep paceNumber of protein-coding genes does not keep pace But the proportion of the genome that is introns But the proportion of the genome that is introns

increasesincreases Humans:Humans: Genome has only about 25,000 coding genesGenome has only about 25,000 coding genes Up to 95% of this DNA genes is intronsUp to 95% of this DNA genes is introns

Possible functions of introns:Possible functions of introns: More bang for buckMore bang for buck Exons might combine in various combinationsExons might combine in various combinations Would allow different mRNAs to result from one Would allow different mRNAs to result from one

segment of DNAsegment of DNA Introns might regulate gene expressionIntrons might regulate gene expression

Exciting new picture of the genome is emergingExciting new picture of the genome is emerging


tRNAtRNA

tRNA molecules come in 64 different kindstRNA molecules come in 64 different kindsAll very similar except thatAll very similar except that

One end bears a specific triplet (of the 64 One end bears a specific triplet (of the 64 possible) called the anticodonpossible) called the anticodon

Other end binds with a specific amino acid Other end binds with a specific amino acid typetype

tRNA synthetases attach correct amino acid to tRNA synthetases attach correct amino acid to the correct tRNA moleculethe correct tRNA molecule

All tRNA molecules with a specific anticodon All tRNA molecules with a specific anticodon will always bind with the same amino acidwill always bind with the same amino acid

18Structure of tRNA


RibosomesRibosomes

Ribosomal RNA (rRNA):Ribosomal RNA (rRNA):Produced from a DNA template in the Produced from a DNA template in the nucleolusnucleolus

Combined with proteins into large and small Combined with proteins into large and small ribosomal subunitsribosomal subunits

A completed ribosome has three binding sites A completed ribosome has three binding sites to facilitate pairing between tRNA and mRNAto facilitate pairing between tRNA and mRNAThe E (for exit) siteThe E (for exit) siteThe P (for peptide) site, andThe P (for peptide) site, andThe A (for amino acid) siteThe A (for amino acid) site

20Ribosomal Structure and Function

Gene ActivityGene Activity 21Steps in Translation:Steps in Translation:#1 - Initiation#1 - Initiation

Components necessary for initiation are:Components necessary for initiation are: Small ribosomal subunitSmall ribosomal subunit mRNA transcriptmRNA transcript Initiator tRNA, andInitiator tRNA, and Large ribosomal subunitLarge ribosomal subunit Initiation factors (special proteins that bring the Initiation factors (special proteins that bring the

above together)above together)Initiator tRNA:Initiator tRNA:

Always has the UAC anticodonAlways has the UAC anticodon Always carries the amino acid methionineAlways carries the amino acid methionine Capable of binding to the P siteCapable of binding to the P site

Gene ActivityGene Activity 22Steps in Translation:Steps in Translation:#1 - Initiation#1 - Initiation

Small ribosomal subunit attaches to mRNA Small ribosomal subunit attaches to mRNA transcripttranscript

Beginning of transcript always has the START Beginning of transcript always has the START codon (AUG)codon (AUG)

Initiator tRNA (UAC) attaches to P siteInitiator tRNA (UAC) attaches to P site

Large ribosomal subunit joins the small Large ribosomal subunit joins the small subunitsubunit

23Steps in Translation:#1 - Initiation

Gene ActivityGene Activity 24Steps in Translation:Steps in Translation:#2 - Elongation#2 - Elongation

““Elongation” refers to the growth in length of Elongation” refers to the growth in length of the polypeptidethe polypeptide

RNA molecules bring their amino acid fares to RNA molecules bring their amino acid fares to the ribosomethe ribosomeRibosome reads a codon in the mRNARibosome reads a codon in the mRNA Allows only one type of tRNA to bring its amino Allows only one type of tRNA to bring its amino acidacidMust have the anticodon complementary to the Must have the anticodon complementary to the mRNA codon being readmRNA codon being read Joins the ribosome at it’s A siteJoins the ribosome at it’s A site

Methionine of initiator is connected to amino Methionine of initiator is connected to amino acid of 2acid of 2ndnd tRNA by peptide bond tRNA by peptide bond

Gene ActivityGene Activity 25Steps in Translation:Steps in Translation:#2 – Elongation (cont.)#2 – Elongation (cont.)

Second tRNA moves to P site (translocation)Second tRNA moves to P site (translocation)Spent initiator moves to E site and exitsSpent initiator moves to E site and exitsRibosome reads the next codon in the mRNARibosome reads the next codon in the mRNA

Allows only one type of tRNA to bring its Allows only one type of tRNA to bring its amino acidamino acidMust have the anticodon complementary to the Must have the anticodon complementary to the mRNA codon being readmRNA codon being read Joins the ribosome at it’s A siteJoins the ribosome at it’s A site

Dipeptide on 2Dipeptide on 2ndnd amino acid is connected to amino acid is connected to amino acid of 3amino acid of 3ndnd tRNA by peptide bond tRNA by peptide bond

26Steps in Translation:#2 - Elongation

Gene ActivityGene Activity 27Steps in Translation:Steps in Translation:#3 – Termination#3 – Termination

Previous tRNA moves to P sitePrevious tRNA moves to P siteSpent tRNA moves to E site and exitsSpent tRNA moves to E site and exitsRibosome reads the STOP codon at the end Ribosome reads the STOP codon at the end of the mRNAof the mRNAUAA, UAG, or UGAUAA, UAG, or UGADoes not code for an amino acidDoes not code for an amino acid

Polypeptide is released from last tRNA by Polypeptide is released from last tRNA by release factorrelease factor

Ribosome releases mRNA and dissociates Ribosome releases mRNA and dissociates into subunitsinto subunits

mRNA read by another ribosomemRNA read by another ribosome

28Steps in Translation:#3 - Termination

Gene ActivityGene Activity 29Protein Synthesis:Protein Synthesis:From DNA to RNA to ProteinFrom DNA to RNA to Protein

The mechanism of gene expressionThe mechanism of gene expressionDNA in genes specify information, but DNA in genes specify information, but information is not structure and functioninformation is not structure and function

Genetic info is expressed into structure & Genetic info is expressed into structure & function through protein synthesisfunction through protein synthesis

The expression of genetic info into structure The expression of genetic info into structure & function:& function:DNA in gene controls the sequence of DNA in gene controls the sequence of nucleotides in an RNA moleculenucleotides in an RNA molecule

RNA controls the primary structure of a RNA controls the primary structure of a proteinprotein

Gene ActivityGene Activity 30Effect of Mutations onEffect of Mutations onProtein ActivityProtein Activity

Point Mutations Point Mutations Involve change in a single DNA nucleotideInvolve change in a single DNA nucleotide Changes one codon to a different codonChanges one codon to a different codon Affects on protein vary:Affects on protein vary: NonfunctionalNonfunctional Reduced functionalityReduced functionality UnaffectedUnaffected

Frameshift MutationsFrameshift Mutations One or two nucleotides are either inserted or deleted One or two nucleotides are either inserted or deleted

from DNAfrom DNA Protein always rendered nonfunctionalProtein always rendered nonfunctional Normal :Normal : THE CAT ATE THE RATTHE CAT ATE THE RAT After deletion:After deletion: THE ATA TET HER ATTHE ATA TET HER AT After insertion:After insertion: THE CCA TAT ETH ERA TTHE CCA TAT ETH ERA T

Gene ActivityGene Activity 31Point Point MutationMutation

Gene ActivityGene Activity

Frameshift MutationsFrameshift Mutations32


Causes of MutationsCauses of MutationsReplication ErrorsReplication Errors

1 in 1,000,000,000 replications1 in 1,000,000,000 replicationsDNA polymeraseDNA polymerase Proofreads new strandsProofreads new strands Generally corrects errorsGenerally corrects errors

Environmental MutagensEnvironmental MutagensCarcinogens - Mutagens that increase the Carcinogens - Mutagens that increase the chances of cancerchances of cancer Ultraviolet RadiationUltraviolet Radiation Tobacco SmokeTobacco Smoke

34Sickle-Cell Disease in Humans

Gene Gene RegulationRegulation 35Prokaryotic Regulation:Prokaryotic Regulation:

The Operon ModelThe Operon ModelOperon consist of three componentsOperon consist of three components

PromoterPromoter DNA sequence where RNA polymerase first DNA sequence where RNA polymerase first attachesattaches Short segment of DNAShort segment of DNA

OperatorOperator DNA sequence where active repressor bindsDNA sequence where active repressor binds Short segment of DNAShort segment of DNA

Structural GenesStructural Genes One to several genes coding for enzymes of a One to several genes coding for enzymes of a metabolic pathwaymetabolic pathway Translated simultaneously as a blockTranslated simultaneously as a block Long segment of DNALong segment of DNA

Gene Gene RegulationRegulation 36Inducible Operons:Inducible Operons:

The lac OperonThe lac OperonThe regulator codes for a repressor The regulator codes for a repressor If lactose (a sugar that can be used for food) is If lactose (a sugar that can be used for food) is

absent:absent: Repressor attaches to the operatorRepressor attaches to the operator Expression is normally “off”Expression is normally “off”

If lactose is present:If lactose is present: It combines with repressor and renders it unable to It combines with repressor and renders it unable to

bind to operatorbind to operator RNA polymerase binds to the promoterRNA polymerase binds to the promoter The three enzymes necessary for lactose catabolism The three enzymes necessary for lactose catabolism

are producedare produced

37The lac Operon

gene activity 1outline function of genes one gene-one enzyme hypothesis genetic code transcription ...

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