gene activity chapter 14 outline
TRANSCRIPT
Gene ActivityChapter 14
2Gene ActivityGene Activity
OutlineOutline
Function of GenesFunction of Genes
One Gene-One Enzyme HypothesisOne Gene-One Enzyme Hypothesis
Genetic CodeGenetic Code
TranscriptionTranscription
Processing Messenger RNAProcessing Messenger RNA
TranslationTranslation
Transfer RNATransfer RNA
Ribosomal RNARibosomal RNA
3Gene ActivityGene Activity
Function of GenesFunction of Genes
Genes Specify EnzymesGenes Specify EnzymesBeadle 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
4Beadle & Tatum Experiment
5Gene ActivityGene ActivityProtein Synthesis:Protein Synthesis:From DNA to RNA to ProteinFrom DNA to RNA to Protein
The mechanism of gene expressionThe mechanism of gene expression
DNA 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
6Sickle-Cell Disease in Humans
7Gene ActivityGene Activity
Types of RNATypes of RNA
RNA is a polymer of RNA nucleotidesRNA is a polymer of RNA nucleotides
RNA 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 DNA
Types of RNATypes of RNAMessenger (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”
8Structure of RNA
9Overview of Gene Expression
10Gene ActivityGene ActivitySteps in Gene Expression:Steps in Gene Expression:TranscriptionTranscription
TranscriptionTranscription
Gene unzips and exposes unpaired basesGene unzips and exposes unpaired bases
Serves as template for mRNA formationServes as template for mRNA formation
Loose 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
11Gene ActivityGene ActivitySteps 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 tRNA
TranslationTranslation 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
12Gene ActivityGene Activity
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”
13Gene ActivityGene Activity
The Genetic CodeThe Genetic Code
Properties 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
14The Genetic Code (mRNA)
15Gene ActivityGene Activity
Transcription of mRNATranscription of mRNA
A 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 genes
The 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
16Transcription
17RNA Polymerase
18Gene ActivityGene Activity
Processing Messenger RNAProcessing Messenger RNA
Primary 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
19mRNA Processing in Eukaryotes
20Gene ActivityGene Activity
Functions of IntronsFunctions of Introns
As 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
21Gene ActivityGene Activity
tRNAtRNA
tRNA molecules come in 64 different kindstRNA molecules come in 64 different kinds
All 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
22Structure of tRNA
23Gene ActivityGene Activity
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) site
The P (for peptide) site, andThe P (for peptide) site, and
The A (for amino acid) siteThe A (for amino acid) site
24Ribosomal Structure and Function
25Gene ActivityGene ActivitySteps 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
26Gene ActivityGene ActivitySteps 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
27Steps in Translation:#1 - Initiation
28Gene ActivityGene ActivitySteps 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 acidacid
Must 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
29Gene ActivityGene ActivitySteps 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 exits
Ribosome 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 acid
Must 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
30Steps in Translation:#2 - Elongation
31Gene ActivityGene ActivitySteps 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 of Ribosome reads the STOP codon at the end of
the mRNAthe 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
32Steps in Translation:#3 - Termination
33Summary of Gene Expression
(Eukaryotes)
34Gene ActivityGene Activity
ReviewReview
Function of GenesFunction of Genes
One Gene-One Enzyme HypothesisOne Gene-One Enzyme Hypothesis
Genetic CodeGenetic Code
TranscriptionTranscription
Processing Messenger RNAProcessing Messenger RNA
TranslationTranslation
Transfer RNATransfer RNA
Ribosomal RNARibosomal RNA
Gene ActivityEnding Slide Chapter 14