Chapter 14
Gene Activity: How Genes Work
14.1 The Function of GenesGenes are segments of DNA that specify amino acids in a protein
Sir Archibald Garrod proposed a link between genes and proteins
Genes Specify EnzymesBeadle and Tatum (1940) experimented with Neurospora (mold similar to Sordaria)
they induced mutations that made the mold only able to grow on enriched medium
Fig. 14.1 Beadle & Tatum experiment
One gene, one enzyme experiment
14.1 The Function of Genes Genes Specify Enzymes, cont.
Beadle and Tatum (1940), cont.concluded that mold must have been missing an enzyme needed to produce missing nutrient
therefore, they proposed the “one gene, one enzyme” hypothesis
14.1 The Function of Genes Genes Specify Enzymes, cont.
Pauling and Itano (1949)demonstrated that sickle cell hemoglobin is different from normal hemoglobin using electrophoresis
showed that a mutation in DNA resulted in a change in the structure of a protein
“one gene, one enzyme” revised to “one gene, one polypeptide”
Fig. 14.2 Sickle cell disease
Fig. 14.2c R group and properties
14.1 The Function of Genes From DNA to RNA to Protein
gene: segment of DNA that specifies the amino acid sequence of a protein
genes pass information to RNA (ribonucleic acid), which is more directly responsible for building proteins
14.1 The Function of Genes DNA to RNA to Protein, cont.
RNA differs from DNA
Fig. 14.3 RNA structure
14.1 The Function of Genes DNA to RNA to Protein, cont.
there are three major classes of RNA, each with its own size, shape, and function
messenger RNA (mRNA): takes message from DNA to ribosomes
transfer RNA (tRNA): carries amino acids to ribosomes
ribosomal RNA (rRNA): makes up ribosomes (along with ribosomal proteins)
14.1 The Function of Genes DNA to RNA to Protein, cont.
steps of protein synthesis (gene expression)
transcription: DNA serves as template to make RNA
translation: mRNA transcript codes for sequence of amino acids
this is Crick’s Central Dogma of Molecular Biology
Overview of gene expression
Overview of gene expression
14.2 The Genetic Code
The genetic code is a triplet codeeach codon consists of three nucleotide bases and codes for one amino acids
three nucleotides is necessary to code for 20 amino acids
14.2 The Genetic Code Finding the Genetic Code
Nirenberg and Matthei (1961) used an enzyme to produce an RNA molecule made entirely of uracil
a protein composed only of phenylalanine resulted from the RNA, therefore UUU = phe
in a similar way, the other amino acids were assigned mRNA codons
14.2 The Genetic Code Finding the Genetic Code
Nirenberg and Matthei (1961)all uracil RNA made a protein that was all phenylalanine
therefore UUU = phein a similar way, the other amino acids were assigned mRNA codons
Fig. 14.5 Messenger RNA codons
14.2 The Genetic Code Finding the Genetic Code, cont.
properties of the genetic code1. it is degenerate (redundant); most amino acids have more than one codon
2. it is unambiguous; each codon has only one meaning
3. it has start and stop signals start codon is AUG; codes for MET UAA, UAG, UGA stop translation
(no amino acid encoded)
The genetic code (easier to read)
14.2 The Genetic Code The Code Is Universal
the code is used by all living thingsevidence for the common ancestry of living things
there are some exceptions to the universality, but they generally involve just a few codons
Overview of transcription
14.3 First Step: Transcription
During transcription, a segment of DNA serves as a template for the production of RNA Messenger RNA Is Formed
initiationa promoter signals the start of a gene, direction of transcription, strand to be transcribed
promoters are specific nucleotide sequences on DNA (often TATA...)
attract RNA polymerase
Initiation of transcription
14.3 First Step: Transcription Messenger RNA Is Formed, cont.
initiation, cont.transcription factors bind to the promoter and help RNA polymerase recognize and bind to DNA
elongationRNA polymerase opens DNA strands and synthesizes mRNA along the template strand
14.3 First Step: Transcription Messenger RNA Is Formed, cont.
elongation, cont.RNA polymerase, cont.
reads 3’-5’ makes mRNA 5’-3’ mRNA grows at a rate of 30-60
nucleotides/secondonly newest portion of mRNA is bound to DNA; most of the new strand dangles off to the side
Fig. 14.6 Transcription
14.3 First Step: Transcription Messenger RNA Is Formed, cont.
elongation, cont.many RNA polymerase molecules might be working at the same time
template/non-template strands of DNA attach back together after RNA polymerase passes
Numerous RNA transcripts
Fig. 14.7b RNA polymerase
14.3 First Step: Transcription Messenger RNA Is Formed, cont.
terminationspecific nucleotide sequences signal “stop” and cause RNA polymerase to release DNA
“stop” sequence is called a terminator, usually AAUAAA on the mRNA in eukaryotes
(do not confuse this with a stop codon)
the product is primary RNA
Transcription animation
14.3 First Step: Transcription RNA Molecules Are Processed
occurs before leaving nucleus5’ cap added to tell ribosome where to attach
3’ poly-A tail added to aid transport out of nucleus and inhibit degradation
introns removed by spliceosomesintrons: in between exons; 95%exons: coding regions that will be expressed
RNA processing
RNA splicing
End result of splicing
Prokaryotes versus eukaryotes
14.4 Second Step: Translation
During translation, the mRNA message is translated to an amino acid sequence The Role of Transfer RNA
tRNA molecules transfer amino acids to ribosomes
tRNA has an anticodon that is complementary to a codon
tRNA also has an amino acid binding site, making it “bilingual”
Fig. 14.9 Structure of transfer RNA
14.4 Second Step: Translation The Role of Transfer RNA, cont.
most cells have 40 different tRNA molecules
this is less than the 61 amino-acid-coding codons and more than the 20 different amino acids
perhaps some tRNAs can pair with more than one codon as long as first two positions are correct
called wobble hypothesis
14.4 Second Step: Translation The Role of Transfer RNA, cont.
aminoacyl-tRNA synthetases attach correct amino acid to each tRNA
requires ATP20 different types (1 for each amino acid)
this means they can accommodate different kinds of tRNA
14.4 Second Step: Translation The Role of Ribosomal RNA
structure of a ribosomeconsists of a small and large subunit
made in nucleolus60% rRNA, 40% proteinhas 3 binding sites for tRNA
A site: amino acid site P site: peptide site E site: exit site
has 1 binding site for mRNA
Fig. 14.10a,b Ribosome structure
14.4 Second Step: Translation The Role of Ribosomal RNA, cont.
function of a ribosometRNA binding sites facilitate base pairing between tRNA anticodons and mRNA codons
rRNA joins amino acids as a polypeptide is synthesized
multiple ribosomes can translate same mRNA simultaneously; complex called a polyribosome
Fig. 14.10c,d Polyribosome
14.4 Second Step: Translation Translation Requires Three Steps
initiation (in prokaryotes)brings all the translation components together
small ribosomal subunit attaches to mRNA at 5' end at initiation sequence (AUG)
tRNA with anticodon UAC (carrying amino acid MET) attaches to initiator
Fig. 14.11 Initiation
14.4 Second Step: Translation Translation Requires Three Steps
initiation, cont.tRNA, mRNA, MET and small subunit form initiation complex
large ribosomal subunit attaches to initiation complex
tRNA is at P siteA site is for next tRNAE site is for tRNAs leaving ribosome
Fig. 14.12 Elongation
14.4 Second Step: Translation Translation Requires Three Steps
elongationpolypeptide increases in length one amino acid at a time
next tRNA with complementary anticodon binds to A site
peptide transferred to this tRNA as ribozyme joins the amino acids at A and P site (forms peptide bond)
14.4 Second Step: Translation Translation Requires Three Steps
elongation, cont.ribosome moves forward
tRNA at A site (with peptide) shifted to P site
tRNA at P site (“empty”) shifted to E site and exits
the next tRNA binds at A site and process repeats
14.4 Second Step: Translation Translation Requires Three Steps
terminationtranslation components separateoccurs at a stop codon (UAG, UAA, UGA)
no tRNA with anticodon for theserelease factors cleave polypeptide from last tRNA and cause ribosome to split, releasing mRNA and new protein
protein is properly folded
Fig. 14.13 Termination
Translation
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Translation animation
Fig. 14.14 Summary
Nucleus
DNA
Replication
RNA
Transcription
Central Dogma of Molecular Biology
Protein
Translation Enzymes
StructuralComponentsof the Cell
Cytoplasm (ribosomes)