Chapter 17 From Gene to Protein. Gene Expression DNA leads to specific traits by synthesizing...
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Transcript of Chapter 17 From Gene to Protein. Gene Expression DNA leads to specific traits by synthesizing...
Chapter 17From Gene to Protein
Gene Expression
•DNA leads to specific traits by synthesizing proteins
•Gene expression – the process by which DNA directs protein synthesis
•Two stages:▫Transcription▫Translation
One Gene – One Polypeptide
•The fundamental relationship between proteins and genes
•The one gene – one polypeptide hypothesis states that each gene dictates production of a specific polypeptide
•Many proteins are composed of multiple polypeptides
Transcription and Translation• RNA (ribonucleic acid) is the intermediate
between genes and proteins
• Transcription – synthesizes RNA under the instruction of DNA▫ mRNA (messenger RNA) is produced
• Translation – synthesis of a polypeptide▫ Occurs at ribosomes under the direction of mRNA
Prokaryotes and Eukaryotes• Transcription and translation occur in both
prokaryotic and eukaryotic cells
In prokaryotes:• mRNA produced by transcription is immediately translated• Bacteria can simultaneously transcribe and translate the
same gene.
In eukaryotes:• Nuclear envelope separates transcription and translation• mRNA goes through RNA processing
TRANSCRIPTION
TRANSLATION
DNA
mRNARibosome
Polypeptide
Nuclearenvelope
TRANSCRIPTION
RNA PROCESSINGPre-mRNA
DNA
mRNA
TRANSLATION Ribosome
Polypeptide
ProkaryoteBacterial Cell
Eukaryotic Cell
Triplet Code• Triplet code – the genetic instructions for a
polypeptide chain are written in the DNA as a series of non-overlapping, three-nucleotide “words”
• Each triplet codes for an amino acid
Triplet Code
Transcription:•The template strand of DNA gives the
sequence of nucleotides in an RNA transcript
Translation:•mRNA triplets (codons) are read in the 5’
to 3’ direction•1 codon = 1 amino acid
DNAmolecule
Gene 1
Gene 2
Gene 3
DNAtemplatestrand
TRANSCRIPTION
TRANSLATION
mRNA
Protein
Codon
Amino acid sequence
Codons and Amino Acids
•Multiple codons can specify one amino acid
•One codon cannot specify multiple amino acids
•One start codon: AUG (specifies methianine)
•Three stop codons: UAA, UAG and UGA
Reading Frame
•Codons must be reading in the correct reading frame, which means:▫5’ to 3’ direction▫Codons must be read as groups of three▫Codons do not overlap
Genetic Code
•The genetic code is nearly universal•It is shared by simple bacteria to complex
animals
•Genes can be transplanted from one species to another
Plant with firefly gene
Pig with jellyfish gene
Transcription
•The enzyme RNA polymerase unzips the two DNA stands
•RNA synthesis base-pairing is similar to DNA synthesis, except uracil substitutes thymine
Transcription
•Promoter – the DNA sequence where RNA polymerase attaches
•Terminator (in bacteria) – sequence that signals the end of transcription
•Transcription unit - The transcribed DNA section
Transcription
Three Stages• Initiation – RNA polymerase binds to DNA•Elongation – RNA polymerase attaches
RNA nucleotides, creating mRNA•Termination – transcription stops
Transcription
•Promoters – signal beginning of RNA synthesis
•Transcription factors (in eukaryotes) – proteins that mediate the binding of RNA polymerase and initiate transcription
•Transcription initiation complex – the complex of transcription factors and RNA polymerase bound to a promoter
TATA Box
•In eukaryotes, a promoter commonly includes a nucleotide sequence containing TATA
•The TATA box is located about 25 nucleotides upstream from the start point
•Transcription factors bind to the DNA first, followed by RNA polymerase and more transcription factors
RNA Modification• In eukaryotes, mRNA is modified after transcription (still
in nucleus)• Both ends are altered
▫ 3’ end gets a poly-A tail▫ 5’ end gets a guanine cap
• These modifications▫ Facilitate mRNA export▫ Protects from hydrolytic enzymes▫ Help ribosomes attach to 5’ end
RNA Splicing
•Introns – noncoding stretches of mRNA •Exons – regions that are coding, they are
expressed•RNA splicing – removes introns and
connects exons▫Carried out by spliceosomes▫snRNPs (small nuclear ribonucleoproteins)
recognize splice sites
Ribozymes• Ribozymes – RNA molecules that function as
enzymes and can splice RNA• Properties that allow RNA to function as an
enzyme:▫RNA can form a three dimensional structure
because it can pair with itself▫Some bases in RNA contain functional groups▫RNA may hydrogen bond with other nucleic
acid molecules
Alternative RNA Splicing
•The segments that are treated as exons in RNA splicing determine the polypeptide
•Variations are called alternative RNA splicing
•The number of proteins an organism can produce is greater than its number of genes
Translation (outside nucleus)
•tRNA (transfer RNA) – translates mRNA message into protein
•Each tRNA molecule carries an amino acid at one end
•There is an anticodon at the other end, which base pairs with mRNA codons
tRNA
Ribosomes
•Ribosomes – facilitate coupling of tRNA anticodons with mRNA codons
•The large and small ribosomal subunits are made of proteins and rRNA (ribosomal RNA)
RibosomesThree bonding sites for tRNA• P site – holds the tRNA that carries the growing
polypeptide chain• A site – holds the tRNA with the next amino acid to be
added to the chain• E site – Exit site, tRNA leaves ribosome
Three Stages of Translation
•Initiation – brings together tRNA, mRNA, small and large subunit
•Elongation – amino acids are added with peptide bonds
•Termination – a stop codon cause the polypeptide to be released
Termination
•When a stop codon is detected, the A site accepts a release factor protein
•A water molecule is added instead of an amino acid
•The polypeptide is released and the translation assembly comes apart
Polyribosomes
•Polyribosome – multiple ribosomes translating a single mRNA simultaneously
•Polyribosomes produce polypeptides quickly
Secretion
•Polypeptides destined for the endomembrane system or secretion are marked with a signal peptide
•A signal recognition particle (SRP) recognizes the signal as it emerges from the ribosome
Point Mutations• Point mutations – occur in just one base pair of
a gene▫Can lead to abnormal protein production
Substitutions• Base-pair substitution – the replacement of one
nucleotide and its partner with another• Missense mutations – change one amino acid to another• Nonsene mutations – a codon for an amino acid is
changed into a stop codon• Silent mutations – no effect on the amino acid produced
by a codon
Insertions and Deletions
•Insertion – addition of nucleotide pairs in a gene
•Deletion – loss of nucleotide pairs in a gene
•Can alter the reading frame, causing a frameshift mutation
•These have disastrous effects on protein more often than substitutions
Mutagens
•Mutagens – physical or chemical environmental agents that cause mutation
•Spontaneous mutation can occur during DNA replication, recombination, or repair
TRANSCRIPTION
RNA PROCESSING
DNA
RNAtranscript
3
5RNApolymerase
Poly-
A
Poly-A
RNA transcript(pre-mRNA)
Intron
Exon
NUCLEUS
Aminoacyl-tRNAsynthetase
AMINO ACID ACTIVATIONAminoacid
tRNACYTOPLASM
Poly-A
Growingpolypeptide
3
Activatedamino acid
mRNA
TRANSLATION
Cap
Ribosomalsubunits
Cap
5
E
PA
AAnticodon
Ribosome
Codon
E
Questions
1. Compare and contrast transcription and translation in eukaryotes and prokaryotes.
2. Finish the chart:
1. Eukaryotes Prokaryotes
Separate transcription and translation
mRNA proceeds directly to translation
mRNA modification occurs No mRNA modification
Nuclear envelope divides transcription and translation
processes
A gene can be transcribed and translated simultaneously
2.
Carries amino acid information from DNA to ribosomes
Translates mRNA codons into amino acids in protein synthesis
Ribosomal RNA (rRNA)