Chapter 17 From 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 mRNA Ribosome Polypeptide Nuclear envelope TRANSCRIPTION RNA PROCESSING Pre-mRNA DNA mRNA TRANSLATION Ribosome Polypeptide Prokaryote Bacterial 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

DNA molecule Gene 1 Gene 2 Gene 3 DNA template strand 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)

Ribosomes Three 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 RNA transcript 3 5 RNA polymerase Poly-A RNA transcript (pre-mRNA) Intron Exon NUCLEUS Aminoacyl-tRNA synthetase AMINO ACID ACTIVATION Amino acid tRNA CYTOPLASM Poly-A Growing polypeptide 3 Activated amino acid mRNA TRANSLATION Cap Ribosomal subunits Cap 5 E P A A Anticodon Ribosome Codon E

Questions 1.Compare and contrast transcription and translation in eukaryotes and prokaryotes. 2.Finish the chart:

1. EukaryotesProkaryotes Separate transcription and translation mRNA proceeds directly to translation mRNA modification occursNo 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)