CH 17 BIO181

transcription elongation

RNA Polymerase II:

untwists and opens a short segment of DNA (gene of interest)

**1 strand serves as a template to make RNA (the strand (sense strand) that has the promoter)**

links incoming RNA nucleotide to the 3’ end of growing chain

transcription initiation

RNA Polymerase binds to promoter (start point of gene)

Promoters recognized by other proteins (transcription factors)

RNAP separates DNA strands and transcription

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Initiation promoter

**Transcription factors**: __proteins__ that __help the RNAP__ __recognize__ the __promoter__

**T**ranscription **f**actors __bind to TATA box__

transcription termination

stops transcription

transcription proceeds until RNAP reaches a __terminator__

Eukaryotes: terminator sequence = AAUAAA (located in mRNA not DNA)

Termination sequence

AAUAAA

translation termination

Ribosome reaches __**stop codon (UAA, UAG, UGA)**__

__***Release factors break the bond between the polypeptide and the tRNA in the P-site***__

polypeptide and tRNA are released from ribosome

Translation complex dissociates

translation elongation

amino acid chain is formed

polypeptide bonds

the extending of the amino acid sequences and the formation of the amino acid chain is formed

p-site

where polypeptide grows

a-site

where incoming polypeptide lands

ribosomes

coordinate the pairing of anticodon to codon

coordinate the pairing of tRNA to mRNA

2 subunits (small and large) separated when not involved in protein synthesis

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aminoacyl-tRNA synthetase

1) amino acid bind to enzyme

2) the right tRNA binds to enzyme

3) the charged complex is released

**Enzyme that catalyzes the attachment of an amino acid to its tRNA (ATP dependent)**

tRNA

the interpreter between the mRNA sequence and amino acid sequence

aligns the correct amino acids to make a protein

transfers amino acids to the protein synthesizing machinery: ribosomes

wobble

the ability of tRNA to recognize 2 or 3 different mRNA codons

occurs when the third base (5’ end) of tRNA has some play

frameshift mutation

insertion or deletion that causes a shift in the readingframe

silent mutation

amino acid stays the same

nonsense mutation

changes an amino acid codon to a stop codon

missense mutation

changes an amino acid to a different amino acid

start codon

initiation codon

the ribosome binds initiator methionyl-tRNA and mRNA with the initiation codon (start codon) base-paired to the anticodon of the former in the p-site

1) small ribosomal subunit binds to mRNA

2) large ribosomal subunit completes the initiation complex

e-site

uncharges tRNA leaves

codons

a 3-nucleotide sequence in mRNA that specifies which amino acids will be added to a growing polypeptide chain

RNA splicing

RNA processing that **removes intron** and **joins extrons** and produces mature mRNA (continuous coding region) that will move into the cytoplasm

cap

tail

splice

spliceosome

a large molecular complex that **catalyzes** RNA splicing reactions

exons

tells mRNA amino acids

**coding sequences** of a gene that are transcribed and translated

coding region tells me which amino acids to use when building my protein

introns

**intervening sequences in DNA** in between coding sequences (exons); initially transcribed but not translated because they are removed

may allow a single gene to direct the synthesis of multiple proteins (alternative splicing)

3’ end modifications

**modified by** the addition of a **poly-a tail** (30-200 a’s) before the RNA exits the nucleus

may inhibit degradation

5’ end modification

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cap - modified GTP that is added to the 5’ end of mRNA after transcription begins

\- protects mRNA from degradation by enzymes

\- helps ribosomal subunit recognize the attachment site on mRNA’s 5’ end

post-transcriptional modifications

mRNA transcripts can be processed in 2 ways:

1) covalent modification of both 3’ and 5’ ends

2) removal of intervening sequences (introns)

RNA

a nucleic acid: polymer of nucleotides

5-carbon sugar RNA is ribose, not deoxyribose

U not T, uracil instead of thymine

transcription overview

the synthesis of RNA using DNA as a template

a gene’s unique nucleotide sequence is transcribed from DNA to a complementary nucleotide sequence in messenger RNA (mRNA)

this mRNA is carried to the cell’s protein-building machinery (ribosomes)

translation overview

RNA-directed synthesis of a polypeptide

tRNA is the interpreter between the mRNA sequence and amino acid sequence

tRNA aligns the correct amino acids to make a protein

\- transfers amino acids from the cytoplasm’s pool of amino acids to a ribosome

\- recognize the correct codons in mRNA

1) transfer RNA

2) aminoacyl-tRNA synthetase

3) ribosomes

translation initiation (initiator tRNA)

initiation brings together mRNA, a tRNA attached to the first amino acid (initiator tRNA)

ITRNA always carries methionine (AUG) and two ribosomal subunits

Large subunit binds to small subunit

Initiator-tRNA fits into the p-site

empty a-site is ready for the next aminoacyl tRNA

transcription unit

promoter, RNA-coding sequence, a terminator

Sequence of nucleotides in DNA that codes for a single RNA molecule, along with the sequences necessary for its transcription

stop codon (translation termination)

UAA, UAG, UGA