Chapter 14 Flashcards
Gene expression to form a specific polypeptide takes two steps:
Transcription
Translation
both are the central dogma of molecular bio
Transcription
copies information from a DNA sequence to a complementary RNA sequence (MRNA)
Translation
converts RNA sequence to amino acid sequence of a polypeptide
(trna lines up amino acids into protein)
RNA (ribonucleic acid) differs from DNA:
- usually only one polynucleotide strand
- sugar is ribose
- contains uracil instead of thymine
Three types of RNA in protein synthesis:
mRNA
tRNA
rRNA
mRNA
carries copy of a dna sequence to site of protein synthesis at the ribosome
tRNA
carries amino acids for polypeptide assembly
rRNA
catalysis of peptide bonds and provides structure
What does mRNA do ?
forms a complementary copy of one Dna strand in gene
travels from nucleus to cytoplasm carrying info as codons
associates with ribosome and tRNA anticodons
transcription component:
A DNA template- one of the two strands
nucleotide triphosphates
RNA polymerase enzyme
what does the RNA polymerase enzyme do
-catalyzed synthesis of RNA
Transcription occurs in 3 phases
- initiation: Ran polymerase binds to promoter
- Elongation: Ran poly. unwinds DNA
- synthesizes transcript - Termination
specific stop sequence
ran poly. disengages
what is the genetic code?
specifies which amino acids will be used to build a protein
redundant - many codons are same amino acids) (not ambiguous - one codon one amino acid
what is a codon/
sequence of three bases- each specifies a specific amino acid
start codon:
AUG - initiation signal for translation
Stop codons:
UAA, UAG, UGA - stop translation
noncoding sequences are:
introns
coding sequences are:
exons
introns and eons appear in
primary mRNA transcript
how are introns removed
by splicosome from the final mRNA
two key events to ensure protein made is the specified one from mRNA:
- tRNAs must read mRNA codons correctly
2. tRNAs must deliver amino acids corresponding to each codon
functions of tRNA
- binds to amino acid
- associate with mRNA
- interact with ribosome
what is the amino acid attachment site?
3’ end - binds covalently
anticodon:
at the midpoint of tRNA sequence- site of base paring with mRNA
-unique for each tRNA
Example for anticodon
DNA codon for arginine: 3′-GCC-5′
Complementary mRNA: 5′-CGG-3′
Anticodon on the tRNA: 3′-GCC-5′. This
tRNA is charged with arginine.
ribosome
hold mRNA and charged tRNAs in correct positions to allow assembly of polypeptide chain
- two subunits -large and small
- held together by ionic and hydrophobic forces
- only held together when active
Large subunit has 3 tRNA binding sites:
A (amino acid) site binds with anticodon of charged tRNA
P (polypeptide) site where tRNA adds its amino acid to the growing chain
E (exit) site where tRNA sits before being released from ribosome
translation occurs in 3 steps:
initiation - complex forms
Elongation - second charged tRNA enters P site
-peptide bond formation
Termination - stop codon enters A site
post translational aspects:
polypeptide emerges and fold into 3-d shape
conformation allows it to interact with other molecules - it may contain a signal sequence indicating where in the cell it belongs
can be chemically modified
