biolA 190 - ch 10

10.1

10.1

at the molecular level, transcription and translation are

key processes of gene expression

central dogma

describes information flow from DNA to RNA to protein

genes provide a

“blueprint” for the characteristics of organisms

10.2

10.2

DNA stores information

This information must be accessed at the molecular level for genes to be expressed

gene

an organized unit of nucleotide sequences that enables a segment of DNA to be transcribed into RNA

results in the formation of a functional product

the functional product is often a

protein

can also be an RNA molecule

a gene is composed of specific base sequences organized in a way that

allows the DNA to be transcribed into RNA

important sequences

promoter

transcribed region

terminator

regulatory sequences

transcription occurs in 3 stages

1. initiation

   a recognition step where the promoter functions as a recognition site

   bacteria: sigma factor binds to RNA polymerase

2. elongation

occurs as RNA polymerase synthesizes RNA

DNA template strand

opposite DNA strand is called the coding strand

3. termination

occurs when RNA polymerase reaches a terminator, releases the transcript, and dissociates from DNA

basic features of transcription are similar among all organisms

promoter, initiation, elongation, and termination stages

transcription of eukaryotic genes involves

more complexity of protein components

10.3

10.3

in eukaryotes, the product of transcription is a

pre-mRNA, an immature precursor that must be processed to form a mature mRNA

pre-mRNAs undergo 3 key modifications

1. A 5’ cap is added

2. a 3’ poly A tail is added

3. during splicing, the introns are removed and the exons are connected together

the 5’ cap is recognized by a variety of proteins

it is needed for the mRNA to exit the nucleus and bind to the ribosome

the poly A tail increases

the stability of the mRNA in the cytosol

spliceosome

complex that precisely removes introns

snRNP

contains small nuclear RNA and a set of proteins

alternating splicing

allows complex eukaryotes to use the same gene to make different proteins

splicing occurs in a series of steps

and the catalytic events of steps 3 and 4 are catalyzed by an RNA component (ribozyme activity)

10.4

10.4

genetic code

specifies the relationship between the sequence of nucleotides in the mRNA and the sequence of amino acids in a polypeptide

the code is read in groups of 3 nucleotides called codons

64 codons, 1 start codon, 3 stop codons

the code is redundant

more than one codon can specify the same amino acid

key components for translation

ribosomal-binding site

start codon

coding sequence

stop codon

start codon

AUG

stop codons

UAA

UAG

UGA

the start codon redefines the reading frame

groups of 3 nucleotides that are “read“ as codons

codons are read in a

sequential and non-overlapping manner in the 5’ to 3’ direction

scan for thet start codon AUG after

the ribosomal binding site to begin

transfer RNAs

tRNAs - are involved in translating the nucleotide base sequence of the mRNA into the amino acid sequence of the polypeptide

Different tRNAs have different

anticodon sequences and each tRNA carries a specific amino acid

10.5

10.5

the tRNAs of all species share common features

anticodon

animo acid attatchment site

cells make many different tRNAs

each encoded by a different gene

The enzymes that catalyze the attachment of amino acids to tRNA molecules are

aminoacyl-tRNA synthetases

ribosomes

the sites of translation

ribosomes contain 3 discrete sites where tRNA may be located

aminoacyl (A) site

peptidyl (P) site

exit (E) site

the gene for small subunits rRNA is found in

all species

if 2 species diverged a long time ago their gene sequences are quite different whereas

if they diverged a short time ago, their gene sequences are more similar

10.6

10.6

like transcription, translation occurs in

3 stages: initiation, elongation, termination

initiation

an mRNA, the first tRNA, and the ribosomal subunits assemble into a complex

initiation factors

proteins that help assemble the mRNA , tRNA, and ribosome int a functional complex; hydrolysis of GTP provides an input of energy

the first tRNA carries a modified

methionine and resides in the P site

initiation of translation differs in a few ways

The cap is recognized by proteins that promote the binding of the mRNA to the small subunit

the elongation stage involved

covalently bonding of amino acids to each other

termination

occurs when a stop codon is reached

(UAA, UAG, UGA) are recognized by a protein called a release factor, NOT a tRNA