Chapter 8c : Molecular Bio Gene Expression

Gene expression

the flows of genetic information from the genotype to the phenotype

DNA —> ________ ——> ______

RNA, Protein

one gene one enzyme hypothesis

a single gene controlled the synthesis of a single enzyme

Beadle and Tatum experiment

genetic mutations in the mold Neurospora crassa

Central Dogma

DNA —→ RNA = translation

RNA—→ Protein = translation

Transcription

info encoded in DNA made into a complementary RNA. The segments can encode proteins are mRNA, others are non-coding RNAs (ncRNA)

Translation

synthesizing a specific polypeptide by using the info encoded in the mRNA on a ribosome

most of the genes are ______ into mRNA, and then the RNA is _____ into a polypeptide

Transcribed, translated

Where does transcription and translation occur in the prokaryotic cell?

The cytoplasm due to the absence of nucleus

Where does transcription and translation occur in eukaryotic cells?

transcription in the nucleus, translation in the ribosomes present on the rough ER membrane in the cytoplasm

Gene

an organized unit of DNA sequences that enables a segment of DNA to be transcribed into RNA and ultimately results in the formation of a functional product

Promotor

the site in the DNA where RNA polymerase binds to begin transcription

Regulatory sequences

the sites binding to regulatory proteins control whether a gene is turned on or off

Transcribed regions

a region of DNA that is transcribed into an RNA molecule

Terminator

a sequence that caused the RNA strand to be released from the transcription complex

Transcription Process

Transcription factors binds to promoter region of a gene, RNA polymerase binds to promoter region. The template strand acts as a template and is read 3 to 5, while RNA is synthesized from 5 to 3.

Eukaryotic promoter region sequence and name

TATA Box

Prokaryotic promotor region sequence and name

TATAAT Pribnow Box

Elongation

RNA polymerase catalyzes the transcription, and U rather than T incorporates into RNA

Transcription ends at ________

terminator site AAUAAA

Product

Transcripts - RNAs (mRNA, tRNA, rRNA)

Rho-independent (intrinsic) termination

the RNA forms a hairpin structure which displaces RNA polymerase and stops transcription

Rho (ATPase) dependent termination

occurs when the rho protein disassociates the RNA polymerase and moves it off of the template

Eukaryotic vs Prokaryotic Transcription

Euk:

Is more complex

In nucleus

5 general transcriptional factors

3 types of RNA polymerase

Pro:

Less complex

In Cytoplasm

a single sigma factor

a single type of polymerase

Exon

a coding portion of a gene

Intron

A non coding portion of a gene that is exercised from the RNA transcript (interrupting Intron gets sliced out)

RNA splicing

the removal of introns and joining of exons in Euk RNA, forming an mRNA molecule with a continuous coding sequence

Where does RNA splicing occur?

mRNA splicing occurs in a spliceosome (large RNA protein complex) before mature mRNA

which RNAs are self splicing ribozymes?

tRNAs and rRNAs

Functions of introns

generates different proteins, contributes to genetic diversity, regulate gene expression

capping

a 7-methylaguanosine attached to a mature euk mRNAs at the 5 prime end

Tailing

A ply A tail is added to the most mature euk mRNAs at the 3 prime end

The genetic code

nucleotide info that specifies a.a. sequence of a polypeptide; comes in triplets (codons). Each codon specifies an a.a., or signals to either start or terminate polypeptides synthesis

how many bases are there? how many possible codons?

4 bases so 64 codons

How many Start Codons, and what does it do?

AUG indicated the starting point of translation and codes for methionine

How many Stop codons and what does it do?

UAA, UGA, UAG indicate the end of translation

___#__ codons code for a particular a.a

60

The genetic code

there are only 20 a.a. The genetic code can be redundant (degeneracy), but the code is not ambiguous

How is the genetic code universal?

The genes can be transcribed and translated after being transplanted from one species to another. ( Ie. Green Fluorescent Protein from Jellyfish)

Codon usage bias

some species prefer some codons over others that code for the same a.a. bc their enzymes are more compatible with them

Reading frame

Codons must be read in the correct reading frame (correct grouping) in order for the specified polypeptide to be produced

what does this strand do? Non-template strand, coding strand, ________ strand

sense; is the strand of DNA that is not used as a template for transcription, bc it corresponds to the same sequence of mRNA that will contain the codons to build proteins in 5—> 3 direction

What does this strand do? Template strand, non coding strand, anti sense strand

mRNAs are built on this strand, runs from 3—> 5 direction

Mutations and most common are?

permanent alteration of the nucleotide sequence of the genome of an organism. Base substitution and insertion or deletion

Base-Pair substitution or point mutation

replacement of 1 nucleotide and its partner in the complementary DNA strand with another pair of nucleotides

Missense mutations

single nucleotide change results in a codon that codes for a different a.a.

Silent mutation

where a change in DNA codon doesn’t result in a change in a.a., specified by the gene

Nonsense mutation

Sense codon that corresponds to one of the 20 a.a. specified by the genetic code is change to a chain terminating codon

Frameshift mutation

insertions or deletions of a number of nucleotides in a DNA sequence is not divisible by 3

Translation requires ? 5 things

Amino acids, ATP and GTP, Enzymes and proteins, RNAs, Ribosomes

Anticodon

the three nucleotide segment that base pairs with a codon in mRNA

aminoacylation

process of adding an a.a. to tRNA

Ribosomes

Machinery for protein synthesis, in cytoplasm or ER, 2 parts the large and small subunits,

Ribosome structure binding sites

A site: Aminoacyl tRNA binding site, P site: Peptidyl tRNA binding site, E site: Exit site

Initiation Process

1. Initiator Met-tRNA(start codon) binds to small subunit and forms a complex

2. Met binds to the 5’cap of the mRNA

3. Large subunit binds and GTP is hydrolized

   ( Grows from N terminus towards C terminus along mRNA 5’ to 3’ direction)

Elongation Process:

1. mRNA meets the “charged tRNAs” entering the A site at a ribosome

2. Ribosome travels in 5’ to 3’ direction and synthesizes a polypeptide

3. The ribosome moves along the mRNA one codon consisting of 3 nucleotides at a time

Termination Process

1. The presence of a stop codon in the A site of the ribosome causes translation to be terminated

2. No tRNA has anticodon that can pair w a stop codon, so a release factor binds in the A site

3. All the components disassemble, releasing a complete polypeptide