Gene Expression - From Gene to protein

What is gene expression

the process by which DNA directs protein synthesis

What are the two stages of gene expression

transcription and translation

What is the central dogma of molecular biology

DNA -> mRNA -> Protein

What is DNA -> mRNA

transcription

What is mRNA -> Protein

translation

What are the links between genotype and phenotype

proteins

What is the intermediate between DNA and protein

RNA

What is transcription

synthesis of an RNA molecule from a DNA template

What does transcription produce

messenger RNA (mRNA)

What is translation

the decoding of an mRNA message into a protein

Where does translation occur

ribosome, in the cytoplasm

Where does transcription / translation occur in prokaryotes

both in cytoplasm, translation can begin before transcription has ended because the mRNA does not have to travel outside of the nucleus, because there is no nucleus.

Where does transcription / translation occur in eukaryotes

transcription: nucleus

translation: cytoplasm / ribosomes

Transcription must be completed before translation begins; ribosomes cannot enter nucleus

What are the three stages of transcription

initiation, elongation, termination

What are the molecular components of transcription

-RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides

-The RNA is complementary to the DNA template strand

-RNA synthesis follows the same base-pairing rules as DNA, except that uracil substitutes for thymine

What is initiation of transcription

RNA polymerase attaches to the DNA on the promoter region; promoters signal the transcription start point and usually extend several dozen nucleotide pairs upstream.

What are transcription factors

They help guide the binding of RNA polymerase and the initiation of transcription

What is elongation in transcription?

RNA polymerase moves along the DNA, untwists the double helix, 10 - 20 nucleotides at a time. Nucleotides are added to the 3' end of the growing RNA molecule.

What is the primary transcript

the initial RNA transcript from any gene prior to processing

What is termination of transcription

RNA polymerase recognizes the genes terminator sequence and stops transcribing.

What is termination of transcription in eukaryotes

Transcription continues until it transcribes a string of A's, known as the Poly A tail, and then continues 10-35 nucleotides past this polyadenylation sequence

What is termination of transcription in prokaryotes

The polymerase stops transcription at the end of the terminator- no modification of the transcript after the terminator codon

What is the template strand

The original strand that is being used to copy and make complementary pairs.

What is the coding strand

The DNA strand that is not copied to synthesize a molecule of RNA

What is pre-mRNA / primary transcript

The first molecule made during elongation. Single stranded RNA made by the transcription of DNA; made into mRNA, tRNA, or rRNA

What are the 5' cap and poly A tail in the transcription process of RNA

The 5' cap is a special modified guanine nucleotide added to the 5' end. The poly A tail is where transcription stops in a eukaryotic cell.

What is RNA splicing

The process by which introns are removed from the pre-mRNA molecule.

What are introns and exons

introns are junk DNA- noncoding sequences. Exons are functional mRNA coding regions.

What is the functional and evolutionary function of introns

They are the no coding segments in a gene. These are cut out during RNA splicing. Introns allow for genes to be spliced which can lead provide a source for new genes

Why is the number of proteins much greater than the number of genes in an organism

Some genes can encode more than one kind of polypeptide depending on which segments are treated as exons and which are treating as introns during splicing ; alternative RNA splicing

What are the functions of the modifications to the ends of the mRNA

Facilitate the export of mRNA to the cytoplasm; protect mRNA from hydrologic enzymes; help ribosomes attach to the 5' end

What is a codon

a sequence of three nucleotides that together form a unit of genetic code in a DNA or RNA molecule.

What is the relationship between the linear sequence of codons on mRNA and the sequence of amino acids in a polypeptide

Each codon, set of three nucleotides, codes for certain proteins.

Why is the genetic code redundant and not ambiguous

Redundant - More than one codon may specify a particular amino acid ( ACU, ACC code for same amino acid)

Not Ambiguous - No codon species more than one amino acid

What is the significance of the reading frame during translation

There are 64 possible codons, all of which have an amino acid or a stop signal that they translate into. However, there are only twenty amino acids, so many of the possible codons share their function with another codon. So the genetic code is unambiguous, yet redundant.

Why is the structure and function of tRNA important

A tRNA molecule is a single strand about 80 nucleotides long. It is L shaped, with a 5' and 3' end both located near one end. The protruding 3' end allows for amino acids to attach. Each anticodon is specific for a certain amino acid

What are the three steps of translation

initiation, elongation, termination

What are the three steps of elongation

codon recognition, peptide bond formation, translocation

What is codon recognition

Anticodon of incoming tRNA pairs with mRNA in A site

What is the peptide bond formation

the new amino acid is joined to the chain

What is translocation

The ribosomes moves to the next codon. A site is empty and is ready to receive the next amino acid

How is tRNA joined to the appropriate amino acid

tRNA has the anticodons that complementary pair with the codons on the mRNA.

What is aminoacyl-tRNA synthesis

An enzyme that adds appropriate amino acid onto the 3' end of the tRNA

Describe the structure and function of ribosomes

Ribosomes are the organelles in the cytoplasm that coordiante the functioning of the mRNA and tRNA and acutally make polypeptides. Structure: the ribosomes consist of two subunits, each made up of proteins and a kind of RNA called ribosomal RNA. Eukaryotes ribosomes are slightly larger and are different in composition. The differences are medically significant.

What are the P, A, and E sites in ribosomes

P site holds the tRNA that carries the growing polypeptide chain

A site holds the tRNA that carries the next amino acid to be added to the chain (start)

E site is the exit site where tRNA leaves the ribosome

What is the significance of start and stop codons

Start codons signals the beginning of protein synthesis. It is AUG on mRNA and represents the amino acids methionine. Stop codons signal the end of protein production.

What is the anticodon

a sequence of three nucleotides forming a unit of genetic code in a transfer RNA molecule, corresponding to a complementary codon in messenger RNA.

When is tRNA 'Charged"

When an amino acid is attached to the 3' end