Genetic Expression

• initiation

• elongation

• termination

What are the three stages of transcription

• DNA is a double helix of two complementary strands

• template strand → provides a template for ordering the sequence of complementary nucleotides in an RNA transcript

  • always the same strand for a given gene

• non-template → aka coding strand; nucleotides are identical to mRNA sequence being produced for gene

What is the difference between the template and non-template strand?

• DNA regions that initiate the transcription of a gene

  • signal the transcription start point

  • usually extend several dozen nucleotide pairs upstream of the gene start point

  • eukaryotes have a promoter called a TATA box that is crucial in forming the initiation complex

What is a promoter?

transcription factors

• proteins that help guide the binding of RNA polymerase

  • in proteins: structure = function

RNA polymerase

• the final piece of the initiation puzzle

  • enzyme that catalyzes the synthesis of RNA from DNA template

  • specialized protein

• as RNA polymerase moves along the DNA, it untwists the double helix, 10-20 nucleotides at a time

• template strand is read 3’ to 5’

• RNA strand is built 5’ to 3’

  • nucleotides are added to the 3’ end of the growing RNA molecule

What is elongation?

They refer to the directionality of nucleic acid strands based on the carbon atoms in the sugar backbone. The 5' end has a phosphate group, and the 3' end has a hydroxyl (-OH) group.

What do 3' and 5' refer to in DNA/RNA?

DNA polymerase can only add nucleotides to the 3' end, so DNA synthesis always occurs in the 5' → 3' direction.

Why is the 3' and 5' direction important in DNA replication?

• The leading strand is synthesized continuously in the 5' → 3' direction.

• The lagging strand is synthesized discontinuously in Okazaki fragments.

How does directionality affect the leading and lagging strands in replication?

The transcribed RNA is identical to the coding strand, except thymine (T) is replaced with uracil (U) in RNA.

Is the RNA strand being transcribed exactly the same as the coding strand?

• RNA strand is released from RNA polymerase

  • releases RNA polymerase from template

• signaled by terminator regions on the template strand

  • e.g. polyadenylation signal in eukaryotes

• ensures proper regulation of gene expression and prevents the production of unwanted transcripts (mRNA sequences)

What is the final step of RNA synthesis?

polyadenylation signal (PAS)

a sequence of nucleotides that marks the end of a messenger RNA (mRNA) molecule

RNA polymerase

• enzyme which pries the DNA strands apart and joins together the RNA nucleotides

What is RNA synthesis catalyzed by?

complementary

• read 3’ to 5’

RNA built is _____ to the DNA template strand.

uracil substitutes for thymine

What is the difference between RNA synthesis and DNA base-pairing rules?

• sites for translation

• use the info in DNA to make proteins

• build proteins in two locations

  • cytosol

  • outside of ER or the nuclear envelope

• made up of ribosomal proteins and ribosomal RNA

What are ribosomes?

• codons into amino acids

• nucleotide sequences of genes in DNA provide a code for constructing a protein

What do ribosomes translate?

• the conversion of the nucleic acid language to the polypeptide (protein) language

  • there is a change in language from the nucleotide sequence of the RNA to the amino acid sequence of the polypeptide

  • based on a triplet code: genetic instructions written as a series of three-base words called codons

What does translation require?

genetic code

the set of rules that dictate the amino acid translations of each of the mRNA nucleotide triplets

• convert mRNA codons transcribed from a gene to the amino acid sequence of a polypeptide

• cytoplasm

• a ribosome attaches to the mRNA and translates its message into a specific polypeptide, aided by transfer RNAs (tRNAs)

• each tRNA is a folded molecule bearing a base triplet called an anticodon on one end and a specific amino acid attachment site at the other end

Where does translation take place? What happens?

• initiation

• elongation

• termination

What are the phases of transcription?

• starts when the small ribosomal subunit binds with mRNA and a special initiator tRNA

• the initiator tRNA carries the amino acid methionine

• then the small subunit moves along the mRNA until it reaches the start codon (AUG)

How does the initiation of translation work?

• translation proceeds along the mRNA in a 5’ to 3’ → direction of mRNA

• the ribosome and mRNA move relative to each other, codon by codon

  • a tRNA with. complementary anticodon pairs with each codon

  • adding its amino acid to the growing polypeptide chain

• elongation continues until a stop codon reaches the ribosome’s A site

How does the elongation of translation work?

• nucleotides in groups of threes

  • 5’ → 3’

• start codon methionine (MET) initiates translation

  • codon = AUG

• three stop codons terminate translation

• there are multiple codon options for several amino acids

How are codons read?

mutations

changes in the genetic information of a cell

point mutations

changes in just one nucleotide pair of a gene

nucleotide-pair substitution

replaces one nucleotide and its partner with another pair of nucleotides

missense mutations

still code for an amino acid, but a different amino acid

nonsense mutations

change an amino acid codon into a stop codon; most lead to a nonfunctional protein

silent mutations

have no effect on the amino acid produced by a codon because of redundancy in the genetic code (more than one codon for each amino acid)

reading frame

Codons must be read in the correct ____ (correct groupings) for the specified polypeptide to be produced