RNA and the genetic code flashcards

types of RNA found in cells

mRNA:

• transcribed from template DNA; translated into a protein

• monocistronic in eukaryotes

tRNA:

• converts nucleic acid to codon form

• found in cytoplasm

rRNA

• synthesized in the nucleus, used during protein assembly

stop codons and start codongs

UAA: U Are Annoying

UGA: U Go Away

UAG: U Are Gone

AUG: start codon

what does it mean when a genetic code is degenerate?

more than one codon can specify a single amino acid

• third base in codon as wobble position; silent or degenerate/ no effects on the expression of amino acid and no adverse effects on sequence

types of mutations

Point mutations:

• missense: one amino acid substitutes for another

• nonsense: codon encodes for premature stop codon

frameshift mutations: some number of nucleotides are added to or deleted from sequence

Transcription

DNA —> mRNA

1. helicase and topoisomerase unwind DNA

2. RNA polymerase II synthesizes RNA

   1. binding site = TATA box

   2. transcription factors help RNA polymerase locate and bind to promoter

3. upstream = 5’ = negative numbers

Three types of RNA polymerase

1. RNA polymerase I: located in the nucleolus and synthesizes rRNA

2. RNA polymerase II: located in nucleus, synthesizes hnRNA and snRNA

3. RNA polymerase III: located in the nucleus, synthesizes tRNA and rRNA

3 post-transcriptional processing

1. intron/exon splicing

   1. remove noncoding (introns)

   2. add coding (exons)

2. 5’ cap

   1. added during transcription, recognized by ribosome as binding site.

   2. protects nRNA from degradation in cyctoplasm

3. 3’ poly A tail

   1. added to end of 3’ on the transcript; protects against degradation

      1. composed of adenine bases

three stages of translation

initiation, elongation, and translation

initiation stage of translation

1. small ribosomal subunit binds to mRNA

• initial amino acid in eukaryote = methionine

2. large subunit binds tot the small subunit

elongation stage of translation

APE, read from 5’ to 3’ , synthesized from N to C terminus

1. A site holds incoming tRNA complex

2. P site holds the tRNA that carries growing polypeptide chain

   1. peptide bond is formed when polypeptide is passed from tRNA in the P to A site

      1. uses GTP

3. E site: inactivated tRNA exits

elongation factors

assist translation by locating and recruiting aminoacyl-tRNA and GTP, and helping remove GDP

termination stage of translation

1. release factor binds to termination codon; water is added

2. hydrolyzes completed polypeptide chain from tRNA

post-translational processing and factors

chaperones: chaperone the folding of the protein

• phosphorylation (addition of phosphate group)

• carboxylation: (addition of carboxylic acid groups, for calcium binding sites)

• glycosylation: (addition of oligosaccharides to determine cellular destination)

• prenylation: addition of lipid groups to membrane-bound enzymes

operon structure and systems

• transcribed as a single mRNA

• operator site = upstream, which binds to the repressor

• regulator gene/repressor = further upstream

inducible: induced when inducer binds to a repressor

• lac operon

repressible: repressor by gene is inactive until it binds to a corepressor

• trp operon

control of gene expression in eukaryotes

1. transcription factors: DNA binding domain and activation domain

2. gene amplification

3. regulation of chromatin structure

Gene amplifiers (2)

1. enhancers: response elements outside normal promotor regions can enhance transcription levels

2. gene duplication: duplicating relevant gene increases expression of a gene

Regulation of chromatin structure (2)

histone acetylation: weakens interaction of histone with DNA, easier access of transcriptional machinery

• increased gene expression levels

DNA methylation: add methyl groups to cytoside and adenine, silencing of gene expression