4.3 Gene Expression Flashcards
what is gene expression
process of using a gene sequence to synthesize a protein, relies on RNA (ribonnucleic acid)
messenger RNA mRNA, ribosomal RNA rRNA, transfer tRNA
compare the sugar, bases, strands, and helix of DNA and RNA
DNA: deoxyribose/ adenine, guanine, thymine, cytosine/ double stranded/ yes helix
RNA: ribose/ adenine, guanine, uracil, cytosine/ single stranded/ no helix
describe the general processes of gene expression
transcription: in nucleus, portion of DNA serves as as a template for mRNA formation
translation: in cytoplasm. sequence of mRNA bases (complementary to those in template) determines sequence of amino acids that form polypeptides
(mRNA has info from DNA) which determines sequence of amino acids in protein, tRNA and rRNA bring amino acids to ribosome
how do proteins differ from each other
sequence of amino acids and proteins determine structure/function of cells and physical characteristics of phenotype of organism
what is a gene
a segment of the DNA that serves as a template for the production of an RNA molecule,
some include instructions for the formation of DNA molecules (mRNA, tRNA, rRNA)
what is the first step of protein synthesis
transcription to form mRNA
what is the purpose of messenger RNA
carry genetic information form the DNA to the ribosome for protein synthesis
what is a promoter
region of DNA that contains a special sequence of nucleotides
dsecribe transcription
- enzyme RNA polymerase binds tightly to a promoter and opens up the DNA helix in front
- RNA polemerase inserts RNA nucleotides and mRNA molecule results
- U,A,C,G now present in DNA template
describe the processing of mRNA
- primary mRNA one end modified by addition of cap made of (altered guanine nucleotide) and other end modified by addition of a poly-A tail (adenosine nucleotides)
- introns removed, exons joined to form a mature mRNA molecule
why is the processing of mRNA important
- after mRNA transcribed, must be processed before entering the cytoplasm
- primary RNA becomes mature RNA after processing
what is an intron and exon
introns: portions of DNA not part of the gene
exons: portion of gene that is expressed
what is a codon and degenerate
each triplet of nucleotides
degenerate: most amino acids are coded for by more than one codon
what are the functions of tRNA
-bring amino acids to the ribosomes
describe the function of tRNA
single stranded polynucleotide that doubles back on itslef such that complementary base pairing creates a book like shape
-one end is amino acid, other end is anticodon
what is an anticodon
triplet of three bases complementary to a codon of mRNA
what is the wobble effect and how does it effect
wobble effect: some tRNA the third nucleotide may vary
-believed to provide additional degeneracy that prevents mutations that could alter amino acid sequence of a protein
what are ribosomes
small sturctural bodies found in the cytoplasm on the endoplasmic reticulum where translation occurs
-composed of proteins and rRNA
where do the rRNA live
rRNA produced in a nucleolus within the nucleus
- then rRNA joins with proteins manufactured from the cytoplasm to form two subunits (large and small)
- subunits leave nucleus and join in cytoplasm to form ribosome just as protein synthesis begins
how are polyribosomes formed
is the entire complex or translation process
why is the order of codons important
determines order of the tRNA molecules at a ribosome and sequence of amino acid of a polypeptide
describe translation
- intiation: initiation factor proteins assemble small ribosomal subunit, mRNA, intiator tRNA, and large ribosomal subunit for start of protein synthesis
- elongation: protein synthesis step in which a polypeptide increases length one amino acid at a time,requires elongation factors
- termination: components of ps separated from one another
- termination occurs at stop codon, requires release factor protein which cleaves polypeptide from last tRNA
- polypeptide is free and is in 3D shape
- ribosome dissociates into its two subunits
describe the three binding sites of a riboosme for tRNA
- peptide site: initiator tRNA binds to P site
- amino acid site: tRNA carries next amino acid
- exit site
describe the steps of elongation
- tRNA with attached peptide is at P site, tRNA carrying next amino acid in the chain is arriving at the A site
- once next tRNA is at A site, peptide chain is transferred to this tRNA
- energy and part of ribosomal subunit needed for this transfer. poptide one amino acid longer by adding the peptide from the A site
- translocation: mRNA moves forward one codon length and peptide bearing tRNA now exits, new codon at A site and ready to receive the next tRNA
