4.3 Gene Expression

Question 1

what is gene expression

Answer 1

process of using a gene sequence to synthesize a protein, relies on RNA (ribonnucleic acid)
messenger RNA mRNA, ribosomal RNA rRNA, transfer tRNA

Question 2

compare the sugar, bases, strands, and helix of DNA and RNA

Answer 2

DNA: deoxyribose/ adenine, guanine, thymine, cytosine/ double stranded/ yes helix
RNA: ribose/ adenine, guanine, uracil, cytosine/ single stranded/ no helix

Question 3

describe the general processes of gene expression

Answer 3

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

Question 4

how do proteins differ from each other

Answer 4

sequence of amino acids and proteins determine structure/function of cells and physical characteristics of phenotype of organism

Question 5

what is a gene

Answer 5

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)

Question 6

what is the first step of protein synthesis

Answer 6

transcription to form mRNA

Question 7

what is the purpose of messenger RNA

Answer 7

carry genetic information form the DNA to the ribosome for protein synthesis

Question 8

what is a promoter

Answer 8

region of DNA that contains a special sequence of nucleotides

Question 9

dsecribe transcription

Answer 9

• 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

Question 10

describe the processing of mRNA

Answer 10

1. 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)
2. introns removed, exons joined to form a mature mRNA molecule

Question 11

why is the processing of mRNA important

Answer 11

• after mRNA transcribed, must be processed before entering the cytoplasm
• primary RNA becomes mature RNA after processing

Question 12

what is an intron and exon

Answer 12

introns: portions of DNA not part of the gene
exons: portion of gene that is expressed

Question 13

what is a codon and degenerate

Answer 13

each triplet of nucleotides

degenerate: most amino acids are coded for by more than one codon

Question 14

what are the functions of tRNA

Answer 14

-bring amino acids to the ribosomes

Question 15

describe the function of tRNA

Answer 15

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

Question 16

what is an anticodon

Answer 16

triplet of three bases complementary to a codon of mRNA

Question 17

what is the wobble effect and how does it effect

Answer 17

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

Question 18

what are ribosomes

Answer 18

small sturctural bodies found in the cytoplasm on the endoplasmic reticulum where translation occurs
-composed of proteins and rRNA

Question 19

where do the rRNA live

Answer 19

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

Question 20

how are polyribosomes formed

Answer 20

is the entire complex or translation process

Question 21

why is the order of codons important

Answer 21

determines order of the tRNA molecules at a ribosome and sequence of amino acid of a polypeptide

Question 22

describe translation

Answer 22

1. intiation: initiation factor proteins assemble small ribosomal subunit, mRNA, intiator tRNA, and large ribosomal subunit for start of protein synthesis
2. elongation: protein synthesis step in which a polypeptide increases length one amino acid at a time,requires elongation factors
3. 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

Question 23

describe the three binding sites of a riboosme for tRNA

Answer 23

• peptide site: initiator tRNA binds to P site
• amino acid site: tRNA carries next amino acid
• exit site

Question 24

describe the steps of elongation

Answer 24

1. tRNA with attached peptide is at P site, tRNA carrying next amino acid in the chain is arriving at the A site
2. once next tRNA is at A site, peptide chain is transferred to this tRNA
3. energy and part of ribosomal subunit needed for this transfer. poptide one amino acid longer by adding the peptide from the A site
4. 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