8- DNA, genes and protein synthesis

Question 1

what is a gene

Answer 1

section of DNA on a chromosome that codes for one or more polypeptides

Question 2

why is the genetic code described as degenerate

Answer 2

most amino acids are coded for by more than one triplet

Question 3

why is the genetic code described as non-overlapping

Answer 3

each base sequence is only read once

Question 4

why is the genetic code described as universal

Answer 4

exists in all organisms

Question 5

what are exons

Answer 5

regions of the gene which are coding

Question 6

what are introns

Answer 6

non-coding sequences

Question 7

structure of chromosome

Answer 7

DNA combine with histones. DNA-histone complex is coiled. Coils fold to form loops. Loops coil and pack together to form the chromosome

Question 8

what are homologous pairs of chromosomes

Answer 8

a pair of chromosomes, one maternal and one paternal that have the same gene loci and therefore determine the same features

Question 9

what is an allele

Answer 9

one of a number of alternative forms of a gene

Question 10

what happens to alleles when there is a mutation

Answer 10

any changes in the base sequence of a gene produces a new allele of that gene and results in a different sequence of amino acids being coded for leading to the production of a different polypeptide

Question 11

genome

Answer 11

the complete set of genes in a cell

Question 12

proteome

Answer 12

full range of proteins produced by the genome

Question 13

structure of RNA

Answer 13

• pentose sugar ribose
• organic base
• phosphate group

Question 14

mRNA

Answer 14

single polynucleotide chain
single helix shape
molecule is smaller than DNA but larger than tRNA
pentose sugar is ribose
organic bases are adenine guanine cytosine and uracil
manufactured in the nucleus but found throughout the cell
less stable than DNA or tRNA

Question 15

tRNA

Answer 15

single polynucleotide chain
clover shape
smallest molecule of the three
pentose sugar is ribose
organic bases are adenine guanine cytosine and uracil
manufactured in the nucleus but found throughout the cell
more stable than mRNA but less than DNA

Question 16

Transcription

Answer 16

Occurs in the nucleus
1. DNA helix unwinds to expose the bases to act as a template
2. only one strand of DNA acts as a template
3.unwinding is catalysed by DNA helices which breaks the hydrogen bonds between the complementary bases
4. free mRNA nucleotides in the nucleus align opposite exposed complementary DNA bases
5. RNA polymerase bonds together the nucleotides to form a new RNA chain which is pre-mRNA
6. when RNA polymerase reaches a stop triplet it detaches

Question 17

why does transcription have to occur instead of DNA leaving the nucleus

Answer 17

DNA is too large to leave the nucleus through the nuclear pores
enzymes in cytoplasm could damage DNA

Question 18

splicing of pre-mRNA

Answer 18

introns are non-coding sections of DNA
they are spliced out by splicesome

Question 19

translation

Answer 19

1. modified mRNA leaves the nucleus through nuclear pores and attaches to a ribosome in the cytoplasm
2. the ribosome attaches at the start codon
3. the tRNA molecule with the complementary anticodon to the start codon will align opposite the mRNA held in place by the ribosome
4. the ribosome will move along the mRNA molecule to allow other complementary tRNA to attach to the next codon on the mRNA
5. the two amino acids that have been delivered by the tRNA molecule are joined by a peptide bond. This is catalysed by an enzyme and requires ATP
6. this continues until the ribosome meets the stop codon at the end of the mRNA molecule. The stop codon does not code for an amino acid and therefore the ribosome detaches
7. polypeptide is now created and enters the golgi apparatus for modification and folding

Question 20

what is a codon

Answer 20

a sequence of 3 adjacent nucleotides in mRNA that code for an amino acid