2.3 NUCLEIC ACIDS

Question 1

General structure of a nucleotide

Answer 1

• a pentose sugar
• a nitrogenous base
• a phosphate group

Question 2

General structure of DNA nucleotide

Answer 2

• a deoxyribose (pentose) sugar
• a nitrogenous base (adenine, cytosine, guanine, thymine)
• phosphate group

Question 3

General structure of RNA nucleotide

Answer 3

• a ribose (pentose) sugar
• a nitrogenous base (adenine, cytosine, guanine, uracil)
• phosphate group

Question 4

State the ring structure of a purine and state the two purine bases.

Answer 4

• 2 carbon rings
• adenine and guanine

Question 5

State the ring structure of a pyrimadine and state the three pyrimadine bases.

Answer 5

• 1 carbon ring
• cytosine, thymine or uracil

Question 6

Describe the bonding between guanine and cytosine

Answer 6

• weak TRIPLE hydrogen bond

Question 7

Describe the bonding between adenine and thymine/uracil

Answer 7

• weak DOUBLE hydrogen bond

Question 8

How are polynucleotides formed from nucleotides?

Answer 8

• condensation reaction (water removal)
• forms phosphodiester bonds between phosphate group and hydroxl group of next pentose sugar

Question 9

Describe the structure of DNA (3)

Answer 9

• two antiparallel strands (5’ to 3’ and 3’ to 5’)
• double helix held by weak hydrogen bonds via complementary base pairing
• sugar phosphate backbone

Question 10

Describe the structure of RNA (2)

Answer 10

• single stranded
• shorter

Question 11

How is DNA precipitated and purified?

Answer 11

1) Crush sample to break down cell walls (plants only)
2) Add detergent to dissolve membranes
2) Add salt to break weak hydrogen bonds between DNA double helix
3) Add protease enzyme to break down proteins associated with the DNA (histone)
4) Filter to remove large debris
5) Add ice cold ethanol to precipitate out the DNA as white strands

Question 12

Define “phosphorylated nucleotide”

Answer 12

• Nucleotide with one or more phosphate groups added to it

Question 13

Describe the structure of ADP

Answer 13

• ribose (pentose) sugar
• adenine nitrogenous base
• 2 phosphate groups

Question 14

Describe the structure of ATP

Answer 14

• ribose (pentose) sugar
• adenine nitrogenous base
• 3 phosphate groups

Question 15

Why is DNA replication “semi-conservative”?

Answer 15

Because the resultant two identicle DNA molecules each have one old strand and one new strand

Question 16

State and explain the enzymes used in semi-conservative DNA replication.

Answer 16

• DNA helicase (unzips DNA strands by breaking hydrogen bonds between nucleotide bases)
• DNA polymerase (catalyses the addition of new nucleotides in the 5’ to 3’ direction)

Question 17

State the steps of semi-conservative DNA replication.

Answer 17

1) DNA double helix unwinds
2) DNA helicase causes the two strands to unzip by breaking the hydrogen bonds between the nucleotide bases to leave exposed bases
3) DNA polymerase catalyses the addition of new phosphorylated nucleotides using complementary base pairing in the 5’ to 3’ direction
4) the leading strand is synthesised continuously where is the lagging strand is synthesised in fragments
5) hydrolysis of the phosphorylated nucleotides supplies the enrgy required to form phosphodiester bonds between each new nucleotide

Question 18

What does “non-overlapping” mean for the genetic code?

Answer 18

• Each base is only part of one triplet base/codon so each triplet base/codon is read separately

Question 19

What does “degenerate” mean for the genetic code?

Answer 19

• the same amino acids are coded for by more than one triplet base

Question 20

What does “universal” mean for the genetic code?

Answer 20

• the same triplet bases code for the same amino acids in all organisms

Question 21

State the three types of RNA

Answer 21

• mRNA (messenger)
• rRNA (ribosomal)
• tRNA (transfer)

Question 22

Function of mRNA?

Answer 22

• carriers the complementary genetic code of DNA from nucleus to ribosome for protein synthesis as DNA is too big

Question 23

Function of rRNA?

Answer 23

• forms the two subunits of ribosomes and helps catalyse the formation of peptide bonds between amino acids

Question 24

Function of tRNA?

Answer 24

• consists of an amino acid binding site on one end and carries an anticodon on other end
• carries the amino acid specific to the anticodon from cytoplasm to ribosome

Question 25

What is DNA transcription?

Answer 25

• Process of transcribing a DNA strand into mRNA in the nucleus

Question 26

State and explain the enzymes used in DNA transcription.

Answer 26

• DNA helicase (unzips two strands by breaks hydrogen bonds between nucleotide bases)
• RNA Polymerase (catalyses the formation of temporary hydrogen bonds between RNA nucleotides and their complementary DNA base)

Question 27

State the steps of DNA transcription

Answer 27

1) in the nucleus, a gene unwinds and unzips
2) DNA helicase breaks hydrogen bonds between nucleotide bases of the two strands
3) RNA polymerase catalyses the formation of temporary hydrogen bonds between RNA nucleotides and their complementary, unpaired DNA base
4) DNA strand is considered the template strand
5) the length of mRNA produced in known as the coding strand and leaves the nucleus through the nuclear envelope pore

Question 28

What is DNA translation?

Answer 28

• Process of assembling amino acids into a particular sequence based on mRNA instructions

Question 29

State the steps of DNA translation

Answer 29

1) mRNA attaches to the ribosome
2) tRNA molecules bring over amino acids specific to the place their anticodon binds to on the mRNA strand with temporary hydrogen bonds
3) A start codon begins the sequence
4) As the ribosome moves along the length of mRNA and rRNA catalyses the formation of covalent peptide bobds when two amino acids are adjacent to each other
5) Once the stop codon is reached the polypeptide chain ends and begins its primary protein structure