3 - NUCLEIC ACIDS

Question 1

What is a nucleotide?

Answer 1

A biological molecule containing a pentose sugar, phosphate group and a nitrogenous base

Question 2

How do nucleotides act as monomers?

Answer 2

They covalently bond together by condensation to form polynucleotide chains such as DNA and RNA

Question 3

What happens when nucleotides become further phosphorylated?

Answer 3

They form molecules such as ADP that contains 2 phosphate groups and ATP that contains 3 phosphate groups which then help regulate many metabollic pathways

Question 4

Describe a DNA nucleotide.

Answer 4

• sugar = deoxyribose
• bases = A T C G
• number of polynucleotide strands = 2
• length of polynucleotide chain = long
• different forms = none
• location in eukaryotic cells = nucleus

Question 5

Describe the structure of a DNA molecule.

Answer 5

Anti-parallel polynucleotide strands, one is 5’ and the other is 3’, that are twisted around eachother forming a double helix shape

Question 6

What are the names of the 4 organic bases and how do they pair up?

Answer 6

A = adenine pairs with T = thymine
C = cytosine pairs with G = guanine

Question 7

What type of bond forms is found between these base pairs?

Answer 7

Hydrogen bonds - 3 between cytosine and guanine and 2 between adenine and thymine

Question 8

What is this process called and why is it important?

Answer 8

Complemetary base pairing; it allows DNA to be replicated without error and reduces the occurence of mutation

Question 9

What are purine bases?

Answer 9

Bases with 2 rings such as adenine and guanine

Question 10

What are pyrimidine bases?

Answer 10

Based with 1 ring such as thymine, U = uracil and cytosine

Question 11

What role does the pairing of purine and pyrimidine bases play in the structure of DNA?

Answer 11

Equal sized rungs are formed between the rings which enable the strands to twist around eachother into the double helix, giving stability

Question 12

What is a phosphodiester bond?

Answer 12

Covalent bond formed by condensation between:

• the sugar and the phosphate group in a nucleotide
• nucleotide monomers in DNA strands

Question 13

What is the importance of the sugar-phosphate backbone?

Answer 13

It maintains the molecule’s stability and ensures that the integrity of the coded information within the base sequences is protected

Question 14

When is DNA replicated? Why?

Answer 14

During the synthesis stage of the cell cycle, in order to make new cells for growth and repair (mitosis) and to make sex cells/ gametes (meiosis)

Question 15

What is the first step in DNA replication?

Answer 15

DNA helicase, which breaks hydrogen bonds between bases in a parent DNA molecule and unwinds the strands so they act as templates

Question 16

What is the second stage of DNA replication?

Answer 16

Free nucleotides with complementary bases are attracted to the exposed bases and DNA polymerase forms hydrogen bonds between them

Question 17

What is the third stage of DNA replication?

Answer 17

DNA polymerase and DNA ligase join nucleotides to form the new strand by phosphodiester bonds and reform the sugar-phosphate backbone

Question 18

Why is this process known as semi-conservative replication?

Answer 18

Because it results in 2 new strands, each containing one parent strand from the original DNA molecule

Question 19

Describe an RNA nucleotide.

Answer 19

• sugar = ribose
• bases = A U C G
• number of polynucleotide strands = 1
• length of polynucleotide chain = short
• different forms: (3) messenger, transfer and ribosomal
• location in eukaryotic cells = nucleolus

Question 20

What is a gene and what is its role?

Answer 20

It is a section of a DNA molecule which provides the sequence of bases which determines the sequence of amino acids and thus a protein

Question 21

What is a base triplet?

Answer 21

A triplet of bases on a DNA molecule

Question 23

What is a codon?

Answer 23

A triplet of bases on a length of an mRNA molecule

Question 23

What is an anti-codon?

Answer 23

A triplet of bases on a tRNA molecule that is complementary to the codon of mRNA

Question 23

What does a tRNA molecule carry?

Answer 23

A base triplet attached to an amino acid on one end and an anticodon on another end

Question 25

What are the two stages of protein synthesis?

Answer 25

Transcription and translation

Question 26

Describe transcription in the nucleus.

Answer 26

• a gene unwinds and unzips so hydrogen bonds between bases break, thus acting as a template strand
• enzyme RNA polymerase catalyses the formation of temporary hydrogen bonds between the exposed DNA bases and free RNA nucleotides with complementary bases
• a length of messenger RNA is formed that acts as a copy of the other (non-template) strand
• mRNA leaves the nucleus through the nuclear pores and attaches to a ribosome

Question 27

How many possible triplet codes are there?

Answer 27

There are 4x4x4=64 possible triplet codes and only 20 amino acids

Question 27

Describe translation in the cytoplasm.

Answer 27

• the anticodon on the tRNA forms temporary hydrogen bonds with complementary bases on the mRNA and amino acids find their place
• the ribosome moves along the mRNA molecule and reads its code
• the ribosome then catalyses the formation of peptide bonds between adjacent amino acids using ATP and enzyme peptidyl transferase
• chaperone proteins help the polypeptide chain fold into its tertiary shape

Question 27

What happens to mRNA once the protein is synthesised?

Answer 27

It is recycled into new lengths of mRNA with different codon sequences

Question 28

Why is the genetic code described as degenerate?

Answer 28

Because for all amino acids there is more than one base triplet, which may reduce the effect of point mutations, as a change in one base of the triplet could produce another base triplet that still codes for the same amino acid