Module 2 - Nucleic Acid

Question 1

What are nucleotides

Answer 1

Biological molecules that participate in all biochemical processes, made of a phosphate group linked to C5 or C3 of the pentose sugar attached to a base

Monomers that form nucleic acid, DNA and RNA

Question 2

What are phosphorylated nucleotides?

Answer 2

Nucleotides that contain more than one phosphate group; eg

ADP (adenosine diphosphate) and ATP ( adenosine triphospate)

Question 3

Where is DNA found

Answer 3

DNA is found in the nuclei of all eukaryotic cells, within the cytoplasm of prokaryotes

Question 4

Explain the structure of DNA

Answer 4

Polymer made of repeating monomeric units called nucleotides
Consists of two polynucleotide strands (antiparallel)
One of four nitrogenous bases: adenine, thymine, cytosine and guanine
Bond between phosphate and sugar is a phosphodiester bond (condensation reaction)

Question 5

Name two purines I

Answer 5

Adenine and guanine ( two rings)

Question 6

Name two pyramidines

Answer 6

Thymine and cytosine (one ring)

Question 7

Explain hydrogen bonds in reference to the four bases

Answer 7

Pyramidines are always paired with purines, so Adenine is with Thymine and Cytosine with Guanine
Adenine and Thymine pair by means of 2 hydrogen bonds
Guanine and Cytosine pair by means of 3 hydrogen bonds

Question 8

What shape is DNA in?

Answer 8

A double Helix

Question 9

How does the sugar phosphate backbone form?

Answer 9

The 5’ end of the molecule is where the phosphate group is attached to the fifth carbon of the pentose sugar
The 3’ end of the molecule is where the phosphate group is attached to another phosphate from the nucleotide below

Question 10

How is DNA organised within cells

Answer 10

Eukaryotes: DNA content (genome) is in nucleus
DNA tightly wound around special histone proteins into chromosomes

Prokaryotes: DNA in loops within cytoplasm
Described as naked

Question 11

What is semi conservative replication?

Answer 11

To make a new copy:
Molecules unwind - double helix untwists, catalysed by gyrase enzyme
Molecules unzip - hydrogen bonds between bases are broken, catalysed by DNA Helicase, resulting in exposed bases and two dna strands

Free phosphorylated nucleotides bond to bases (complementary)
DNA polymerase catalyses new bases in 5’ to 3’ direction using unzipped dna as template

Question 12

What are the leading strand and lagging strand?

Answer 12

Leading strand is in the 5’ to 3’ direction so is continuously synthesised
Lagging strand is in the 3’ to 5’ directions so cannot be continuous but is fragmented (discontinuous), later joined catalysed by ligase enzyme

Question 13

Why is it called semi conservative replication?

Answer 13

This is because each molecule contains one old strand and one new strand, they are identical to their parent and to each other

Question 14

What are mutations?

Answer 14

Errors that are made during replication eg wrong nucleotide is inserted
This can change the genetic code (point mutation)
Not all are harmful and some give advantages like helping animals hide in winter (changing fur to white like snow)

Question 15

How does the body prevent mutations occurring

Answer 15

There are enzymes that proof read and edit out incorrect nucleotides, reducing the rate of mutations being produced

Question 16

How is RNA different from DNA structurally?

Answer 16

Sugar molecule is ribose 
Uracil replaces thymine 
Polynucleiotide is single stranded
Shorter chain
Three forms of RNA - messenger (mRNA), transfer (tRNA) and ribosomal (rRNA)

Question 17

What are genes?

Answer 17

Genes are proteins that contain codes which determine the sequence of amino acids in a particular polypeptide/protein

Question 18

What are the structures of genes?

Answer 18

Sequence of DNA base triplets that determine amino acid sequence (primary structure) so it folds correctly
Instructions in genes cannot pass out the nucleus thus must be transcribed into a length of mRNA - codons (base triplets) pass out nucleus to ribosomes ensuring the instructions are translated and the protein is assembled properly

Question 19

Describe the nature of the genetic code

Answer 19

Near universal as all living things have the same triplet copse of dna bases codes for the same amino acid
Described as degenerate, as for many there are more than one base triplet reducing point mutations
Described as non overlapping, read starting from a fixed point and bases added or deleted causes a shift changing the amino acid sequence

Question 20

Transcription n Translation

Answer 20

Transc-

• gene unzips
• hydrogen bonds between complementary nucleotide bases break
• RNA Polymerase catalysed formation of temporary hydrogen bonds between RNA nucleotides and their complementary unpaired DNA bases
• A and T, C and G, U and A
• length of RNA complementary to the template strand is produced
• mRNA passes out of nucleus through the nuclear envelope and attaches to a ribosome

Question 21

What is tRNA?

Answer 21

• tRNA molecules are made in nucleus
• single stranded polynucleotides twisted into a hairpin shape
• has anticodons on one end and amino acids on the other
• found in the cytoplasm

Question 22

Translation?

Answer 22

• tRNA molecule brings amino acids and finds its place when the anticodon bonds by temporary hydrogen bonds to the complementary codon on the mRNA molecule
• as ribosome moves along length of mRNA, reads code and when two amino acids are beside each-other a peptide bond forms between them
• ATP is needed
• after polypeptide has been made, mRNA breaks down, molecules can b recycled into new lengths of mRNA with different codon sequences
• new polypeptide is helped by chaperone proteins in the cell to fold into 3D shape