2.1.3 Nucleic Acids

Question 1

Explain the process of DNA replication

Answer 1

1. DNA molecule unwinds - catalysed by GYRASE enzyme
2. unzips- bonds between nucleotide bases are broken. catalysed by DNA HELICASE.
3. free phosphorylated nucleotides are bonded to the exposed bases via complementary base pairing
4. DNA POLYMERASE joins the nucleotides by catalysing formation of phosphodiester bonds between them, in 5’ to 3’ direction.
5. LEADING STRAND: synthesised continuously
   LAGGING STRAND: in fragments and then these are joined. DNA LIGASE catalyses phosphodiester bond formation between these okazaki fragments

Question 2

What is tRNA? Where’s it made, structure, function.

Answer 2

• made in nucleolus
• smaller than mRNA and DNA
  -single stranded polynucleotides that twists into clover shape
  -one end has trio of nucleotide unpaired bases that attaches to a specific amino acid
  -opp end hasanticodon complementary to specific codon on mRNA.

Question 3

What is the structure of RNA?

Answer 3

ribose sugar, nitrogenous base uracil instead of thymine
polynucleotide chain is usually single stranded and shorter
3 forms: mRNA, tRNA, rRNA

Question 4

What is the structure of DNA?

Answer 4

polymer made of nucleotides
2 polynucleotide strands that are antiparallel, twisted into double helix. with phosphodiester bonds
each DNA nucleotide consists of a phosphate group, deoxyribose, and one of the four nitrogenous bases adenine, guanine, cytosine, thymine.
long: carry lots of encoded genetic informartion

Question 5

What is the structure of ADP?

Answer 5

• pentose sugar ( ribose), nitrogenous base (adenine) and 2 inorganic phosphates

Question 6

What is the structure and function of ATP?

Answer 6

pentose sugar (ribose), nitrogenous base (adenine) and inorganic phosphates

function:
used in most energy-requiring metabolic processes

Question 7

What is the product of DNA replication?

Answer 7

2 DNA molecules identical. Each contains 1 old strand and 1 new strand = semi- conservative replication

Question 8

What is the importance of hydrogen bonding in DNA?

Answer 8

between nitrogenous bases
complementary base pairing: A pairs with T via 2 H BONDS, C pairs with G via 3 H BONDS.
uniform width across molecule due to pairing of purines with pyrimidines and twists into double helix.
large no. of H bonds = stability, but can be broken for transcription and replication.

Question 9

What is the importance of DNA replication?

Answer 9

so that each new daughter cell receives the same full set of instructions
DNA in chloroplasts and mitochondria also replicates

Question 10

What is DNA?

Answer 10

• a nucleic acid macromolecule (very large polynucleotide)
  -deoxyribonucleic acid

Question 11

what is dispersive replication?

Answer 11

• molecules are a mix of old and new - original molecule breaks up into nucleotides

Question 12

What is conservative replication?

Answer 12

• original molecule acts as template and a new molecule is made with new nucleotides ( 2 new strands in one molecule)

Question 13

what is adenosine?

Answer 13

• nucleoside: sugar and base

Question 14

What helps ribosomes to bind together? What are ribosomes made of?

Answer 14

mag ions help bind two ribosome subunits
rRNA and protein

Question 15

Explain the process of transcription

Answer 15

1. Gene unwinds and unzips. H bonds between nucleotide bases break. RNA helicase
2. Formation of temporary hydrogen bonds between free RNA nucleotides and their complementary unpaired DNA bases on the template strand occurs.
3. RNA polymerase catalyses the joining of RNA nucleotides by forming phosphodiester bonds
4. A length of complementary RNA to the template strand is produced. Copy of DNA coding strand.
5. Introns are removed from this mRNA, to leave only exons
6. it pases out of nucleus via nuclear envelope and attaches to ribosome.

Question 16

Explain the process of translation

Answer 16

RNA is attached to ribosome in cytoplasm or RER
tRNA molecules bring amino acids and find their place when its anticodon binds by temporary H bonds to the complementary codon on the mRNA
2 adjacent amino acids = peptide bond formed
a polypeptide assembled. mRNA breaks down and its component molecules can be recycled.
newly synthesised protein is helped by chaperone proteins to begin folding

Question 17

What evidence is there for semi-conservative replication?

Answer 17

Grew E. coli in medium containing a “heavy” isotope of nitrogen (N15), used it to synthesize DNA. Gen 0 = only N15 (one single heavy band)

Then after Gen 0, the bacteria were switched to medium containing a “light” N14 isotope and allowed to grow for several generations. They then measured the density of the DNA.

Generation 1
Higher single band- intermediate in density between the heavy and light. DNA molecules was a hybrid of light and heavy DNA. Doesnt fit with the conservative model.

Generation 2
Showed DNA was being replicated semi-conservatively. The pattern of two distinct bands—one at the position of a hybrid molecule and one at the position of a light molecule. in dispersive replication, it would be impossible to get a “purely light” molecule.

Generations 3 and 4
over the third and fourth generations, we’d expect the hybrid band to become progressively fainter and the light band to become progressively stronger

Question 18

What does translation require?

Answer 18

ATP

Question 19

What does it mean by the antiparallel sugar phosphate backbone?

Answer 19

‘opp directions’ of 2 strands = direction that C3 or C5 are facing.
one end: C5 of pentose is nearest to end = 5’ end
other end: C3 of pentose is nearest to end= 3’ end

Question 20

What does each gene contain?

Answer 20

sequence of DNA base triplets that determines the sequence (primary structure) of amino acids in a particular polypeptide

Question 21

what do they become when they have more than one phosphate group?

Answer 21

PHOSPHORYLATED nucleotides
for example, ADP ( adenosine diphosphate)

Question 22

What are some features of the genetic code?

Answer 22

1. near universal: in almost all living organisms the same triplet of DNA bases code for the same amino acid
2. degenerate: for all amino acids except tryptophan and methionine, there is more than one base triplet that can code for it -reduces effect of point mutations
3. non overlapping + read starting from a fixed point in groups of 3 bases, with no bases being part of more than one triplet.

Question 23

what are purines and pyrimidines?

Answer 23

purine: two rings, A or G
pyrimidines: one ring, T or C or U

Question 24

What are nucleotides? (structure)

Answer 24

• nitrogenous base linked to C1 of sugar, and a phosphate group linked to C5 or C3 by covalent bonds
• monomers of nucleic acids

Question 25

What are nucleotides used for?

Answer 25

help regulate many metabolic pathways e.g. ATP, ADP and AMP

may be components of many co-enzymes - adenine nucleotides = components of NADP which is used in photosynthesis and NAD involved in respiration.

Question 26

How is DNA purified?

Answer 26

Macerating tissue to get individual cells and break cell walls etc in plants
Adding strong detergent (including salts too) to break down phospholipid bilayer and act as an emulsifier
Add protease enzyme= breaks down proteins associated with DNA
Add ethanol- DNA precipitates

Question 27

How is DNA organised in cells?

Answer 27

eukaryotic: genome in nucleus. each DNA molecule is wound around histone proteins into chromosomes. loop of DNA without proteins in mitochondria and chloroplasts too
prokaryotic: loop in cytoplasm without proteins. also have plasmid.
- viruses that contain DNA also have it as a loop