nucleic acids + DNA replication

Question 1

DNA

Answer 1

deoxyribonucleic acid - used to store genetic information - all the instruction an organism needs to grow and develop from a fertilised egg to a fully grown adult

Question 2

RNA

Answer 2

ribonucleic acid - one of its main functions is to transfer genetic information from the DNA to the ribosomes - ribosomes are the body’s protein factories - they read the RNA to make polypeptides in a process called translation
ribosomes are made from RNA and proteins

Question 3

what are DNA and RNA made of

Answer 3

polymers of nucleotides

Question 4

what is a nucleotide

Answer 4

a type of biological molecule which is made from pentose sugar, a nitrogen-containing organic base and a phosphate group

Question 5

structure of DNA

Answer 5

the pentose sugar in a DNA nucleotide is called deoxyribose
each DNA nucleotide has the same sugar and a phosphate group
the base on each nucleotide may vary
the four possible bases are adenine, thymine, cytocine and guanine

Question 6

structure of RNA

Answer 6

RNA contains nucleotides with a ribose sugar
also has a phosphate group and one of four different bases
adenine, cytocine and guanine are the same as DNA
has uracil instead of thymine

Question 7

how are nucleotides joined together

Answer 7

through a condensation reaction between the phosphate group of one nucleotide and the sugar of another
this forms a phosphodiester bon
the chain of sugars and phosphates is known as the sugar-phosphate backbone

Question 8

DNA and double helix structure

Answer 8

two DNA polynucleotide strands join together by hydrogen bonding between the bases
each base can only join with one particular partner - complementary base pairing

two antiparallel polynucleotide strands twist to form the DNA double-helix

Question 9

what binds with adenine

Answer 9

thymine (uracil in RNA)

Question 10

what binds with cytocine

Answer 10

guanine

Question 11

how many hydrogen bonds

Answer 11

two between A and T

three between C and G

Question 12

how does DNA replicate

Answer 12

semi-conservative replication

the enzyme DNA helicase breaks the hydrogen bonds between bases on the two polynucleotide DNA strands. This makes the helix unwind to form two single strands

Each original single strand acts as a template for a new strand. Complementary base pairing means that free-floating DNA nucleotides are attracted to their complementary exposed bases on each original template

Condensation reactions join the nucleotides of the new strands together - catalysed by the enzyme DNA polymerase. Hydrogen bonds form between the bases on the original and new strands

Each new DNA molecule contains one strand from the original DNA molecule and one new strand

Question 13

how does DNA polymerase move

Answer 13

in opposite ways along antiparallel DNA strands

each end of a DNA strand is slightly different in its structure
One end is called the 3’ (3 prime) end and one end is called the 5’ end
In a DNA helix, the strands run in opposite directions - they’re antiparallel

The active site of DNA polymerase is only complementary to the 3’ end of the newly formed DNA strand - so the enzymes can only add nucleotides to the new strand at the 3’ end causing the polymerase working on one of the template strands to move in the opposite direction to the DNA polymerase working on the other template strand

Question 14

who found evidence for semi-conservative replication

Answer 14

Meselson and Stahl

Question 15

evidence for semi-conservative replication

Answer 15

• two samples of bacteria were grown - one in a nutrient broth containing light nitrogen one and one in a nutrient broth containing heavy nitrogen - as the bacteria reproduced, they took up nitrogen from the both to help form the nucleotides, so the nitrogen gradually because part of the bacteria’s DNA
• a ample of DNA was taken from each batch of bacteria, and spun in a centrifuge - the DNA from the heavy nitrogen bacteria settled lower down in the centrifuge tube than the DNA from the light nitrogen - because its heavier
• the bacteria grown in the heavy nitrogen broth were taken out and put in a broth containing only light nitrogen - the bacteria were left for one round of DNA replication and then another DNA sample was taken out and spun in the centrifuge

if conservative - the heavy DNA would still be together
if semi-conservative - new bacteria would contain one strand of heavy nitrogen and one strand of light nitrogen

The DNA settled in the middle, showing that the DNA molecules contained a mixture of heavy and light nitrogen - the bacterial DNA had replicated semi-conservatively in the light nitrogen

Question 16

how is atp an immediate source of energy in a cell

Answer 16

plant and animal cells release energy from glucose - this process is called respiration

a cell can’t get its energy directly from glucose
so, in respiration, the energy released from glucose is used to make ATP (adenosine triphosphate)

once made, ATP diffuses to the part of the cell that needs energy
the energy in ATP is stored in high energy bonds between the phosphate groups
it’s released via hydrolysis reactions

Question 17

structure of ATP

Answer 17

made from the nucleotide base, adenine, combined with a ribose sugar and three phosphate groups
it’s known as a nucleotide derivative because it’s a modified form of a nucleotide

Question 18

hydrolysis of ATP

Answer 18

when energy is needed by a cell, ATP is broken down into ADP (adenine diphosphate) and Pi (inorganic phosphate)

this is a hydrolysis reaction - a phosphate bond is broken and energy is release - the reaction is catalysed by the enzyme ATP hydrolase

ATP hydrolysis can be ‘coupled’ to other energy-requiring reactions in the cell so energy can be used directly to make the coupled reactions occur, rather than being lost as heat

Question 19

inorganic phosphate

Answer 19

can be added to another compound (phosphorylation), which often makes the compound more reaction

Question 20

ATP re-synthesis

Answer 20

by a condensation reaction between ADP and Pi

this happens during both respiration and photosynthesis, and is catalysed by the enzyme ATP synthase

Question 21

how can a genetic code be degenerate

Answer 21

there are more possible combinations of triplets than there are amino acids

this means that some amino acids are coded for by more than one base triplet