2. Nucleic acids

Question 1

Which 2 scientists were able to provide the best/most reliable evidence to support Watson and Crick’s hypothesis for the semi-conservative replication of DNA?

Answer 1

Meselson and Stahl

Question 2

What are the 3 theoretical possible methods of DNA replication?

Answer 2

semi-conservative
conservative
dispersive

Question 3

name the 2 types of nucleic acids

Answer 3

RNA

DNA

Question 4

considering the structure of DNA what is it classed as?

Answer 4

a polymer (polynucleotide)

Question 5

what are individual units of DNA known as

Answer 5

nucleotides

Question 6

A typical simple diagram of a DNA nucleotide consists of a circle, a pentagon and a rectangle. Name what these shapes represent

Answer 6

Phosphate
Pentose sugar (deoxyribose)
hydrogenous base

Question 7

name the 2 base pairing rules

Answer 7

A to T (2 hydrogen bonds)

C to G (3 hydrogen bonds)

Question 8

why is DNA considered as molecular genetic code?

Answer 8

It is a hereditary information-carrying molecule

Question 9

why was it theorized that a minimum of 3 bases is needed to code for an amino acid?

Answer 9

due to their only being 4 bases and pairs would not produce the range of amino acids that we know of.

Question 10

list the features of genetic code

Answer 10

degenerate code, amino acids can be coded for by more than one triplet
triplets always read the same way
have start and stop codons in DNA sequence
each sequence is read only once
universal code, all organisms use the same sequence to code for specific amino acids

Question 11

What information do genes carry?

Answer 11

the information to produce the amino acid sequence of polypeptides

Question 12

regarding chromosomes, describe the contents of a eukaryote cell?

Answer 12

each normal cell contains 2 copies of each type of chromosome (homologous pairs)

Question 13

How many chromosomes do humans have

Answer 13

46 chromosomes (diploid number)
i.e 23 homologous pairs

Question 14

How much of a cells content is made up of water?

Answer 14

80%

Question 15

What is a metabolic reaction?

Answer 15

a chemical reaction that happens in a living organism to keep it alive.

Question 16

What is a metabolite?

Answer 16

A substance involved in a metabolic reaction

Question 17

Water is an important metabolite, why?

Answer 17

Many metabolic reactions involve a condensation or hydrolysis reaction.
A hydrolysis reaction requires a molecule of water to break a bond.
A condensation reaction releases a molecule of water as a new bond is formed

Question 18

Why is it essential that water is a solvent?

Answer 18

substances dissolve in it.

most metabolic reactions take place in a solution, e.g. cytoplasm of eukaryotic and prokaryotic cells

Question 19

water has a high latent heat of vaporisation, why is this useful?

Answer 19

It takes a lot of energy to break the hydrogen bonds between water molecules
So water has a high latent heat of vaporisation - a lot of energy is used up when water evaporates.
This is useful for living organisms because it means they can use water loss through evaporation to cool down without losing too much water

Question 20

Describe the structure of a water molecule.

Answer 20

One atom of oxygen joined to two atoms of hydrogen by shared electrons

Question 21

How is water a polar molecule?

Answer 21

Because the shared negative hydrogen electrons are pulled towards the oxygen atom, the other side of each hydrogen atom is left with a slight positive charge
The unshared negative electrons on the oxygen atom give it a slight negative charge
This makes water a polar molecule - it has a partial negative charge on one side and a partial positive charge on the other.

Question 22

water can buffer (resist) changes in temperature, why is this useful?

Answer 22

the hydrogen bonds between water molecules can absorb a lot of energy
so water has a high specific heat capacity - it takes a lot of energy to heat it up

Question 23

Why is water’s high specific heat capacity useful for living organisms?

Answer 23

this is useful for living organisms because it means that water doesn’t experience rapid temperature changes.
this makes water a good habitat because the temperature under water is likely to be more stable than on land.
the water inside organisms also remains at a fairly stable temperature - helping them to maintain a constant internal body temperature.

Question 24

Water is a good solvent, why is this useful?

Answer 24

a lot of important substances in metabolic reactions are ionic.
because water is polar, the positive end of a water molecule will be attracted to the negative ion, and the negative end of a water molecule will be attracted to the positive ion.
This means the ions will get totally surrounded by water molecules - in other words, they’ll dissolve
so water’s polarity makes it a useful solvent

Question 25

There is strong cohesion between water molecules, why is this useful?

Answer 25

Cohesion is the attraction between molecules of the same type.
water molecules are very cohesive, they tend to stick together, this is because they’re polar.
Strong cohesion helps water flow, making it great for transporting substances.
Strong cohesion also means that water has a high surface tension when it comes into contact with air. This is the reason why sweat forms droplets, which evaporate from the skin to cool an organism down.

Question 26

Name some examples of cohesion tension theory at work in the natural world.

Answer 26

Water traveling up the xylem in a plant

Pond skaters and other insects being able to ‘walk’ on the surface of a pond.