Topic 2B - Proteins And Genetics

Front 1

What is a mononucleotide?

Back 1

A mononucleotide is a type of biological molecule. It's made from a pentose sugar (has 5 carbon atoms), a nitrogen containing organic base and a phosphate group.

Front 2

What is the sugar in DNA?

Back 2

The pentose sugar in a DNA mononucleotide is called deoxyribose. Each DNA mononucleotide has the same sugar and phosphate group. The base cam vary though. The four bases are Adenine, Thymine, Cytosine and Guanine.

Front 3

What is the sugar in RNA?

Back 3

RNA contains mononucleotides with a ribose sugar. It has a phosphate group and one of four different bases. These bases are Adenine, Uracil, Cytosine and Guanine.

Front 4

How are polynucleotides formed from mononucleotides?

Back 4

A polynucleotide is a polymer of mononucleotides. Both DNA and RNA mononucleotides form polynucleotides. They are joined through condensation reactions between the phosphate of one mononucleotide and the sugar group of another. DNA is made of two polynucleotide strands, RNA has just one.

Front 5

What is the structure of DNA?

Back 5

Two DNA polynucleotide strands bond together by hydrogen bonding between bases. Each base has a complementary base partner. Adenine pairs with Thymine and Cytosine bonds with Guanine. Two polynucleotide strands twist to form the DNA double helix.

Front 6

How is the genetic code non-overlapping?

Back 6

In the genetic code, each base triplet is read in sequence, separate from the triplet before it and after it. The base triplets don't share their bases, thus it is non-overlapping.

Front 7

How is the genetic code degenerate?

Back 7

There are more possible combinations of triplets than there are amino acids (20 amino acids but 64 triplets). This means that some amino acids are coded for by more than one base triplet. Tyrosine can be coded by UAU or UAC.

Front 8

What is a gene?

Back 8

A gene is a sequence of mononucleotide bases on a DNA molecule that codes for the sequence of amino acids in a polypeptide. The sequence of amino acids in a polypeptide chain form the primary structure of a protein.

Front 9

What is the general structure of an amino acid?

Back 9

Amino acids have the general structure of a carboxyl group, an amine group (-NH2) and a carbon containing R group. The difference between the amino acids are the R group.

Front 10

How are polypeptides formed?

Back 10

Amino acids are linked together by condensation reactions to form polypeptides. A molecule of water is released during the reaction. The bonds are called peptide bonds.

Front 11

What are the primary and secondary structures of a protein?

Back 11

• The primary structure is the sequence of amino acids in the polypeptide chain. It is held together by the peptide bonds between amino acids.

• The secondary structure has hydrogen bonds forming between the amino acids in the chain, making it coil into an alpha helix or fold into a beta pleated sheet.
  

Front 12

What is the tertiary structure of a protein?

Back 12

The coiled or folded chain of amino acids is often coiled and folded further. More bonds form between the different parts of the chain.

• Bonds include hydrogen bonds
• Ionic bonds
• Disulfide bridges.

For proteins made from a single polypeptide chain, the tertiary structure forms the final 3D structure.

Front 13

What bonds are present in the tertiary structure of a protein?

Back 13

Ionic Bonds - attractions between negative and opposite charges on different parts of the molecule.

Disulphide bridges - When two molecules of cysteine come close together, the sulphur atom in one bonds to the sulphur in the other,

Hydrophobic and hydrophobic interactions - hydrophobic groups clump together, meaning hydrophilic groups are more likely to be pushed outside, which affects how the proteins fold up.

Front 14

What is the quaternary structure of a protein?

Back 14

Proteins that are made of several different polypeptide bonds are held together by bonds. The quaternary structure is the way these chains are assembled together. For proteins made from more than one chain (insulin, collagen etc.) the quaternary structure is the final 3D structure.

Front 15

What are globular proteins?

Back 15

1. Globular proteins are round, compact and made of multiple polypeptide chains. They are coiled up so the hydrophilic parts are on the outside of the molecule and hydrophobic parts face inwards.
   
2. This makes the proteins soluble so they're easily transported.
3. Haemoglobin is a globular protein made of four polypeptide chains. It carries oxygen around the body in the blood. Soluble so can easily be transported in the blood.

Front 16

What are fibrous proteins?

Back 16

1. Fibrous proteins are made up of long, insoluble polypeptide chains that are tightly coiled around each other to form a rope shape.
   
2. The chains are held by lots of bonds such as disulfide and hydrogen bonds which make the proteins strong
   
3. This means they're often found in supportive tissue, such as collagen.
   

Front 17

How do enzyme properties relate to their tertiary structure?

Back 17

• Enzymes are very specific - they catalyse one reaction usually. This is because only one complementary substrate fits in the active site.
  
• The active site shape depends on the tertiary structure which is determined by the primary structure.
  
• Each different enzyme has a different tertiary structure and so has a different shaped active site. If the substrate shape doesn't match the active site, an enzyme-substrate complex wont form and the reaction wont be catalysed.
  

Front 18

Why are enzymes said to be biological catalysts?

Back 18

Look in topic 2 text book.

Front 19

What was the first model to explain enzymes?

Back 19

The lock and key model states the substrate fits into the enzyme the same way the key fits into a lock.

Front 20

What is the 'induced fit' model?

Back 20

• Enzyme-substrate complex's change shape slightly to complete the fit.
• This locks the substrate even more tightly to the enzyme.
• It explains why enzymes are so specific and only bond to one substrate.
• The substrate doesn't have to be the same shape as the active site, but has to make the active site change shape in the right way.
  

Front 21

How do enzymes affect rate of reaction?

Back 21

1. If two substrate molecules need to be joined, being attached to the enzyme holds them close together, reducing any repulsion so they can bond more easily.
   
2. If the enzyme is catalysing the breakdown reaction, fitting into the active site puts a strain on bonds in the substrate so the molecule breaks up more easily.
   

These factors reduce the activation energy so the reaction happens faster.

Front 22

How does enzyme concentration affect rate of reaction?

Back 22

1. The more enzyme molecules there are in solution, the more active sites are present and therefore the more likely a substrate molecule is to collide with an active site and form an enzyme substrate complex and thus increases rate of reaction.
   
2. If the amount of substrate is limited, there will be more enzymes than substrate, so adding more enzymes has no further affect.
   

Front 23

How does substrate concentration affect rate of reaction?

Back 23

• The higher the substrate concentration, the faster the reaction as there is more of a chance of a collision between substrate and enzyme and so more active sites will be used.
  
• This happens until the saturation point where there are so many substrate molecules that the enzymes will have filled active sites and adding more substrate will have no affect.