B1.2 What happens in cells?

Question 1

State the role of DNA in the body

Answer 1

• each long molecule of DNA is a chromosome
• most people have 46 chromosomes in each cell
• you inherit half your chromosomes from your mother and half from your father
• everyone’s DNA is unique except identical twins and clones
• DNA is arranged into short sections that code for a characteristic - genes
• the code that a gene contains caused specific proteins to be made, which determine the cell’s function
• the combination of genes in an organism controls how the organism functions and its appearance

Question 2

Describe the structure of DNA

Answer 2

• made up of 2 strands which are joined together by bases, then twisted together to form the shape of a double helix
• made of lots of small units called nucleotides which are joined together - DNA is a polymer
• each nucleotide is made of a sugar (deoxyribose), a phosphate group and a base - 2 strands of DNA are held together by bonds between the bases
• 4 different types of nucleotide in DNA - each contains a different bases
• 4 bases are adenine, thymine, cytosine, and guanine

Question 3

Explain what is meant by complementary base pairing

Answer 3

• to hold strands of DNA together a base from 1 strand bonds with a base on the other strand - forming a base pair
• the base pairs always bond together in the same formation - complementary base pairing
  A - T
  C - G

Question 4

Describe the process of transcription (1st stage of protein synthesis)

Answer 4

• DNA cannot leave the nucleus of your cells as it’s too big so instead a copy of the DNA called mRNA is made, which is like a strand of DNA

Process:

• DNA around a gene unzips so that both strands are separated
• One of the strands acts as a template
• complementary bases attach to the strand being copied
• C - G and as there isn’t any base T in mRNA, a base called uracil (U) binds with base A
• when complete the strand of mRNA detaches itself from the DNA template and the DNA zips back up
• mRNA is small enough to move out of the nucleus - it travels to the ribosomes in the cytoplasm where the protein will be made

Question 5

Describe the process of translation (2nd stage of protein synthesis)

Answer 5

• proteins are made from amino acids - the type + order of amino acids is determined by order of nucleotides in DNA, which determines which proteins are produced (each type of protein has a specific shape)
• The mRNA attaches to a ribosome
• the ribosome ‘reads’ the nucleotides on the mRNA in groups of 3 - called base triplets (or codons) - each triplet codes for a specific amino acid
• the ribosome continues to ‘read’ (interpret) the triplet code, adding more and more amino acids
• the amino acids join together in a chain - this is a protein

Question 6

State what an enzyme is

Answer 6

• made of protein
• biological catalysts - they speed up a reaction without being used up themselves and once a reaction is finished they can be used to catalyse the same type of reaction again
• involved in many reactions in your body e.g. :
  
  • building larger molecules from smaller ones (protein synthesis)
  • breaking down large molecules into smaller ones (digestion)

Question 7

Describe the structure of an enzyme

Answer 7

• made up of long chains of amino acids (like all proteins) which are folded together to form a specific shape
• the active sites shape is very important, as here the molecules of other substances bind to the enzyme
• the molecule that binds to the enzyme is the substrate - when it binds to enzyme it fits inside enzyme

Question 8

Explain what is meant by enzyme specificity (lock and key hypothesis)

Answer 8

• enzymes are highly specific - they can only bind to one type of substrate molecule, and the substrate must exactly fit into the active site for the reaction to occur
• the enzyme is like the lock, and the substrate is the key - only 1 key will fit the lock and be able to turn to open the door (lock & key hypothesis)
• when substrate binds to enzyme, an enzyme-substrate complex is formed
• the reaction then happens quickly and products are released from enzyme
• the enzyme is them ready to catalyse another reaction

Question 9

Explain how temperature affects enzyme-controlled reactions

Answer 9

• at high temps enzyme and substrate molecules move faster and collide more often
• in general, the higher the temp, the faster the reaction
• the optimum temp is when the reaction works as fast as possible
• if temp becomes too high, the amino acid chains in protein start to unravel, changing the shape of the active site
  
  • the enzyme is now denatured - the substrate can no longer bind so the rate of reaction decreases, and once all enzymes have denatured the reaction stops
  • most denatured enzymes cannot return to their original shape - irreversible

Question 10

Explain how pH affects enzyme-controlled reactions

Answer 10

• each enzyme has its optimum pH
• a change in pH affects the interactions between amino acids in a chain
• which may make the enzyme unfold, changing the shape of the active site
• the enzyme is denatured

Question 11

Explain how other factors (enzyme and substrate concentration) affect enzyme controlled reactions

Answer 11

Substrate concentration:

• in general, the higher the substrate concentration is, the faster the rate of reaction
• but at a certain substrate concentration, all the enzyme molecules are bound to substrate molecules and so the rate of reaction is at its maximum
• any further increase in the no. of substrate molecules will not increase the rate of reaction as there are no enzymes for them to bind to

Enzyme concentration:
- in general, the higher the enzyme concentration is, the faster the rate of reaction. But this is limited by substrate concentration, as if no new substrate molecules are add the reaction will stop