B1.2 What Happens In Cells

Question 1

Explain what a chromosome is

Answer 1

• A chromosome is the name for each long molecule of DNA.
• Most people have 46 chromosomes in each of their cells, half from your mother and half from your father.

Question 2

Explain what genes are

Answer 2

• DNA is arranged into sections. Short sections of DNA that code for a characteristic are called genes.
• The code that a gene contains causes specific proteins to be made, which determines the cell’s function.

Question 3

What is the structure of DNA

Answer 3

• DNA is made up of two strands which are twisted together. This forms a shape known as a double helix.
• DNA is made of lots of small units called nucleotides, which are joined together meaning that DNA is a polymer.
• Each nucleotide is made of a deoxyribose sugar, a phosphate backbone, and a base. The two DNA strands are held together by bonds between the bases.
• There are four different types of nucleotide in DNA: Adenine, Thymine, Cytosine, and Guanine.

Question 4

How is a copy of DNA made?

Answer 4

• DNA cannot leave the nucleus as it is too big so a copy of the DNA is made called mRNA.
• The mRNA is produced in a process called transcription.
• This is where the DNA around a gene unzips so that both strands are separated.
• One of the strands acts as a template as complementary bases attach to the strand being copied.
• Cytosine joins to guanine, uracil joins to adenine, and adenine joins to thymine.
• Finally, the strand of mRNA detaches from the DNA and travels to the ribosomes in the cytoplasm, where protein is made.

Question 5

How is a protein made?

Answer 5

• Proteins are made by a process called translation.
• The mRNA attaches to a ribosome. The ribosome reads the nucleotides on the mRNA in groups of three. These groups are called codons or triplets.
• Each codon codes for a specific amino acid.
• As the ribosome continues to read the code, the amino acids join together in a chain, this is a protein.

Question 6

State a difference between transcription and translation

Answer 6

• Transcription is when a strand of DNA is copied into mRNA, but translation is when the ribosomes reads the mRNA and produces proteins.

Question 7

State three characteristics of enzymes

Answer 7

• Enzymes are made of protein.
• They are biological catalysts, meaning they speed up a reaction without being used up.
• Once a reaction is finished they can be used to catalyse the same type of reaction again.

Question 8

Describe what enzymes look like

Answer 8

• Enzymes are made up of long chains of amino acids.
• The shape of the active site is particularly important, since here, molecules of other substances bind to the enzyme.
• The molecule that binds to the enzyme is called the substrate.

Question 9

What factors affect enzymes?

Answer 9

Temperature - As temperature increases the rate of reaction also increases, however once the temperature gets too high, the amino acid chains start to unravel and the enzyme becomes denatured.

pH- A change in pH affects the interactions between amino acids in a chain. This may make the enzyme unfold, changing the shape of the active site, and making the enzyme denatured.

Enzyme and substrate concentration - In general, the higher the substrate concentration, the higher the rate of reaction. But at a certain concentration when the rate of reaction gets to its maximum, any further increase will not increase the rate of reaction as there are no enzymes for them to bind to.
The same is true for enzyme concentration. The reaction will stop as substrate molecules run out.