DNA

Question 1

What is the structure of DNA?

Answer 1

2 strands coiled together in the shape of a double helix DNA is a polymer made from 4 nucleotides;
Adenine, Thymine, Guanine and Cytosine
Adenine always pairs with Thymine
Guanine always pairs with Cytosine
This is called complementary base pairing
These base pairs are joined together by weak hydrogen bonds between them
A-T = 2 hydrogen bonds
G-C = 3 hydrogen bonds
One of the bases is attached to a stand made of sugar and phosphate. DNA has a sugar phosphate backbone.

Question 2

What is the process for extracting DNA? What do certain stages do?

Answer 2

1) Mash up fruit
2) Add salt and detergent into the mashed up fruit
- The detergent breaks the cell membrane to release the DNA
- The salt clumps the DNA together
3) Gently heat the mixture at 60 degrees for 15 minutes
- This speeds up the rate of reaction of breaking the cell and nuclear membranes
4) Filter the mixture, leaving the residue behind and removing the large, insoluble parts of the cell.
5) Gently add some ice- cold ethanol to the filtered mixture.
- DNA is insoluble in ice - cold ethanol so it moves to the top and looks like a string white precipitate that can be carefully extracted out with a glass rod.

Question 3

What does sexual reproduction produce?

Answer 3

Genetically different cells.

Question 4

What are gametes?

Answer 4

Gametes are haploid, meaning they contain 23 chromosomes.

Question 5

What happens at fertilisation?

Answer 5

The male gamete penetrates the outer layer of an egg cell and burrows itself in the egg. The sperm cell fuses with the egg cell to create a fertilised egg. This is called a zygote. The zygote is diploid as when the gametes fuse, both the 23 chromosomes add up to produce the full 46.

Question 6

What happens to the zygote in order for it to grow?

Answer 6

The zygote undergoes mitosis and develops into an embryo

Question 7

What does the embryo inherit?

Answer 7

It inherits characteristics from both parents, as it has received a mixture of chromosomes (and therefore genes) from its mum and dad.

Question 8

What are the advantages of sexual reproduction?

Answer 8

• Creates variation
• Makes it more likely for a population to survive as their are more combinations of alleles, some of which will be more resistant to changes in the environment etc.

Question 9

What are the disadvantages of sexual reproduction?

Answer 9

• It takes time and energy

- It is difficult to find a mate if an individual is isolated

Question 10

What is asexual reproduction?

Answer 10

The process of reproducing without the fusion of 2 gametes

Question 11

What are the advantages of asexual reproduction?

Answer 11

• It’s much quicker
• It’s easier to colonise a new area rapidly
• Only one parent is needed

Question 12

What are the disadvantages of asexual reproduction?

Answer 12

• There’s no genetic variation between offspring in the population
• If the environment changes or disease strikes, the chances are unfavourable so the entire population could go extinct

Question 13

What is meiosis?

Answer 13

A type of cell division that only happens in the reproductive organs. It produces 4 genetically different haploid daughter cells.

Question 14

How does meiosis produce genetic variation?

Answer 14

When the cell divides, some of the chromosomes from the organism’s father and some chromosomes from the organism’s mother go into each new cell. The mixing up of the chromosomes/ genes creates genetic variation.

Question 15

What is a genome?

Answer 15

The entire genetic material (including non- coding DNA) of an organism

Question 16

What is a gene?

Answer 16

A section of DNA that codes for a specific protein

Question 17

How do the order of bases in a section of DNA decide the order of amino acids in a protein?

Answer 17

An order of 3 bases is called a codon. Each codon codes for an amino acid. The order of codons in a DNA base sequence decides the order of amino acids in a protein.

Question 18

What is the process of transcription?

Answer 18

RNA polymerase binds to the non- coding region of DNA located in the front of a gene. The 2 strands of DNA unzip. The RNA polymerase moves along the coding strand of DNA and uses it as a template to make the mRNA. Complementary base pairing occurs and the DNA nucleotides pair with the complementary RNA nucleotides to produce mRNA. The mRNA is identical to the other strand of DNA that isn’t being used. The mRNA then moves out of the nucleus, through nuclear pores and moves into the cytoplasm where it joins with a ribosome.

Question 19

What is the process of translation?

Answer 19

Transfer RNA (tRNA) carries specific proteins. It also has the anti-codon to codons in mRNA. The order in which tRNA brings amino acids to the ribosome matches the order of codons in the mRNA strand. The pairing of the anti-codon to the codon ensures that the amino acids are brought to the ribosome in the correct order. The ribosome joins the amino acids together to form a polypeptide chain which then folds into the specific shape of a protein.

Question 20

How does a genetic variant in the coding DNA of a gene affect the activity of the protein produced?

Answer 20

A genetic variant could mean that one base is substituted for another, altering the base sequence in the DNA. This means that that codon in the gene could now code for a new amino acid therefore meaning a different shape is produced so a new protein is made. This protein could have a higher or lower level of activity.

Question 21

How can a genetic variant in the non - coding region of DNA affect the binding of RNA polymerase and the quantity of protein produced?

Answer 21

A genetic variant could cause the RNA polymerase to not bind as well or bind better to the non- coding region of DNA located in the front of the gene. This means if less binds, less mRNA is produced and so less mRNA binds to the ribosome and therefore less protein is produced. However, if RNA polymerase binds better, more mRNA is produced and so more binds to the ribosomes, therefore more protein is produced.

Question 22

How did Mendel discover the basis of genetics?

Answer 22

Mendel bred 2 pea plants together. A tall one and a short one. He found that all the offspring were tall. He then bred 2 of the tall offspring together and found that for every 3 tall plants, there was 1 short one. He discovered that the tall plant was dominant over the short plant.

Question 23

What 3 conclusions did Mendel come to?

Answer 23

• Characteristics are determined by hereditary units
• Hereditary units are inherited
• Hereditary units can be dominant or recessive

Question 24

What were the difficulties of understanding inheritance?

Answer 24

• His ideas were new and scientists didn’t fully understand his ideas
• Scientists had no idea about genes or chromosomes or DNA
• He didn’t communicate his ideas well
• He published his work in a small diary.

Question 25

Why are there differences in inherited characteristics as a result of alleles?

Answer 25

There are different allele combinations, resulting in different phenotypes.

Question 26

What is single gene inheritance?

Answer 26

A pair of alleles, one from mum and dad, that code for one particular gene. The phenotype is affected in the offspring depending on whether the alleles are homozygous recessive, homozygous dominant or heterozygous.

Question 27

How are sex- linked genetic disorders inherited?

Answer 27

There may be a mutation on the x chromosome or the y chromosome depending on whether you are female or male. They are inherited on either of the sex chromosomes.

Question 28

What are 2 causes of variation that influence phenotype?

Answer 28

Genetic variation - there are different characteristics as a result of a mutation or sexual reproduction with different allele combinations.
Environmental variation- there are different characteristics caused by an organism’s characteristics. This is called an acquired characteristic.

Question 29

How is continuous data represented?

Answer 29

By a histogram.

Question 30

How is discontinuous data represented?

Answer 30

By a bar chart