4.6.1.5 DNA Structure (biology only)

Question 1

DNA:

Answer 1

• deoxyribonucleic acid
• polymer made from 4 different nucleotides that contains instructions for the body

Question 2

DNA structure diagram:

Answer 2

Question 3

What does each nucleotide consist of?

Answer 3

• each nucleotide consists of a common sugar molecule and one phosphate molecule (which form the backbone)
• one of four different organic bases attached to the sugar

Question 4

How many bases does DNA contain?

Answer 4

DNA contains 4 types of organic bases - A, C, G and T

Question 5

Complementary (in DNA):

Answer 5

• describes how the chemical bases in DNA pair up with each other
• A pairs up with T and C pairs up with G

Question 6

What is needed to code for an amino acid?

Answer 6

• a sequence of 3 bases is the code for a particular amino acid
• the order of bases controls the order in which amino acids are assembled to produce a particular protein

Question 7

What do long strands of DNA consist of?

Answer 7

• long strands of DNA consist of alternating sugar and phosphate section
• attached to each sugar is 1 of the 4 bases
• the DNA polymer is made up of repeating nucleotide units

Question 8

How are bases linked to each other?

Answer 8

• each DNA molecule is made up of 2 DNA strands which are twisted together
• each base is connected to another base in the other strand
• in the complementary strands C bases are always linked to G bases on the opposite strands and A basses are always linked to T bases in the opposite strands

Question 9

How does DNA determine a protein?

Answer 9

• each group of 3 bases (found on DNA) codes for an amino acids
• the amino acids are joined together to make a protein
• it is the different types of order of amino acids that determine which type of protein it is
• therefore, it is the order of bases in DNA that determine which proteins are produced

Question 10

How many types of amino acids are there?

Answer 10

20 types of amino acids

Question 11

Coding DNA:

Answer 11

a sequence of DNA that codes for the production of a protein

Question 12

Non-coding DNA:

Answer 12

• not all parts of DNA code for proteins
• non-coding parts of DA cans witch genes on and off so variations in these areas of DNA may affect how genes are expressed

Question 13

How may genetic variants influence phenotype in coding DNA?

Answer 13

• in a coding DNA by altering the activity of a protein
• a change in the type/sequence of amino acids will affect the way the protein folds and therefore the structure
• mutations occur continuously - most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed
• a few mutations do code for an altered protein with a different shape
  
  • an enzyme (substrate) may no longer fit the substrate binding site (active site)
  • a structural protein may lose its strength (if its shape changes)
• therefore causing a change in the phenotype

Question 14

How may genetic variants influence phenotype in non-coding DNA?

Answer 14

• there are also non-coding parts (introns) of DNA that do not code for proteins
• instead, some of them are responsible for acting as switches to switch genes on or off - control whether a gene is used to form a protein or not
• genetic variants in non-coding DNA may influence phenotype as they may alter how genes are expressed

Question 15

Protein synthesis:

Answer 15

the process of producing a protein from a gene (DNA)

Question 16

mRNA:

Answer 16

messenger RNA - a different type of nucleic acid

Question 17

Amino acids:

Answer 17

small molecules from which proteins are assembled

Question 18

Ribosomes:

Answer 18

sub-cellular structures where protein synthesis takes place

Question 19

When has a gene been expressed?

Answer 19

if a gene is coded to make a protein it has been expressed

Question 20

Process of protein synthesis:

Answer 20

1. DNA contains the genetic code for making a protein, but it cannot move out of the nucleus as it is too big
2. DNA unzips - the 2 strands pull apart from each other
3. mRNA nucleotides match to their complementary base on the strand - the mRNA nucleotides themselves are then joined together creating a new strand called the mRNA strand - so a template/copy of the DNA (gene) is made into mRNA
4. mRNA leaves the nucleus and goes into the cytoplasm where the mRNA attaches to a ribosome
5. The ribosome reads the mRNA code (bases) in threes to code for an amino acid (the first 3 bases code for 1 amino acid, the second 3 bases code for another etc.) - proteins are synthesised on ribosomes according to the template
6. Carrier molecules in the cytoplasm bring the corresponding (specific) amino acids which they have attached
7. The base pairs on the carrier molecules and mRNA are complementary so they attach
8. A second carrier molecule brings in another amino acid
9. A peptide bond forms between the 2 amino acids
10. A third carrier molecule brings in another amino acid and another peptide bond forms
11. A polypeptide chain is formed
12. Carrier molecules continue to bring specific amino acids that they have attached to add to the growing protein chain in order
13. When the protein chain is complete it folds up to form a unique shape - this unique shape enables the proteins to do their job as enzymes, hormones or forming structures in the body such as collagen

Question 21

Roles of protein in the body:

Answer 21

• enzymes - biological catalysts that speed up the rate of reaction
• hormones - chemical messengers that send signals around the body
• structural protein - strong proteins in order to form structures such as collagen

Question 22

How can mutations change the sequences of bases in DNA?

Answer 22

1. Insertion
   
   1. base is inserted into the code
   2. as they are read in threes, this changes the way it is read
   3. it may change all the amino acids coded after this insertion
2. Deletion
   
   1. a base is deleted from the code
   2. changes the way the bases are read
   3. it may change all the amino acids coded after this deletion
3. Substitution
   
   1. this will only change one amino acid in the sequence or it may not change the amino acid (as the new sequence can sometimes still code for the same amino acid)
4. Duplication
   
   1. a section of DNA bases are repeated

Question 23

How may a change in DNA structures (e.g. mutations) result in a change in the protein synthesised by a gene?

Answer 23

• a change in the type/sequence of amino acids will affect the way the protein folds and therefore the structure
• a few mutations do code for an altered protein with a different shape
  
  • a substrate may no longer fit the substrate binding site (active site) on an enzyme (if its shape has changed)
  • a structural protein may lose its strength (if its shape changes)

Question 24

Why may mutations in DNA not all have serious affects?

Answer 24

• mutations occur continuously - most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed
  
  • sometimes if a base is changed in the DNA is does not code for a different protein - e.g. if it is coding for an enzyme it may not lose its function
• we have 2 copies of every gene so even if one is faulty one may be okay

Question 25

Why does variation between 2 organisms arise?

Answer 25

1. The coding DNA that determines the proteins and their activity
2. The non-coding DNA that determines which genes are expressed

Question 26

Function of DNA:

Answer 26

code sequence of amino acids which form a specific protein

Question 27

How are amino acids used in protein synthesis?

Answer 27

• amino acids make up a protein
• so a protein is a particular sequence of amino acids
• bases form a code
• bases work in threes and a triplet of bases code for one amino acid