1.3 - Biological Molecules 2

Question 1

What are nucleotides?

Answer 1

molecules with three parts - 5-carbon pentose sugar (DNA - deoxyribose, RNA - ribose) , a nitrogen-containing base and a phosphate group - joined by condensation reactions

Question 2

What is the nucleotides function?

Answer 2

they provide the energy currency of cells in the form of ATP

Question 3

What are the common types of nucleotides?

Answer 3

Purines - contain 1 nitrogen-containing ring (eg. Adenine/Guanine)

Pyrimidine - contains 2 nitrogen-containing rings (eg. Cytosine, Uracil, Thymine)

Question 4

Explain the structure of DNA.

Answer 4

Two strands of DNA are antiparallel and twisted around each other in a double helix, held together by hydrogen bonds between complementary base pairs. These hydrogen bonds form between the amino acid group of one base and the carbonyl group of the complementary base pair.
Complementary base pairings are Adenine Thymine/ Guanine and Cytosine.
The nucleotides are linked together by phosphodiester bonds between a C3 sugar of one nucleotide and a phosphate group of another nucleotide in a condensation reaction. This forms the sugar-phosphate backbone of DNA
The pentose sugar for DNA is deoxyribose

Question 5

Explain why C-G and A-T can only pair together.

Answer 5

*Adenine (A) can only pair with Thymine (T) via two hydrogen bonds
*Guanine (G) can only pair with Cytosine (C) via three hydrogen bonds

Question 6

How is DNA semi-conservatively replicated?

Answer 6

1. Parent DNA molecule unwinds and unzips into a DNA ladder using the enzyme DNA helicase
2. Hydrogen bonds are broken and the parent strands separate
3. Each parent strand acts as a template for the formation of the new strands
4. Free-floating nucleotides in the cytoplasm into complementary base pair with the exposed bases on the template strands by hydrogen bonding
5. The nucleotides are brought together by DNA ligase and joined together by phosphodiester bonds in a condensation reaction which is catalysed by DNA polymerase in a 5^1 to 3^1 direction forming the sugar-phosphate backbone of the new strand
6. The two strands twist to form a double helix
7. Each new double-stranded DNA molecule contains one parent and one new strand hence why its semi-conservative replication

Question 7

How did the banding patterns after centrifugation prove that semi-conservative replication was the correct replication?

Answer 7

Semi-conservative is the correct form of DNA replication that supports the evidence provided as the second and third generations have this banding as it is a hybrid of the original and new strand.

Question 8

What is a gene?

Answer 8

A sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain

Question 9

What is the structure of RNA?

Answer 9

• RNA molecules form a single polynucleotide strand that can fold into complex shapes
• RNA nucleotides contain the nitrogenous bases adenine, guanine, cytosine and uracil instead of thymine.
• Pentose sugar in RNA is ribose.
• The nucleotides are linked together by phosphodiester bonds between the sugar group of one nucleotide and the phosphate group of another nucleotide in a condensation reaction. This forms the sugar-phosphate backbone of RNA.

Question 10

Examples of RNA molecules

Answer 10

• mRNA (messenger RNA)
• tRNA (transfer RNA)
• rRNA (ribosomal RNA)

Question 11

What is the structure and function of mRNA?

Answer 11

• It’s formed in the nucleus and used in transcription to make copies of the parent strand and carries copies out of the nucleus through nuclear pores to the ribosome.
• mRNA is a single-stranded molecule
• It’s made up of a sugar-phosphate backbone and exposed unpaired bases.
• RNA polymerase catalyses the formation of phosphodiester bonds forming the sugar-phosphate backbone.
• Uracil bases are present instead of thymine bases
• Exposed bases are paired with complementary free RNA nucleotides by hydrogen bonds.
  Complementary base pairs (A-U/G-C)

Question 12

What is the structure and function of mRNA?

Answer 12

• Used in translation to carry complementary amino acids to ribosomes to form a protein
• found in the cytoplasm
• Is a single-stranded molecule
• in a clover leaf shape with an amino acid site and an anticodon site.
• the anticodon is complementary to the corresponding codon in mRNA and will decide the exact order of amino acid in the resulting polypeptide chain.
• the clover shape allows hydrogen bonding between complementary base pairs.

Question 13

Explain the transcription.

Answer 13

TRANSCRIPTION:
1. In the nucleus, the enzyme DNA helicase unzips and unwinds DNA into 2 parent strands
2. These parent strands become the template strands
3. mRNA has complementary base pairs between the RNA nucleotides and DNA nucleotides (U-A, C-G) on the antisense strand (non-coding strand)
4. RNA polymerase moves along and assembles the RNA nucleotides + catalyses phosphodiester bonds between the RNA nucleotides
5. Once all the complementary base pairs are bonded the RNA polymerase reaches the stop codon and the mRNA moves out of the nucleus through nuclear pores to the ribosomes for the production of protein.

Question 14

Explain the process of translation

Answer 14

TRANSLATION:
1. In ribosomes, mRNA binds to ribosomes + finds start codon
2. tRNA that has an anticodon complementary base pairing
3. tRNA carries an amino acid in amino acid site
4. Second tRNA attaches to the next codon in the same way as the first tRNA
5. The amino acid carried by first and second tRNA are bonded together by peptide bonds
6. RNA polymerase catalyses peptide bonds in condensation reaction to eventually create a long polypeptide chain which folds into specific shapes to produce a protein.

Question 15

What is the nature of the genetic nature?

Answer 15

1. degenerate: many amino acids are coded for by more than one codon (degenerate codon). This is a protective feature as it reduces the chance of a mutation and has a great impact if it codes for the same amino acid.
2. universal: the genetic code is universal as specific codons are the same across all organisms
3. non-overlapping: triplet do not share their bases, each triplet is read in a sequence. This is a protective feature as since it is not over-lapping any mutations will not affect three different amino acids only a single base mutation occurs

Question 16

What is a start and stop codon?

Answer 16

The start codon (AUG) signifies the start of a particular long polypeptide chain.

The stop codon signifies the end of a particular long polypeptide chain. Has no complementary base pairings to show the end of the production of that particular polypeptide chain.

Question 17

What is a mutation?

Answer 17

permanent change in the DNA of an organism, can occur whenever DNA replicates itself like mitosis and meiosis

Question 18

Examples of mutagens…

Answer 18

• UV radiation
• ionising
• x-ray

Question 19

What is the chance of a mutation happening during DNA replication?

Answer 19

2.5 x 10^5

Question 20

How are point mutations caused?

Answer 20

• substitution: where one base substitutes for another
• deletion: when a base is lost completely
• insertion: where an extra base is added

Question 21

How does deletion and insertion affect the triplet?

Answer 21

the longer the repeats, the more difficult for proteins to fold = insoluble proteins

Question 22

Examples of chromosomal mutations:

Answer 22

• gene deletion
• gene duplication
• whole chromosome mutation where an entire chromosome is either lost or duplicated

Question 23

SICKLE CELL

Answer 23

• caused by a point mutation, a single substitution mutation in the beta-haemoglobin gene, which changes an amino acid in the polypeptide resulting in the polypeptide folding incorrectly
• haemoglobin molecules stick together to form rigid rod that give red blood cells a sickle shape
• this shape means that they do not carry oxygen efficiently and can cause pain + small blockages in blood vessels