Chapter 6: Nucleic Acids And Protein Synthesis

Question 1

What are nucleotides made up of?

Answer 1

1. A nitrogen containing base
2. A pentose sugar
3. A phosphate group

Question 2

What are the four nitrogen containing bases found in DNA?

Answer 2

Adenine, guanine, thymine, cytosine

Question 3

What are the four nitrogen containing bases found in RNA?

Answer 3

Adenine, guanine, uracil, cytosine.

Question 4

What can the pentose sugar be in DNA and RNA?

Answer 4

DNA-deoxyribose

RNA- ribose

Question 5

Where does the linking of nucleotides take place and when?

Answer 5

This takes place in the nucleus, during the interphase of the cell cycle.

Question 6

What are the purine bases?

Answer 6

Adenine and guanine

Question 7

What are the pyrimidines?

Answer 7

Cytosine and Thymine

Question 8

What are the complementary base pairs?

Answer 8

A with T and C with G

Question 9

Describe the structure of a polynucleotide.

Answer 9

Each polynucleotide strand contains covalent sugar-phosphate bonds linking the 5-carbon of one sugar molecule and the 3-carbon of the next. The polynucleotide strand is said to have 3’ and 5’ ends.

Question 10

Describe the structure of DNA.

Answer 10

DNA is made up of two antiparallel polynucleotide strands lying side by side, held together by hydrogen bonds. These strands are arranged into a double helix structure. The way the two strands line up is very precise.

Question 11

What is the process of semi-conservative DNA replication?

Answer 11

It occurs during S phase of interphase.

1. The DNA double helix unwinds and ‘unzips’ as the hydrogen bonds between the bases break by helicase.
2. In the nucleus, there are nucleotides to which two extra phosphates have been added to activate the nucleotides.
3. Each of the bases pairs up with its complementary base on each of the old DNA strands. The enzyme DNA polymerase links the nucleotides to each other by catalysing phosphodiester bonds. The two extra phosphates are broken off and released into the nucleus.

Question 12

What is a gene?

Answer 12

A part of a DNA molecule where the nucleotide sequence codes for just one polypeptide, is called a gene.

Question 13

What is a mutation with reference to nucleotides?

Answer 13

A change in the nucleotide sequence of a gene, which may then result in an altered polypeptide is called a mutation.

Question 14

What is a triplet code?

Answer 14

It is a sequence of three bases standing for one amino acid.

Question 15

What causes sickle cell anaemia?

Answer 15

The gene which codes for one of the amino acid sequence in the B-globin polypeptide chain in haemoglobin is substituted with the Hbs(sickle cell) allele of the gene, as opposed to the Hba (normal allele). This causes the outer surface of haemoglobin to become less soluble due to the substitution being a non-polar amino acid.

Question 16

How does DNA affect the primary structure of a protein?

Answer 16

DNA codes for multiple proteins and controls the exact order in which amino acids join together when proteins are made.

Question 17

How does DNA control the activities of a cell?

Answer 17

All chemical reactions are controlled by enzymes. Enzymes are proteins. Genes code for proteins, controlling which proteins are made. Thus, DNA controls the cell’s activities.

Question 18

When does transcription of a gene begin? ref. promoter

Answer 18

Transcription of a gene begins when RNA polymerase binds to a control region of the DNA called a promoter and ends when the enzyme reached a terminator sequence.

Question 19

Describe the transcription process of protein synthesis.

Answer 19

In the nucleus, the DNA strands unwind and ‘unzip’ as the hydrogen bonds between them break. Free activated RNA nucleotides pair up with the exposed bases of one strand only. As the RNA nucleotides pair up with their complementary ones, their sugar-phosphate groups are bonded together by RNA polymerase. The transcription begins when RNA polymerase binds to a control region of the DNA known as a promoter and ends when the enzyme has reached a terminator sequence. The hydrogen bonds holding DNA and RNA together are broken and the double stranded DNA reforms. The new single stranded molecule formed is known as mRNA. It leaves the nucleus through a nuclear envelope.

Question 20

Describe the translation process of protein synthesis.

Answer 20

In the cytoplasm, there are free amino acids and transfer RNA (tRNA) molecules. At one end of a tRNA molecule is a site to which an amino acid can bind. At the other end are three unpaired bases. This base triplet is called an anticodon. Each tRNA molecule bonds with a particular amino acid. Meanwhile, also in the cytoplasm, the mRNA molecule binds to the small subunit of the ribosome. The ribsome is made up of RNA and protein. A tRNA molecule enters the large subunit of the ribosome and attaches to the mRNA. The first three exposed bases or CODON are always AUG with the complementary anticodon UAC on the tRNA. This tRNA molecule has the amino acid methionine attached to it. The anticodon on the tRNA and the codon on the mRNA form hydrogen bonds. Only two tRNA molecules can fit in the large subunit of the ribosome at the same time. A second tRNA molecule bonds with the next three bases bringing a different amino acid. The two amino acids are held closely together and a peptide bond is formed between them catalysed by the enzyme peptidyl transferase which is found in the small subunit of the ribosome. The ribosome moves along the mRNA , reading the next codon and bringing a third amino acid which bonds with the second one. The first tRNA leaves and is reused. The polypeptide chain continues to grow until a ‘stop’ codon is exposed on the ribosome. This is UAA, UAC or UGA.