2.1.3 - Nucleotides and nucleic acids (set B - DNA + Protein Synthesis)

Question 1

Explain the process of DNA replication - what happens to the strand, explain briefly?

Answer 1

When cell prepares to divide, the two strands of DNA double helix separate and each strand serves as a template for the creation of a new double-stranded DNA molecule - complementary base pairing rules ensure the two new strands are identical to the original

Question 2

Explain the process of semi-conservative replication?

Answer 2

DNA unwinds and separates into two strands (hydrogen bonds holding complementary bases are broken) - free DNA nucleotides will pair with their complementary bases (exposed during separation of strands) and form hydrogen bonds (nucleotides form phosphodiester bonds with neighbouring nucleotides)

Question 3

Explain the product of semi-conservative replication?

Answer 3

Two new molecules of DNA are produced - each one consists of one old strand of DNA and one new strand

Question 4

Explain the bonds formed and broken during semi-conservative replication?

Answer 4

Hydrogen bonds holding complementary bases are broken - new H bonds formed between DNA nucleotides and their complementary bases

• phosphodiester bonds formed between adjacent nucleotides

Question 5

Explain the role of the enzyme Helicase in replication?

Answer 5

unwinds the DNA double helix to form 2 single strands - by breaking hydrogen bonds between complementary nucleotide bases - creates a replication fork, allowing access to DNA template strands

Question 6

Explain the role of the enzyme polymerase in replication?

Answer 6

Catalyses the formation of phosphodiester bonds between nucleotides

Question 7

Explain why DNA replication is described as semi-conservative?

Answer 7

Strands from original DNA molecule act as templates

• new DNA molecule consists of 1 old strand and 1 new strand (specific base paring enables genetic material to be conserved accurately)

Question 8

Explain two enzymes involved in DNA replication?

Answer 8

• DNA helicase
• DNA polymerase

Question 9

Explain how a mutation can occur during DNA replication?

Answer 9

Sequences of bases are not always matched exactly (an incorrect sequence may occur in the newly-copied strand) - lead to a change in the sequence of bases

Question 10

Explain what a triplet code is, give the number of possible triplet codes?

Answer 10

A sequence of three bases, nucleotides (Called a codon) which codes for a specific amino acid

• 64 possible codons (61 that specificy amino acids and three codons which signal the termination of translation)

Question 11

What is a gene?

Answer 11

The section of DNA that contains the complete sequence of bases (codons) to code for an entire protein

Question 12

Explain where the instructions that are carried by DNA are located, explain genetic code?

Answer 12

Instructions are contained in the sequences of bases along the chain of nucleotides that make up the two strands of DNA

• genetic code is universal for all organisms - however sequences of bases coding for each individual protein differs among organisms

Question 13

Identify three features of the genetic code?

Answer 13

• non-overlapping - each triplet is only read once
• degenerate - more than one triplet codes for the same amino acid (64 possible triplets)
• universal - same bases and sequences used by all species

Question 14

Explain what is meant by genetic code being degenerate?

Answer 14

refers to the redundancy in the genetic code, where multiple codons can code for the same amino acid - 64 possible codons and 20 amino acids in proteins therefore some amino acids are specified by more than one codon

Question 15

What is a significance of genetic code being degenerate?

Answer 15

provides a degree of flexibility and robustness to the genetic information - It reduces the impact of mutations, as changes in some codons may not alter the amino acid sequence

Question 16

Explain how the genetic code being redundant can increase efficiency during translation?

Answer 16

multiple codons can be recognized by the same tRNA molecule during protein synthesis

Question 17

Explain why the genetic code is non-overlapping, reference codons?

Answer 17

Having a single codon to signal the start of a sequence ensures that the codons are read ‘in frame’ - meaning the base sequence is ‘read’ from base 1, so the genetic code is non-overlapping

• can result in mutations otherwise

Question 18

Explain the complete process of semi-conservative replication?

Answer 18

1) DNA helicase causes two strands of DNA to separate (hydrogen bonds broken

2) free nucleotides which have been activated are attracted to their complementary bases

3) nucleotides line up and are joined by DNA polymerase (phosphodiester bonds formed) - remaining unpaired bases continue to attract complementary nucleotides

4) complete polynucleotide chain finally formed (composed of two identical molecules of DNA, a new strand and the original strand)

Question 19

How does a gene determine the sequence of amino acids in a protein?

Answer 19

Consists of base triplets (codon) that code for a specific amino acid

Question 20

Briefly explain the two stages of protein synthesis, give the name of the molecules involved?

Answer 20

Transcription - occurs in the nucleus and involves DNA and mRNA

Translation - involves mRNA, tRNA and ribosomes

Question 21

Outline the process of transcription?

Answer 21

1) RNA polymerase binds to promoter region on a gene

2) hydrogen bond between complementary bases broken (DNA helicase) - DNA uncoils into two strands

3) antisense acts as template to make mRNA

4) free nucleotides are attracted to their complementary bases - RNA polymerase joins adjacent nucleotides by phosphodiester bonds

5) mRNA moves out of nucleus through pores - attaches to ribosome

Question 22

What does transcription produce and where does it occur?

Answer 22

Produces mRNA

• occurs in nucleus - mRNA molecules move out of nucleus through pores and attach to ribosomes in the cytoplasm

Question 23

Explain the need for transcription?

Answer 23

Protein synthesis occurs in the ribosomes located in the cytoplasm, however chromosomal DNA molecules are too large to leave the nucleus, and supply the coding information for synthesising proteins

• transcription produces messenger RNA (complementary to the DNA template) which move through nuclear pores

Question 24

Explain how transcription is similar to DNA replication, what enzyme is involved in both processes?

Answer 24

In both section of DNA that contains the gene unwinds and unzips, under the control of a DNA helicase, beginning at start codon - involves breaking of hydrogen bonds between bases

• enzyme helicase involved in transcription and DNA replication

Question 25

Outline the role of the sense strand in transcription?

Answer 25

One of the two strands which contains the code for the protein - runs from 5’ to 3’, its complementary strand is the antisense strand

• does not act as template strand

Question 26

Outline the role of the antisense strand in transcription?

Answer 26

Complementary to the sense strand and runs from 3’ to 5’ - acts as the template strand during transcription so that the complementary RNA strand carries same base sequence as sense strand

• does not code for a protein

Question 27

Explain how mRNA differs from DNA?

Answer 27

mRNA is a singe stranded molecule where as DNA is double-stranded molecule

• the DNA base thymine is replaced by uracil in mRNA (uracil binds to adenine on the DNA template strand)

Question 28

Give the names of the nitrogenous bases for the molecule mRNA?

Answer 28

Adenine, uracil, cytosine and guanine

• same for DNA except uracil is replaced with thymine

Question 29

Outline the role of RNA polymerase in transcription?

Answer 29

The enzyme forms phosphodiester bonds between the RNA nucleotides, forming a single stranded molecule of mRNA

Question 30

Outline the process of translation?

Answer 30

1)mRNA attaches to a ribosome, tRNA collects amino acids from the cytoplasm and transports them to the ribosomes

2)tRNA attaches by complementary base paring to mRNA (2 molecules of tRNA attach to mRNA at a time)

3)amino acids attached to two tRNA molecules join by a peptide bond - tRNA molecules detach leaving amino acid behind

4) process repeated leading to formation of polypeptide chain until a stop codon is reached on mRNA, process of protein synthesis ends

Question 31

Explain what happens to the amino acids during the process of translation?

Answer 31

amino acids are joined together forming the primary structure of the protein, they fold into secondary and tertiary structure (Sequence of amino acids in p structure determine how it folds and the type of bonds formed)

• may undergo further modification at the Golgi apparatus

Question 32

Give the name of the enzyme which catalyses the formation of a peptide bond during translation?

Answer 32

Peptidyl transferase, an rRNA component of the ribosome - catalyses the formation of a peptide bond

Question 33

Explain the role of the two subunits which make up ribosome?

Answer 33

Made up of one large and one small subunit - composed of equal amounts of protein and rRNA (form of RNA)

• important in maintains structure stability of the protein synthesis sequence and involved in catalysing the reaction

Question 34

Outline the structure and role of tRNA?

Answer 34

Necessary for translation of mRNA

• Transfer RNA is composed of a strand of RNA folded so that three bases (anticodon) are at one end of the molecule - bind to complementary codon on mRNA

Question 35

Explain the function of anticodons on tRNA molecules and the role they play during translation?

Answer 35

Anticodons located at one end of tRNA molecule (which carry an amino acid corresponding to that codon) will bind to a complementary codon on an mRNA mocluelce - amino acids are brought together (peptide bond) in the correct sequence forming the primary structure of the protein

Question 36

Outline what a mutation is, give an example?

Answer 36

An alteration to the DNA base sequence - often arise spontaneously during DNA replication

• eg the mutation which leads to the production of sticky mucus and causes cystic fibrosis

Question 37

Give two examples of mutation?

Answer 37

• the mutation which leads to the production of sticky mucus and causes cystic fibrosis
• sickle cell anaemia - mutated form of Haemoglobin which distorts the shape of the red blood cell