1.4 - DNA & protein synthesis

What 3 things does a nucleotide contain?

• Phosphate group

• Pentose sugar

• Nitrogenous base

What are the pentose sugars in DNA & RNA?

• DNA: deoxyribose

• RNA: ribose

How do polynucleotide strands form?

Condensation reactions between nucleotides to form strong phosphodiester bonds (sugar-phosphate backbone)

Describe the structure of DNA

• Double helix of 2 deoxyribose polynucleotide strands → 2 sugar-phosphate backbones

• Hydrogen bonds between complementary base pairs on opposite strands

  • adenine & thymine → 2 hydrogen bonds

  • guanine & cytosine → 3 hydrogen bonds

What are the purine bases & describe their structure?

• Adenine & Guanine

• Two-ring molecules

How to remember → pure As Gold (purines are large & gold is valuable)

What are the pyrimidine bases & describe their structure>

• Thymine, Cytosine & Uracil

• One-ring molecules

How to remember → CUT the Pie (smaller bases; easier to CUT)

What are the complementary base pairs in DNA?

• Adenine (A) + Thymine (T)

  • 2 hydrogen bonds between

• Guanine (G) + Cytosine (C)

  • 3 hydrogen bonds between

Name the complementary base pairs in RNA

• Adenine (A) + Uracil (U)

  • 2 hydrogen bonds between

• Guanine (G) + Cytosine (C)

  • 3 hydrogen bonds between

Why is DNA replication described as semi-conservative?

• Strands from original DNA molecule act as templates

• New DNA molecule contains 1 old strand & 1 new strand

Describe Meselson & Stahl’s experiment

1. Generation 0

• bacterium E.coli grown in heavy nitrogen medium (¹⁵N) & centrifuged

• band is at the bottom of test tub → 100% DNA is ¹⁵N ¹⁵N

2. First generation

• bacterium E.coli grown in light nitrogen medium (¹⁴N) & centrifuged

• band is in middle of test tube → 100% is ¹⁵N ¹⁴N

3. Second generation

• bacterium E.coli grown in light nitrogen medium (¹⁴N) & centrifuged

• one band is in the middle of test tube, another band is at the top → 50% of DNA is ¹⁴N ¹⁴N & 50% of DNA ¹⁵N ¹⁴N

4. Third generation

• bacterium E.coli grown in light nitrogen medium (¹⁴N) & centrifuged

• one thicker band at the top of test tube & one thinner band in the middle → 75% of DNA is ¹⁴N ¹⁴N & 25% of DNA is ¹⁵N ¹⁴N

Explain the role of DNA ligase in DNA replication

Joins Okazaki fragments on the lagging strand by forming phosphodiester bonds between adjacent nucleotides, using ATP, to produce a continuous DNA molecule

What is the role of DNA helicase in semi-conservative replication?

Breaks hydrogen bonds between complementary base pairs to form 2 single strands, each of which can act as a template

Outline the process of semi-conservative replication

1. DNA double helix is ‘nicked’ & hydrogen bonds between complementary base pairs are broken

2. This occurs due to DNA helicase & results in the unwinding of the two polynucleotide strands

3. Bases are now exposed on each strand. Separated polynucleotide strands both act as templates for the addition of free, activated nucleotides

4. These pair with complementary bases that have been exposed by the unwound DNA (A+T & G+C)

5. DNA polymerase catalyses the linking together of activated nucleotides through covalent phosphodiester bonds. Two new nucleotide strands are formed

6. There are now two daughter DNA molecules, each of which contains one of the original strands of DNA & one new one

What is the definition of a gene?

A sequence of DNA bases that code for a polypeptide or a functional RNA

Describe the structure of mRNA

• Single-stranded, linear molecule made of ribonucleotides

• Each ribonucleotide consists of ribose sugar, phosphate group & one nitrogenous base: A, U, G or C

• Nucleotides are joined by phosphodiester bonds, forming a sugar-phosphate backbone

• The sequence of bases forms triplets called codons → each codon codes for one specific amino acid

Describe the structure of tRNA

• Single-stranded RNA molecule made of ribonucleotides, containing: ribose sugar, phosphate group & one nitrogenous base (A, U, C, or G)

• Folded into clover-leaf shape due to hydrogen bonding between complementary bases

• One end has an anticodon (triplet of bases complementary to an mRNA codon)

• The opposite end has an amino acid binding site (where a specific amino acid binds)

What does transcription produce & where does it occur?

• Produces mRNA (more specifically pre-mRNA)

• Occurs in the nucleus

Outline the process of transcription

1. DNA helicase unwinds the DNA double helix

2. Free RNA nucleotides pair complementarily with exposed bases on the antisense strand (A+U & G+C)

3. As nucleotides are added, the DNA double helix rewinds behind it

4. RNA polymerase joins RNA nucleotides together, catalysing the formation of phosphodiester bonds

5. RNA polymerase detaches once stop codon is reached

What happens after a strand of mRNA is transcribed?

• Splicing removes introns (leaving only exons) from pre-mRNA in eukaryotic cells

• mRNA moves out of nucleus via nuclear pore & attaches to ribosome in cytoplasm

What is the function of mRNA?

Transfers the genetic code from DNA in the nucleus to ribosomes for translation into a specific polypeptide

What is the anti-sense strand of DNA?

The template strand of DNA which is transcribed

What does translation produce & where does it occur?

• Produces proteins

• Occurs in the cytoplasm on ribosomes

Outline the process of translation

1. mRNA attaches to a ribosome in the cytoplasm

2. A tRNA molecule with an anticodon complementary to the codon on the mRNA attaches (occurs at the start codon)

3. Another tRNA molecule with a complementary anticodon attaches to the next codon on the mRNA → a peptide bond forms between the amino acids

4. The ribosome moves along the mRNA to the next codon & the process repeats

5. This continues until a stop codon is reached

What is the role of ATP during translation?

ATP hydrolysis provides energy to form peptide bonds

What are 3 features of the genetic code?

• Non-overlapping: each triplet is only read once

• Degenerate: more than one triplet codes for the same amino acid (64 possible triplets for 20 amino acids)

• Universal: same bases & sequences used by all species

What are DNA triplets?

Sequence of 3 bases that code for a particular amino acid

What is a start codon?

Initiates the translation of a polypeptide

What is a stop codon?

Terminates translation of a polypeptide & does not code for an amino acid

What are introns & exons?

• Introns: non-coding sections of DNA (removed during splicing)

• Exons: regions of DNA that code for amino acid sequences

What is a mutation?

Any change in the base sequence of DNA (often arise spontaneously during DNA replication)

What is a substitution mutation?

One nucleotide in the DNA sequence is replaced by another → (likely to be a silent mutation which does not change amino acid sequence)

What is a deletion mutation & what is its effect?

• A nucleotide in the DNA sequence is removed, leading to a frameshift

• Significant since entire amino acid sequence downstream of mutation will be different

What is an insertion mutation & what is its effect?

• Addition of one or more nucleotides to the DNA sequence, which causes a frameshift

• Significant since entire amino acid sequence downstream of mutation will be different

What is sickle cell anaemia?

• Genetic condition that is common among people originating from malaria-stricken areas that results in abnormal haemoglobin

• Impaired ability to transport oxygen (blocks small capillaries) causing rapid heart rate, fatigue & dizziness

What causes sickle cell anaemia in humans?

Single point mutation in gene that affects protein chains making up haemoglobin:

• in DNA: GTG (Glutamic acid) → GAG (Valine)

• change in primary structure, meaning different tertiary structure → results in abnormal haemoglobin molecules that clump together & form long fibres that are sickle-shaped red blood cells