Mistakes in topic questions

Errors in the precise location of splicing in the DNA molecule an lead to mutations

Explain why

• Deletion of bases

• Changes in base sequence of exons

• May result in a shortened (truncated) and non functional polypeptide produced

Describe the role of haemoglobin (Hb) in the loading, transport and unloading of oxygen

• Haemoglobin has a high affinity for oxygen where PO2 is high

• Therefore it binds to oxygen in the lungs to form oxyhaemoglobin (oxygen is loaded)

• Haemoglobin has a low affinity for oxygen when the pO2 of oxygen is low

• Therefore it releases oxygen at muscle tissues (oxygen is unloaded).

• Haemoglobin binds by cooperative binding, where the binding of one oxygen molecule changes the tertiary structure of haemoglobin , making it easier for the second and third oxygen molecules to bind rapidly.

• Haemoglobin also contains four polypeptide chains, each with a heme group, which contains an Fe2+, which can bind reversibly to oxygen to form oxyhaemoglobin

• This allows it to transport oxygen around the body and release or pick up oxygen

Describe how a polypeptide is produced from the translation of mRNA

• mRNA binds to a free ribosome in the cytoplasm or in the rough endoplasmic reticulum

• The tRNA binds to mRNA with a codon complementary to its anticodon

• tRNA carries an amino acid corresponding to the anticodon

• A second tRNA brings a specific amino acid to an adjacent codon

• A peptide bond is formed in a condensation reaction, where energy is released from the hydrolysis of a phosphate bond in ATP

• tRNA then detaches and goes and collects another amino acid

• The ribosome moves along the mRNA sequence one codon at a time, causing a polypeptide chain to grow one one amino acid at a time

• The polypeptide is released and translation stops when stop codon is reached

Give three structural differences between an mRNA molecule and a tRNA molecule

• mRNA contains codons, whereas tRNA contains anticodons

• mRNA is longer than tRNA

• mRNA does not contain an amino aicd binding site whereas tRNA does
  
  Extra points

• mRNA doesn’t contain hydrogen bonds whereas tRNA does

• mRNA is a linear single helix whereas tRNA is a folded clover leaf shape

The rate of translation is increased in T cells activated by antigens

Describe the role of tRNA in translation

tRNA binds to an mRNA molecule in the ribosome with a codon complementary to its anticodon

tRNA carries an amino acid to the ribosome

Condensation reaction occurs between adjacent amino acids using energy released from hydrolysis of ATP to form a peptide bond

Give 4 structural differences between mRNA and DNA

• DNA contains thymine, whereas mRNA contains uracil

• DNA is longer than mRNA

• DNA is double-stranded (double helix), whereas mRNA is single stranded (single helix)

• DNA contains hydrogen bonds, whereas mRNA does not

Extra points

• DNA contains deoxyribose sugar, whereas mRNA contains ribose sugar.

What is a gene?

A sequence of DNA nucleotide bases which result in the production of a specific functional polypeptide

Describe how the production of messenger RNA (mRNA) in a eukaryotic cell is different from the production of mRNA in a prokaryotic cell

In eukaryotic cells only, pre-mRNA is produced

This is because eukaryotic cells only contain introns and prokaryotic cells do not.

Therefore, in eukaryotic cells, splicing must occur to remove these introns

Describe the role of ATP in the process of translation of protein synthesis

Energy is released from the hydrolysis of ATP (phosphate bonds)

This provides energy for the formation of a peptide bond in a condensation reaction between amino acids