Edexcel GCSE Biology (1BI0) — Genetics
A massive architectural blueprint is useless if it cannot be taken out of the architect's office to the building site. Similarly, the original DNA blueprint is a massive, double-stranded molecule that is too large to exit the nucleus. Instead, the cell uses a smaller, single-stranded copy called mRNA (messenger RNA).
First, the newly formed mRNA molecule leaves the nucleus through a tiny opening in the nuclear envelope called a nuclear pore. Next, the mRNA strand enters the fluid-filled space of the cell known as the cytoplasm. Finally, the mRNA moves towards and attaches to a specific binding site on a ribosome, which acts as the cellular factory where protein synthesis will take place.
How can an alphabet of only four letters create instructions for thousands of different complex molecules? The secret lies in the triplet code. Every sequence of three bases on the mRNA strand forms a codon, and each codon provides the exact instruction for one specific amino acid.
Because there are four different RNA bases (A, U, C, and G), a sequence of three creates 64 possible combinations (). This is more than enough to code for the 20 different amino acids used by living organisms. In fact, the genetic code is , meaning some amino acids are coded for by more than one codon.
The ribosome reads the mRNA strand in discrete, non-overlapping blocks of three, starting at a specific 'start' codon and continuing until it hits a 'stop' codon. Stop codons do not code for an amino acid; instead, they signal the ribosome that the sequence is complete.
If a specific functional protein consists of 145 amino acids, how many bases must be in the coding region of the mRNA strand?
Step 1: Identify the mathematical relationship between amino acids and bases.
Step 2: Multiply the number of amino acids by 3.
Step 3: Calculate the final answer.
Think of a postal worker matching the address on a package exactly to the number on a door. Once the mRNA is attached to the ribosome, specific amino acids must be brought from the cytoplasm to the assembly line. This delivery job is carried out by small, "clover-leaf" shaped molecules called tRNA (Transfer RNA).
One end of the tRNA molecule carries a specific amino acid, while the other end features a sequence of three unpaired bases called an anticodon. As the ribosome reads the mRNA strand, a tRNA molecule approaches.
The tRNA anticodon binds to the matching mRNA codon using complementary base pairing. Under these strict rules, adenine (A) always pairs with uracil (U), and cytosine (C) always pairs with guanine (G). This complementary matching ensures that the amino acids are delivered in the precise order dictated by the genetic code.
Have you ever threaded individual beads onto a string to make a necklace? The ribosome functions just like a pair of hands threading those beads. It is large enough to hold two tRNA molecules at the exact same time, bringing two amino acids perfectly side-by-side.
The ribosome then acts as a catalytic site to join these adjacent amino acids together with a strong covalent bond known as a peptide bond. This joining process is a condensation reaction, meaning a molecule of water is released every time a bond is formed.
Next, the ribosome moves along the mRNA strand (translocation) to read the next codon, making room for a new tRNA molecule to arrive. This cycle continues, linking amino acids one by one into a long, growing polypeptide chain. This entire assembly process is called translation.
Finally, when a stop codon is reached, the completed polypeptide is released. It immediately folds into a highly specific 3D shape to become a fully functional protein, such as an enzyme or hormone. Because the order of bases determines the exact sequence of amino acids, any mutation in the DNA sequence can alter the final shape and ruin the function of the protein.
Students often just say mRNA 'leaves the nucleus' — you must explicitly state that it exits through a 'nuclear pore' and binds to a specific 'binding site' on the ribosome to secure full marks in Edexcel exams.
In 6-mark questions describing translation, examiners expect you to explicitly use the phrase 'complementary base pairing' to describe how tRNA interacts with mRNA.
Never confuse the locations of triplets: remember that 'codons' are strictly found on the mRNA strand, while 'anticodons' are located on the tRNA molecules.
When explaining the effect of a mutation, make sure to follow the logical sequence: a change in base sequence leads to a different codon, which attracts a different amino acid, which ultimately changes the 3D shape of the protein.
mRNA (messenger RNA)
A single-stranded RNA molecule that carries a copy of the genetic code from the DNA in the nucleus to the ribosome.
Nuclear pore
Small holes in the nuclear envelope that allow mRNA to exit the nucleus and enter the cytoplasm.
Cytoplasm
The jelly-like substance inside a cell where the ribosomes are located and translation takes place.
Binding site
A specific location on the ribosome where the mRNA strand attaches to begin translation.
Ribosome
The organelle in the cytoplasm where translation occurs and amino acids are joined together.
Triplet code
The system where three consecutive bases provide the specific instruction for one amino acid.
Codon
A sequence of three nucleotides (bases) on an mRNA molecule that codes for a specific amino acid.
Nucleotide
The subunit of DNA and RNA, consisting of a sugar, a phosphate group, and a nitrogenous base.
Amino acid
The building blocks of proteins, which are joined together in a specific order during translation.
Degenerate
A property of the genetic code meaning that some amino acids are coded for by more than one possible codon.
tRNA (Transfer RNA)
Small RNA molecules that transport specific amino acids from the cytoplasm to the ribosome.
Anticodon
A sequence of three bases on a tRNA molecule that is complementary to a specific mRNA codon.
Complementary base pairing
The strict rules of bonding between specific bases, where A pairs with U, and C pairs with G during translation.
Peptide bond
The strong covalent bond that joins two adjacent amino acids together during protein synthesis.
Condensation reaction
A chemical reaction where two molecules are joined together with the release of a water molecule.
Translocation
The movement of the ribosome along the mRNA strand to read the next codon.
Polypeptide chain
A long sequence of amino acids linked together by peptide bonds.
Translation
The process of synthesizing a polypeptide chain at the ribosome using the sequence of codons on an mRNA template.
Protein
A functional molecule formed when a polypeptide chain folds into a highly specific 3D shape.
Mutation
A change in the base sequence of DNA, which can alter the resulting protein.
Put your knowledge into practice — try past paper questions for Biology
mRNA (messenger RNA)
A single-stranded RNA molecule that carries a copy of the genetic code from the DNA in the nucleus to the ribosome.
Nuclear pore
Small holes in the nuclear envelope that allow mRNA to exit the nucleus and enter the cytoplasm.
Cytoplasm
The jelly-like substance inside a cell where the ribosomes are located and translation takes place.
Binding site
A specific location on the ribosome where the mRNA strand attaches to begin translation.
Ribosome
The organelle in the cytoplasm where translation occurs and amino acids are joined together.
Triplet code
The system where three consecutive bases provide the specific instruction for one amino acid.
Codon
A sequence of three nucleotides (bases) on an mRNA molecule that codes for a specific amino acid.
Nucleotide
The subunit of DNA and RNA, consisting of a sugar, a phosphate group, and a nitrogenous base.
Amino acid
The building blocks of proteins, which are joined together in a specific order during translation.
Degenerate
A property of the genetic code meaning that some amino acids are coded for by more than one possible codon.
tRNA (Transfer RNA)
Small RNA molecules that transport specific amino acids from the cytoplasm to the ribosome.
Anticodon
A sequence of three bases on a tRNA molecule that is complementary to a specific mRNA codon.
Complementary base pairing
The strict rules of bonding between specific bases, where A pairs with U, and C pairs with G during translation.
Peptide bond
The strong covalent bond that joins two adjacent amino acids together during protein synthesis.
Condensation reaction
A chemical reaction where two molecules are joined together with the release of a water molecule.
Translocation
The movement of the ribosome along the mRNA strand to read the next codon.
Polypeptide chain
A long sequence of amino acids linked together by peptide bonds.
Translation
The process of synthesizing a polypeptide chain at the ribosome using the sequence of codons on an mRNA template.
Protein
A functional molecule formed when a polypeptide chain folds into a highly specific 3D shape.
Mutation
A change in the base sequence of DNA, which can alter the resulting protein.
