Transcription and Translation
- Junessa Masaya
- Apr 15
- 3 min read
HSC Biology | Free Study Notes
In this lesson
what transcription is
what translation is
the roles of mRNA and tRNA
what codons are
how ribosomes are involved in polypeptide synthesis
Why transcription and translation matter
DNA stores genetic information, but proteins are the molecules that carry out many important cell functions.
Cells need a way to:
use the code in DNA
transfer that code to the ribosome
build a chain of amino acids in the correct order
This happens through:
transcription
translation
Together, these stages are part of polypeptide synthesis.
What is transcription?
Transcription is the process in which a section of DNA is used to make a complementary strand of mRNA.
What happens in transcription
the DNA unwinds and unzips
one DNA strand acts as a template
a complementary mRNA strand is formed
In transcription, DNA is copied to produce a single strand of mRNA, while DNA replication produces two identical DNA strands.
Where transcription happens
At this level, students should know that transcription happens in the nucleus.
What is mRNA?
mRNA stands for messenger RNA.
Role of mRNA
mRNA carries a complementary copy of a section of DNA that codes for a polypeptide to the ribosomes.
Why mRNA matters
mRNA is important because:
DNA stays in the nucleus
the ribosomes are in the cytoplasm
mRNA carries the genetic instructions from DNA to the ribosome
What is translation?
Translation is the process in which the code on mRNA is used to build a polypeptide.
The syllabus glossary defines translation as the process by which a sequence of nucleotide triplets in mRNA gives rise to a specific sequence of amino acids during synthesis of a polypeptide or protein.
What happens in translation
mRNA attaches to a ribosome
the ribosome reads the mRNA codons in order
tRNA molecules bring amino acids
amino acids are joined together
a polypeptide chain is formed
Codons
A codon is a sequence of three nucleotides on mRNA.
Why codons matter
Each codon codes for:
one specific amino acid
or, in some cases, a start or stop signal
Each codon, three nucleotides, on the mRNA results in the addition of a correct amino acid to form a polypeptide chain.
Key idea
The order of codons on the mRNA determines the order of amino acids in the polypeptide.
What is tRNA?
tRNA stands for transfer RNA.
Role of tRNA
tRNA carries specific amino acids to the ribosome during translation.
Each tRNA has:
an anticodon
a specific amino acid attached
Anticodon pairing
The anticodon on tRNA pairs with a complementary codon on the mRNA strand.
Each tRNA molecule has an anticodon that pairs with a complementary codon on mRNA, and as each tRNA brings its specific amino acid to the ribosome, the polypeptide is assembled.
Ribosomes
What ribosomes do
Ribosomes:
bind to mRNA
read the codons in sequence
help position tRNA molecules correctly
join amino acids together to form a polypeptide
Why ribosomes matter
Without ribosomes, the amino acids would not be assembled into the correct sequence.
Polypeptide synthesis
Polypeptide synthesis is the production of a chain of amino acids based on the genetic code.
Simple sequence
A section of DNA is transcribed into mRNA.
mRNA leaves the nucleus and goes to a ribosome.
The ribosome reads the mRNA codons.
tRNA brings the correct amino acids.
Amino acids are joined together.
A polypeptide is formed.
Why sequence matters
The amino acid sequence is important because it determines the structure and function of the final protein.
Transcription and translation compared
Process | What happens | Main product |
Transcription | DNA is copied into a complementary RNA strand | mRNA |
Translation | mRNA code is used to assemble amino acids | Polypeptide |
Worked example
Exam-style question
Explain the roles of mRNA and tRNA in polypeptide synthesis.
Worked answer
mRNA carries the genetic instructions from DNA in the nucleus to the ribosome in the cytoplasm. During translation, the ribosome reads the mRNA codons in sequence. tRNA molecules then bring specific amino acids to the ribosome, and each tRNA anticodon pairs with a complementary codon on the mRNA so the amino acids can be assembled into a polypeptide.
Why this works
This answer:
explains both molecules clearly
includes codons and anticodons
links both to polypeptide synthesis
Common mistakes
Confusing transcription with translation.
Saying mRNA carries amino acids. tRNA carries amino acids.
Forgetting that codons are found on mRNA.
Mixing up codons and anticodons.
Saying ribosomes make mRNA rather than translate it.
Quick quiz
What is transcription?
What is translation?
What does mRNA do?
What does tRNA do?
What is a codon?
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