Protein synthesis

What this reading is about

In the last block, you learned that dnaThe molecule that carries the genetic instructions for life. Shaped like a twisted ladder. is the molecule that carries the genetic instructions for life. But how does a cell actually use those instructions? How do the bases A, T, C, and G turn into eyes, muscles, blood, and bone?

The answer is: cells use DNA to build proteins, and proteins do almost every job in the body. The process of building proteins from DNA is called protein synthesis.

This reading covers two steps:

1. Transcription — copying a section of DNA into mrnaMessenger RNA — a single-stranded copy of a gene that carries the message from DNA to the ribosome..
2. Translation — reading the mRNA at a ribosomeThe cell structure where proteins are built. The ribosome reads mRNA and links amino acids together. to build a protein.

The central dogma

The big rule for this whole topic is called the central-dogmaThe flow of genetic information in a cell: DNA → RNA → protein.:

DNA → mRNA → protein

Information flows in one direction. DNA gets copied into mRNA, and mRNA gets read to make a protein. This is true for nearly every living thing on Earth, from bacteria to plants to you. It is one of the strongest pieces of evidence that all life on Earth shares a common ancestor.

The two steps each happen in a different place inside the cell:

• Transcription happens in the nucleus, where the DNA lives.
• Translation happens at a ribosome, outside the nucleus.

The mRNA is the messenger that travels between the two — it carries the message from the gene out to the ribosome.

Step 1: Transcription (DNA → mRNA)

transcriptionThe first step of protein synthesis: a DNA gene is copied into a strand of mRNA. is the first step. It happens inside the nucleus. A section of DNA — a geneA section of DNA that holds the instructions to build one protein. — gets copied into a strand of mRNA.

The steps are:

1. The DNA unzips in the region of the gene. The two strands separate.
2. One strand acts as a template. RNA bases float in and pair up with the exposed DNA bases.
3. The pairing rule for transcription has one big change from DNA pairing: T becomes U.

The pairing chart for transcription:

• DNA A pairs with mRNA U
• DNA T pairs with mRNA A
• DNA C pairs with mRNA G
• DNA G pairs with mRNA C

RNA does not have thymine (T). It uses uracil (U) instead. Wherever the DNA template strand has a base, the mRNA has its complement — but with U replacing T.

When transcription is done, the new mRNA strand leaves the nucleus through a pore in the nuclear membrane. The DNA zips back up unchanged. The original gene is still there, ready to be transcribed again the next time the cell needs that protein.

Step 2: Translation (mRNA → protein)

translationThe second step of protein synthesis: mRNA is read at the ribosome and used to build a protein. is the second step. It happens at a ribosomeThe cell structure where proteins are built. The ribosome reads mRNA and links amino acids together., in the cytoplasm of the cell, outside the nucleus. The ribosome reads the mRNA and builds a protein from An amino acid is a small molecule that is the **building block of a protein**. Proteins are long chains of amino acids linked together. There are about 20 different amino acids that living things use. Each codon on an mRNA strand codes for one amino acid.s.

The steps are:

1. The mRNA arrives at the ribosome. The ribosome locks onto one end.
2. The ribosome reads the mRNA three bases at a time. Each group of three bases is called a codonA group of three mRNA bases that codes for one amino acid..
3. For each codon, a matching trnaTransfer RNA — a small RNA molecule that carries an amino acid to the ribosome during translation. arrives, carrying one amino acid.
4. The amino acids link together in the order set by the codons. This growing chain is the protein.
5. When the ribosome reads a stop codon (UAA, UAG, or UGA), it releases the finished protein.

Each codon codes for one amino acid. To find which amino acid a codon codes for, you look it up on a codon chart. You will have a codon chart for the test if you need to translate a sequence — you do not memorize codons.

Codon math: 3 bases = 1 amino acid

The most important number rule for protein synthesis is:

3 mRNA bases = 1 amino acid

This means:

• Number of codons = number of mRNA bases ÷ 3.
• Number of amino acids = number of codons.

For some questions you may also need to start from DNA. If a DNA gene has 24 bases, it transcribes into an mRNA with 24 bases, which gets read as 24 ÷ 3 = 8 codons, which builds a protein with 8 amino acids.

Why this matters

Protein synthesis matters because proteins do almost every job in your body. They are enzymes that speed up chemical reactions. They are antibodies that fight infection. They are hormones that send signals. They are the muscle fibers that let you move and the keratin in your hair and nails. Without proteins, life as we know it does not work.

And proteins all start the same way: a gene in your DNA gets transcribed into mRNA, the mRNA travels to a ribosome, and the ribosome translates the codons into a chain of amino acids. The order of bases in the gene decides the order of amino acids in the protein, which decides the protein's shape, which decides its job.

This is also why mutations matter — which is the next topic. A small change in the DNA bases can change one codon, which can change one amino acid, which can change the shape of a protein, which can change what it does (or whether it works at all). Sickle-cell disease, for example, comes from a single-base change in one gene. The next block builds on this one.