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Biology Fundamentals · Genetics and DNA

DNA, genes, and proteins

Reading13mLesson 1 of 3 in this module

Describe how the information in DNA is transcribed into RNA and translated into protein.

Why it matters

Most of modern biotechnology works on the path from DNA to protein. PCR copies DNA. RT-PCR (the basis of many viral tests) starts from RNA. Recombinant insulin is made by putting a human gene into cells so they translate it into protein. Biochemists study how proteins fold and work. To follow any of it, you need the central dogma: DNA makes RNA makes protein.

The core idea: information flow

  • DNA stores the information long-term in the nucleus.
  • Transcription copies one gene into messenger RNA (mRNA).
  • Translation at the ribosome reads the mRNA and builds a chain of amino acids, which folds into a protein.

DNA structure. DNA is a double helix of two strands. Each strand is a chain of nucleotides, each with a sugar (deoxyribose), a phosphate and one of four bases: adenine (A), thymine (T), guanine (G) and cytosine (C). The strands are held together by base pairing: A pairs with T, G pairs with C. Because pairing is fixed, each strand is a template for the other, which is how DNA is copied before a cell divides.

RNA differs in three ways: it is usually single-stranded, its sugar is ribose, and it uses uracil (U) in place of thymine.

A gene is a stretch of DNA that codes for a functional product, usually a protein. Humans have roughly 20,000 protein-coding genes.

Transcription

In the nucleus, the enzyme RNA polymerase binds to a promoter in front of the gene, unwinds the DNA and builds an mRNA strand complementary to the template strand. The mRNA ends up with the same sequence as the other DNA strand (the coding strand), except U replaces T.

In eukaryotes, the first transcript is processed before it leaves the nucleus: a cap and a poly-A tail are added, and non-coding introns are cut out (splicing), leaving the exons joined together.

Translation and the genetic code

At the ribosome, the mRNA is read three bases at a time. Each triplet is a codon, and each codon stands for one amino acid or a stop signal.

  • There are 4 × 4 × 4 = 64 codons for 20 amino acids, so most amino acids have several codons. The code is redundant.
  • AUG codes for methionine and is the start codon.
  • UAA, UAG and UGA are stop codons.
  • Transfer RNA (tRNA) molecules carry amino acids. Each has an anticodon that pairs with a codon, so the right amino acid is added in the right order.

Worked example: from gene to peptide

Coding strand of a tiny gene (5' to 3'):

DNA coding strand:   ATG GGC TTT AAA TGA
DNA template strand: TAC CCG AAA TTT ACT
mRNA:                AUG GGC UUU AAA UGA
Amino acids:         Met Gly Phe Lys STOP

Step by step:

  1. Write the mRNA by copying the coding strand and swapping T for U (or by pairing with the template strand: A→U, T→A, G→C, C→G).
  2. Split the mRNA into codons from the start codon AUG.
  3. Look each codon up in a codon table: AUG = Met, GGC = Gly, UUU = Phe, AAA = Lys, UGA = stop.
  4. The peptide is Met-Gly-Phe-Lys. Translation stops at UGA, which adds no amino acid.

From chain to working protein

The order of amino acids (the primary structure) determines how the chain folds into its 3D shape, and the shape determines what the protein does: an enzyme's active site, an antibody's binding pocket, a channel in the membrane. Heat, extreme pH or some chemicals can unfold (denature) a protein and destroy its function. That is why many lab enzymes and samples are kept cold.

Common mistakes

  • Using T in RNA. RNA uses U.
  • Reading codons from the wrong place. Translation starts at AUG and reads in non-overlapping triplets; shifting by one base gives a completely different protein.
  • Thinking one gene always gives one protein. Alternative splicing lets one gene make several related proteins.
  • Mixing up transcription and translation. Transcription makes RNA (in the nucleus in eukaryotes); translation makes protein (at ribosomes).

Quick recap

  • DNA (A-T, G-C) stores information; RNA uses U instead of T.
  • Transcription: RNA polymerase copies a gene into mRNA; introns are spliced out in eukaryotes.
  • Translation: ribosomes read codons; tRNAs bring amino acids; AUG starts, UAA/UAG/UGA stop.
  • 64 codons code for 20 amino acids, so the code is redundant.
  • The amino acid sequence determines the fold, and the fold determines the function.
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