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Biology Unit 6: Gene expression and regulation

DNA stores instructions, RNA carries them, and ribosomes build proteins. Cells control which genes are expressed, and mutations change the instructions.

Covers: DNA structure and replication, transcription, translation, gene regulation, mutations, biotechnology

Study guide

DNA is a double helix of two antiparallel strands. Each nucleotide has a deoxyribose sugar, a phosphate and one of four bases. Bases pair by hydrogen bonds: adenine with thymine and cytosine with guanine. Replication is semiconservative: helicase unzips the helix, DNA polymerase adds nucleotides to build each new strand from 5′ to 3′ (continuously on the leading strand and in Okazaki fragments on the lagging strand), and ligase seals the gaps. Each new molecule keeps one original strand.

Gene expression follows the central dogma: DNA → RNA → protein. In transcription, RNA polymerase reads a gene and builds a complementary mRNA, using uracil instead of thymine. In eukaryotes this happens in the nucleus, and the mRNA is processed: introns are removed by splicing, and a 5′ cap and poly-A tail are added. In translation at the ribosome, mRNA is read in codons (three-base units). Each tRNA carries a specific amino acid and has an anticodon that pairs with a codon. Translation starts at AUG and ends at a stop codon, and the genetic code is nearly universal.

Gene regulation controls which genes are expressed. Bacteria use operons, such as the lac operon, which is turned on when lactose is present. Eukaryotes regulate with transcription factors, chromatin packing, RNA processing and more, which is how different cells with the same DNA become specialized.

Mutations change DNA. Point mutations can be silent, missense (a different amino acid) or nonsense (an early stop). Insertions and deletions cause frameshifts that change every codon downstream. Biotechnology tools include PCR to copy DNA, gel electrophoresis to separate fragments by size, DNA sequencing, recombinant DNA with plasmids, and CRISPR gene editing.

Small RNAs such as microRNAs and siRNAs regulate genes after transcription. They pair with matching mRNA and guide a protein complex to destroy it or block its translation, which lowers the amount of protein made. Researchers use synthetic siRNAs to knock down a chosen gene. Most amino acids are coded by more than one codon, so the code is redundant, and that is why many silent mutations have no effect. The tRNAs that read different synonymous codons can be more or less plentiful, so genes rich in common-tRNA codons are translated faster.

An operon is a group of bacterial genes under one promoter and operator. In the inducible lac operon, a repressor sits on the operator until lactose binds to it and removes it. In a repressible operon like trp, the end product (tryptophan) activates the repressor, which switches the genes off. In gel electrophoresis, DNA is negatively charged, so it moves toward the positive electrode, and smaller fragments travel farther. Epigenetic changes, such as DNA methylation or histone modification, change how much a gene is expressed without changing its sequence, and some can be inherited.

Key ideas

• DNA base pairs: A–T and C–G. RNA uses U instead of T.
• Replication is semiconservative: each new DNA has one old and one new strand.
• Transcription (nucleus) makes mRNA; translation (ribosome) reads codons to make protein.
• Each codon is 3 bases and codes for one amino acid.
• Mutations: substitutions may be silent, missense or nonsense; insertions and deletions cause frameshifts.

Formulas and rules

Common mistakes

• Writing T in mRNA. RNA uses U.
• Thinking every mutation is harmful. Many are silent.

Words to know

Uracil (U)

The RNA base that pairs with A, replacing thymine.

Transcription

Making mRNA from DNA; happens in the nucleus in eukaryotes.

Codon

Three mRNA bases that code for one amino acid.

Semiconservative replication

Each new DNA molecule keeps one original strand.

Frameshift mutation

An insertion or deletion that shifts the reading frame.

Double helix

DNA's shape: two antiparallel strands with paired bases.

Base pairing rules

A–T and C–G in DNA; A–U in RNA.

DNA polymerase

Builds new DNA strands 5′ → 3′.

Helicase

Unzips the DNA helix.

Okazaki fragments

Short DNA pieces made on the lagging strand.

Translation

Building a protein from mRNA at the ribosome.

Anticodon

The tRNA sequence that pairs with an mRNA codon.

Intron / exon

Noncoding segments removed by splicing / coding segments kept.

Operon

A bacterial gene cluster with one promoter, like the lac operon.

Point mutation

A single-base change: silent, missense or nonsense.

PCR

Polymerase chain reaction: copies DNA many times.

Gel electrophoresis

Separates DNA fragments by size.

CRISPR

A gene-editing tool that cuts DNA at a chosen sequence.

Biotechnology

Using DNA tools like PCR, sequencing, recombinant plasmids and CRISPR.

Sample practice questions

21 more questions like these, from level 1 to 5, are in the practice set.