Unit 6: Gene Expression and Regulation

22 terms. Say what each one means, then open the card to check.

• Nucleotide

  The building block of DNA and RNA: a five-carbon sugar, a phosphate group and a nitrogen base. Nucleotides join sugar to phosphate to form a strand.

  Example. A DNA nucleotide might hold deoxyribose, a phosphate and the base guanine.

• Antiparallel strands

  The two strands of a DNA double helix run in opposite directions: one runs 5' to 3' and its partner runs 3' to 5'.

  Example. If one strand reads 5'-GATC-3', the strand paired with it reads 3'-CTAG-5'.

• Complementary base pairing

  In DNA, A pairs with T by two hydrogen bonds and G pairs with C by three. So in double-stranded DNA the amount of A equals T and G equals C.

  Example. A double-stranded DNA sample that is 22 percent A is also 22 percent T, leaving 28 percent each for G and C.

• Semiconservative replication

  DNA copying in which each new double helix keeps one original strand and gains one newly built strand.

  Example. After one round of copying, each of the two daughter DNA molecules has one old strand paired with one new strand.

• DNA polymerase

  The enzyme that builds new DNA by adding nucleotides that pair with a template strand. It can add only to the 3' end of a growing strand.

  Example. DNA polymerase reads a template base A and adds a T to the 3' end of the new strand.

• Okazaki fragments

  Short pieces of DNA made on the lagging strand, which is built away from the fork in pieces. Ligase later joins the pieces into one strand.

  Example. As a fork opens, the lagging strand gets a new primer every so often, and a short fragment is built from each.

• DNA ligase

  An enzyme that seals breaks in the sugar-phosphate backbone, joining pieces of DNA into one continuous strand.

  Example. After primers are replaced, ligase links neighboring lagging-strand fragments. It also joins a gene into a plasmid in the lab.

• Promoter

  A DNA sequence just upstream of a gene where RNA polymerase, with help from other proteins, binds to start transcription.

  Example. A mutation that destroys a gene's promoter can stop the gene from being transcribed at all.

• RNA processing

  The changes a eukaryotic pre-mRNA goes through in the nucleus: a 5' cap is added, a poly-A tail is added, and introns are spliced out.

  Example. A pre-mRNA of 8,000 nucleotides becomes a mature mRNA of 2,500 nucleotides once its introns are removed.

• Alternative splicing

  Joining different sets of exons from one pre-mRNA, so one gene can code for more than one version of a protein.

  Example. A fictional gene's mRNA keeps exon 4 in nerve cells but drops it in liver cells, giving two related proteins.

• Codon

  A group of three mRNA nucleotides that stands for one amino acid or a stop signal. Most amino acids have more than one codon.

  Example. The codons GGA and GGC both stand for glycine, so either one adds glycine to the chain.

• Transfer RNA (tRNA)

  An RNA that carries a specific amino acid and has an anticodon that pairs with a matching mRNA codon on the ribosome.

  Example. A tRNA with the anticodon 3'-UAC-5' pairs with the start codon 5'-AUG-3' and brings methionine.

• Translation

  Building a polypeptide from mRNA. A ribosome reads codons from the start codon AUG to a stop codon, and tRNAs bring the matching amino acids.

  Example. A ribosome reading AUG AAA GGA UGA makes the chain methionine, lysine, glycine and then stops.

• Operon

  In bacteria, a group of genes under one promoter and one operator, so they are switched on or off together.

  Example. Three genes for enzymes in one fictional sugar pathway sit side by side and are copied into one mRNA.

• Repressor

  A protein that blocks transcription by binding to an operator, keeping RNA polymerase from moving along the genes.

  Example. When a repressor sits on its operator, the enzymes of that operon are made only in tiny amounts.

• Transcription factor

  A protein in eukaryotes that binds DNA near a gene and helps or hinders RNA polymerase. The set of factors in a cell decides which genes it uses.

  Example. A factor found only in fictional lens cells binds near the genes for clear lens proteins and turns them on.

• Epigenetic change

  A change in gene expression that does not change the DNA sequence, such as adding methyl groups to DNA or acetyl groups to histones, and that can pass to daughter cells.

  Example. Heavy methylation near a gene's promoter keeps the gene silent in skin cells and in the skin cells that come from them.

• Substitution mutation

  A change of one nucleotide for another. It may be silent (same amino acid), missense (a different amino acid) or nonsense (an early stop codon).

  Example. The codon AAA becoming AAG is silent, since both stand for lysine.

• Frameshift mutation

  An insertion or deletion of a number of nucleotides that is not a multiple of three. It changes how every codon after it is read.

  Example. Deleting one nucleotide early in a gene turns the rest of the message into different codons and often an early stop.

• Polymerase chain reaction (PCR)

  A lab method that copies one region of DNA many times by repeated cycles of heating to separate strands, cooling so primers bind, and building with a heat-stable polymerase.

  Example. Starting from one copy of a gene, 10 cycles of PCR can make about 2 to the power 10, or 1,024, copies.

• Gel electrophoresis

  A method that sorts DNA fragments by size. DNA is negative, so it moves toward the positive electrode, and shorter fragments move farther.

  Example. A fragment of 300 base pairs ends up closer to the positive end of a gel than a fragment of 1,200 base pairs.

• Bacterial transformation

  The uptake of outside DNA, such as a plasmid, by a bacterium, which can then express the genes it took up.

  Example. Bacteria that take up a plasmid carrying a resistance gene survive on a plate with that antibiotic.