Genetics: DNA, Genes and Chromosomes

Introduction to Genetics

• Genetics examines inheritance, specifically how traits are passed from parents to their offspring.
• Inherited traits include things such as dimpled chins.

DNA: The Genetic Material

• DNA (Deoxyribonucleic Acid) stores genetic information.
• Nucleotides compose DNA, which contain:
  • A 5-carbon sugar called deoxyribose.
  • A phosphate group.
  • A nitrogenous base, which can be Adenine, Thymine, Cytosine, or Guanine.
• Adenine (A) pairs with Thymine (T) in DNA.
• Guanine (G) pairs with Cytosine (C) in DNA.

Structure of DNA

• DNA has a double helix structure, like a twisted ladder.
• The sugar-phosphate backbone makes up the sides of the ladder.
• Nitrogen bases form the rungs.
• Franklin, Watson, and Crick are among those who discovered DNA structure.

Genes and Chromosomes

• A gene is a DNA segment that codes for a protein.
• DNA wraps around histones to create nucleosomes.
• Nucleosomes condense into chromosomes and provide protection.

DNA Replication

• DNA replication occurs before cell division.
• The steps of replication include:
  • DNA unwinds from histones.
  • Helicase unzips the DNA strands at the replication fork.
  • DNA polymerase matches nucleotides to create new strands.
  • One strand is continuously copied and called the leading strand.
  • The other strand is copied in fragments termed the lagging strand.
• DNA replication is semi-conservative because each new DNA molecule has one original and one new strand.

RNA and Transcription

• RNA compared to DNA:
  • RNA is single-stranded.
  • RNA contains ribose sugar.
  • RNA uses uracil (U) instead of thymine (T).
• Three types of RNA:
  • Messenger RNA (mRNA) carries the genetic code.
  • Ribosomal RNA (rRNA) is a component of ribosomes.
  • Transfer RNA (tRNA) brings amino acids to ribosomes.
• Transcription involves:
  • RNA polymerase unwinding DNA and copying a gene into mRNA.
  • mRNA following base-pairing rules, replacing T with U.
  • mRNA splicing, which removes introns and keeps exons, before it leaves the nucleus.

Translation: Protein Synthesis

• mRNA travels to ribosomes where proteins are synthesized.
• The steps include:
  • mRNA is read in codons (three-base sequences).
  • tRNA matches codons with amino acids.
  • Linked amino acids form a polypeptide chain through peptide bonds.
  • The process stops at a "stop" codon.

Gene Regulation

• Cells regulate gene activity.
• Prokaryotic regulation uses operons, turning genes on or off, like the lac operon.
• Eukaryotic regulation uses transcription factors to control DNA accessibility.

Mutations and Genetic Changes

• Point mutations affect a single nucleotide.
  • Silent mutations cause no change in the protein.
  • Missense mutations changes one amino acid.
  • Nonsense mutations create a stop codon, halting protein production.
• Frameshift mutations: insertion or deletion shifts the reading frame, so alters all downstream amino acids.
• Chromosomal mutations include deletion, duplication, inversion, and translocation.
• Mutations originate from replication errors and mutagens such as radiation and chemicals.
• Mutations can be harmful, beneficial, or neutral.

Summary

• DNA stores genetic information and replicates prior to cell division.
• RNA is involved in protein synthesis through transcription and translation.
• Gene expression is regulated to control protein production.
• Mutations can change genes and proteins, which influences traits and evolution.

Vocabulary

• Insertion: Addition of one or more nucleotide base pairs into a DNA sequence.
• Deletion: A mutation in which one or more pairs of nucleotides are removed from a gene.
• Frameshift: A genetic mutation caused by indels (insertions or deletions) of a number of nucleotides in a DNA sequence that is not divisible by three
• Point mutation: A mutation affecting only one or very few nucleotides in a gene sequence.
• Nonsense mutation: A mutation in which a sense codon that corresponds to one of the twenty amino acids specified by the genetic code is changed to a chain-terminating codon.
• Missense mutation: A genetic mutation where a single nucleotide change results in a codon that codes for a different amino acid.
• Silent mutation: Mutations in DNA that do not have an observable effect on the organism's phenotype.
• Mutagen: A chemical or physical agent that increases the rate of mutation.
• Codon: A sequence of three nucleotides that together form a unit of genetic code in a DNA or RNA molecule.
• Peptide: A compound consisting of two or more amino acids linked in a chain.
• Polypeptide: A chain of amino acids linked together by peptide bonds.
• Anticodon: A sequence of three nucleotides forming a unit of genetic code in a transfer RNA molecule, corresponding to a complementary codon in messenger RNA.
• Ribose: A sugar of the pentose class that occurs widely in nature as a constituent of nucleosides and nucleotides.
• Ribosome: A structure within a cell that is responsible for protein synthesis.
• mRNA: Messenger ribonucleic acid; a type of RNA that carries genetic information from DNA in the nucleus to the cytoplasm, where it is used to produce proteins.
• tRNA: Transfer ribonucleic acid; a type of RNA that carries amino acids to the ribosome for protein synthesis.
• rRNA: Ribosomal ribonucleic acid; a type of RNA that is a component of ribosomes.
• Enzyme: A substance produced by a living organism that acts as a catalyst to bring about a specific biochemical reaction.
• Amino acid: A simple organic compound containing both a carboxyl and an amino group.
• Cytosine: A pyrimidine base found in DNA and RNA.
• Guanine: A purine base found in DNA and RNA.
• Thymine: A pyrimidine base found in DNA.
• Uracil: A pyrimidine base found in RNA.
• Adenine: A purine base found in DNA and RNA.
• Deoxyribose: A sugar derived from ribose by replacing a hydroxyl group with hydrogen.
• DNA: A self-replicating material present in nearly all living organisms as the main constituent of chromosomes. It is the carrier of genetic information.
• Double helix: The form of native DNA, referring to its two adjacent polynucleotide strands wound into a spiral shape.
• Chromosome: A threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.
• Transcription: The process by which the information in a strand of DNA is copied into a new molecule of messenger RNA (mRNA).
• Translation: The process by which a protein is synthesized from the information contained in a molecule of messenger RNA (mRNA).
• Gene: A distinct sequence of nucleotides forming part of a chromosome, the order of which determines the order of monomers in a polypeptide or nucleic acid molecule which a cell (or virus) may synthesize.
• Nucleotide: A compound consisting of a nucleoside linked to a phosphate group. Nucleotides form the basic structural unit of nucleic acids such as DNA.
• Mutation: The changing of the structure of a gene, resulting in a variant form that may be transmitted to subsequent generations, caused by alteration of single base units in DNA, or the deletion, insertion, or rearrangement of larger sections of genes or chromosomes.
• Start codon: The first codon of a messenger RNA (mRNA) transcript translated by a ribosome.
• Stop codon: A nucleotide triplet within messenger RNA that signals a termination of translation into proteins.
• Nucleic acid: A complex organic substance present in living cells, especially DNA or RNA, whose molecules consist of many nucleotides linked in a long chain.