Meiosis, Inheritance, and Gene Expression: Comprehensive Notes

Meiosis and Sexual Life Cycles

  • Genes and Genomes
    • Blueprints: Physical structures composed of DNA.
    • Genes: Segments/regions within genomes that encode something (i.e., they can be read or "transcribed" to make mRNA).
    • Genome Contents:
      • Coding: Genes (~2% of human genome).
      • Non-coding: 'Junk,' viruses, repeat sequences, transposable elements.
    • Genome Types:
      • Prokaryotes and viruses: Bacterial and viral (DNA or RNA) genomes.
      • Eukaryotes:
        • Nuclear genome (located in the nucleus).
        • Organelles: Mitochondrial and chloroplast genomes (located in mitochondria and chloroplasts).
    • Chromosomes:
      • Long DNA molecule.
      • Part or all of the genetic material of an organism (i.e., a genome is made up of one or more chromosomes).
      • Contains coding (genes) and non-coding DNA.
    • Genomes: An organism's complete set of DNA (or RNA in many viruses).
      • Full set of chromosomes.
      • Each genome contains all the information (genes) needed to build and maintain an organism.
    • Genomics: Investigate the properties of a genome, often in comparison with genomes of other species (comparative genomics).
    • Genetics: Investigate properties of one or more genes to understand heredity and variation.
    • Human Genome:
      • 22 paired chromosomes plus X and Y (=46 total).
      • 3 billion base pairs.
      • Coding sequence = 2%; non-coding = 98%.
    • Genes: Molecular unit of heredity.
      • Region of DNA that encodes for a functional RNA or protein product.
      • Every person has two copies of each gene, one inherited from each parent.
      • Alleles: Tens of same gene with small differences in DNA sequence.
        • Differences contribute to unique physical features.
      • Genes specify proteins via transcription and translation.
    • Evolution of Genetic Code:
      • Genetic code is nearly universal; all living things have DNA (RNA for some viruses).
    • Heredity: Transmission of genes from one generation to the next.
    • Variation: Differences in offspring from parent and siblings.

Reproduction

  • Asexual Reproduction: Single individual passes its genes to its offspring.
    • Clone: Group of genetically identical individuals.
  • Sexual Reproduction: Two parents give rise to offspring having unique combinations of genes inherited from parents.
    • Gamete: Cell that fuses with another cell to form a new organism.
      • Contains a single set of chromosomes → haploid (n).
        • Unfertilized egg (ovum): Sex chromosome is X.
        • Sperm cell: Sex chromosome can be X or Y.
    • Somatic Cell: Any cell other than a gamete.
      • Humans have 46 chromosomes in each cell → called diploid (2n) (n=23; 2n=46).
    • Karyotype: An ordered display of chromosome pairs from a cell.
      • Human somatic cells have 23 chromosome pairs.
        • Two chromosomes in each pair are homologous.
      • Sex Chromosomes: Determine the sex of individual (X and Y).
        • Human females: Homologous pair of X chromosomes (XX).
        • Human males: One of each (XY).
      • Autosomes: Non-sex chromosomes

Gametes

  • How they are made
    1. Interphase: Chromosomes duplicate forming sister chromatids.
    2. Meiosis I: Homologous chromosomes pair up and separate → two haploid daughter cells containing replicated chromosomes.
    3. Meiosis II: Sister chromatids separate → four haploid daughter cells containing un-replicated chromosomes.

Meiosis I

  1. Prophase I: Chromosomes condense, homologous chromosomes loosely pair up, homologous chromosomes physically connect and exchange information (synapsis and crossing over).
  2. Metaphase I: Paired homologs line up at the metaphase plate with one chromosome facing each pole; microtubules (spindle fibers) from the poles attach to each pair.
  3. Anaphase I: Pairs of homologous chromosomes separate, chromosomes move toward each pole; sister chromatids remain at the centromere and move as one unit to the pole.
  4. Telophase I and Cytokinesis: Each half of the cell has a haploid set of chromosomes → cell division usually occurs simultaneously → 2 haploid daughter cells.

Meiosis II

  1. Prophase II: Early → spindle apparatus forms; late → chromosomes move towards the metaphase plate.
  2. Metaphase II: Sister chromatids arrange at the metaphase plate, no longer identical; sister chromatids attach to spindle fibers from opposite poles.
  3. Anaphase II: Sister chromatids separate and move as 2 new individual chromosomes towards opposite poles.
  4. Telophase II and Cytokinesis: Chromosomes arrive at opposite poles, nuclei form and chromosomes de-condense, cytokinesis separates the cytoplasm → each daughter cell is distinct from the others and the parent cell.

Mitosis

  • Cells replicate themselves.
  • Results in the production of two genetically identical daughter cells.

Mitosis vs. Meiosis

  • Mitosis: Conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell.
  • Meiosis I: Reduces the number of chromosome sets in half, producing cells that differ genetically from each other and the parent cell.

Genetic Variation

  • Mutations: Changes in organism's DNA.
    • Original source of genetic diversity.
    • Create different versions of genes (alleles).
    • Reshuffling of alleles during sexual reproduction produces genetic variation.
    • Single-nucleotide polymorphism (SNP): Substitution of a single nucleotide at a specific position in the genome.
    • Homeotic genes → control pattern formation
      • Bicoid gene → after front half of body → Supernodulation
      • Nodule number regulated by host plant → balance need for nitrogen vs. ability to expend energy.
      • Mutations in key genes in the autoregulation pathway lead to loss of modulation control and supernodulation phenotype.
      • Clubbed thumb → mirrored organs
  • 3 Main Mechanisms Contributing to Genetic Variation
    1. Independent Assortment of Chromosomes
      • Homologous pairs of chromosomes orient randomly at metaphase I.
      • The number of combinations is 2n2^n.
    2. Crossing Over
      • Combines DNA inherited from each parent.
      • Begins very early in prophase I.
      • Homologous portions of two non-sister chromatids trade places.
    3. Random Fertilization
      • Any sperm can fuse with any ovum.
      • The fusion of two gametes produces a zygote with any of 70 trillion diploid combinations.
  • Evolutionary Significance
    • Natural selection results in the accumulation of genetic variations favored by the environment.
    • Sexual reproduction contributes to genetic variation in a population, which originates from mutations.

Mendel and the Gene Idea

  • Laws of Inheritance
    • Mendel discovered principles of heredity by breeding garden peas.
    • Mendel's Experiments
      • Only tracked characteristics that occurred in 2 distinct forms.
      • True-breeding → produced offspring of the same variety when self-pollinated.
      • Hybridization → mated two contrasting, true-breeding varieties.
      • Observed purple flower color was dominant; white was recessive.
      • ‘Heritable factor’ → now called gene.
    • Concept One
      • Alternative versions of a gene account for variations in inherited factors.
      • Alternative versions of a gene → now called alleles.
    • Concept Two
      • For each characteristic, an organism invents two alleles, one from each parent.
      • Two alleles may be identical (true-breeding) or they may differ.
    • Concept Three
      • The dominant allele determines the organism's appearance, and the recessive allele has no noticeable effect.
    • Concept Four
      • Two alleles separate (segregate) during gamete formation.
      • An egg or a sperm only gets one of the two alleles that are present in the parent organism.
  • Punnett Square
    • Shows possible combinations resulting from sperm and egg.
    • Capital = dominant allele.
    • Lowercase = recessive allele.
    • Homozygous = 2 identical alleles (true-breeding).
    • Heterozygous = 2 different alleles.
    • Monohybrid = heterozygous for only one character/gene.
    • Law of Independent Assortment = each pair of alleles segregates independently of the others.
      • Crossing F1 dihybrids → phenotypic ratio = 9:3:3:1

Complex Genes

  • Non-Mendelian Inheritance

    • When alleles are not completely dominant or recessive.
    • A gene has more than two alleles.
    • When a gene produces multiple phenotypes.

  • Degrees of Dominance

    • Complete dominance: Phenotype of heterozygote and dominant homozygote are identical.
    • Incomplete dominance: F1 hybrid phenotype is somewhere between the phenotypes of the two parents.
    • Co-dominance: Two dominant alleles affect the phenotype in separate, distinguishable ways.
    • Pleiotropy: One gene influences multiple, seemingly unrelated phenotypic traits.
    • Epistasis: One gene alters the phenotypic expression of another gene.

Chromosomal Inheritance

  • Locating Genes Along Chromosomes

    • A gene's location can be seen by tagging isolated chromosomes with fluorescent dye.

  • Morgan's Experiments

    • Chromosomes contain Mendel's heritable factors (genes).
    • Traits alternative to wild type (normal) are called mutant phenotypes.

  • Sex-Linked Genes

    • Genes located on sex chromosomes.

  • Recombination

    • Generates new allele combinations.
    • Genes that are far apart on the same chromosome can have a recombination frequency of nearly 50%.

  • Nondisjunction

    • Homologous chromosomes do not separate normally during meiosis.
    • One gamete receives two of the same chromosome.

  • Aneuploidy

    • Fertilization of gametes where nondisjunction occurred.
    • Offspring have an abnormal number of a particular chromosome.
      • Monosomic zygote: Has only one copy of a particular chromosome.
      • Trisomic zygote: Has three copies of a particular chromosome.

  • polyploidy

    • Organism with two or more complete sets of chromosomes.
      • Triploidy (3n) → 3 sets of chromosomes
      • Tetraploidy (4n) → 4 sets of chromosomes
      • Hexaploidy (6n) → 6 sets of chromosomes

  • Alteration of Chromosomes

    • Deletion: Removes a chromosomal segment.
    • Duplication: Repeats a segment.
    • Inversion: Reverses the orientation of a segment within a chromosome.
    • Translocation: Moves a segment from one chromosome to another.

Gene Expression

  • Transcription and Translation

    • Info in DNA is in the form of specific nucleotide sequences → code for RNA (typically encode for synthesis of proteins (made of amino acids))
    • Proteins link genotype and phenotype
    • Transcription → synthesis of RNA (e.g., mRNA (messenger RNA)) under the direction of DNA
      1. Initiation
      2. Elongation
      3. Termination
        • Promoter → DNA sequence where RNA polymerase attaches
        • Transcription factors mediate the binding of RNA polymerase and initiation of transcription
    • Translation → synthesis of a protein under the direction of mRNA
      • tRNA → carries a specific amino acid on one end and has an anticodon on the other → pairs with a complementary codon on the mRNA
        1. Initiation
        2. Elongation
        3. Termination
    • Ribosomes → translate mRNA into proteins
      • Facilitate the coupling of tRNA with mRNA colons

  • Genetic Code

    • During transcription → one of two DNA strands (template strands) provides the template for mRNA synthesis
    • During translation → mRNA base triplets (codons) are read in the 5' to 3' direction for protein synthesis
    • Each codon specifies the amino acid to be placed along the polypeptide
    • DNA bases → adenine (A), cytosine (C), guanine (G), thymine (T)
    • RNA bases → adenine (A), cytosine (C), guanine (G), uracil (U)

DNA Tools and Biotechnology

  • Biotechnology Tools

    • PCR - Polymerase Chain Reaction
      • Rapid and relatively inexpensive
      • Amplifies DNA and determines presence/absence
      • Generates large volumes of target sequence
      • Uses thermal cycling
    • cDNA Synthesis - Complimentary DNA
      • Very stable compared to mRNA
      • Coding sequence of the gene without introns
    • Real-Time PCR
      • Determines transcript abundance
      • Combines amplification and detection
    • Next-Gen Sequencing
      • Sequences genomes or RNA (RNAseq)
    • Gene Cloning
      • Amplifies sequence of interest

  • Bioinformatics

    • Heaps of programs and bioinformatics tools are available to identify genes, genome and gene structures, introns, promoter regions, establish and visualize gene expression profiles.