BIOB11 - L1 AUDIO

Units of Inheritance

  • Experiments led to the concept of units of inheritance, crucial for understanding Darwinian evolution.

  • Units of inheritance are passed down through generations, dictating traits.

  • Early scientists hypothesized an inheritable code for traits, predating the discovery of DNA.

Chromosomes as Heritable Material

  • Early microscopists observed chromosomes lining up and moving during cell division.

  • Chromosomes were identified as the likely heritable material.

  • Chromosomes get copied and separated during cell division.

  • Genetic material gets scrambled during gamete production (sperm and egg).

  • Crossing over reshuffles genetic material before it's passed to offspring.

  • Mutagenesis experiments in Drosophila (fruit flies) showed that specific genes have specific positions on chromosomes.

  • Genes at specific locations on chromosomes are associated with specific traits.

  • Scientists mapped gene locations on chromosomes before understanding DNA.

DNA as the Genetic Material

  • The chemical structure of DNA revealed that it is the genetic material encoding genes (units of inheritance).

  • The structure of the DNA molecule is the basis for understanding genetics.

Chromosomes and Genetic Information

  • Chromosomes carry genetic information.

  • Every cell originates from another cell, and nuclear material within cells organizes into visible threads (chromosomes).

  • During cell division, a full set of chromosomes is passed to daughter cells.

Experiments Proving DNA's Role

  • Experiments with bacteriophages (viruses that infect bacteria) demonstrated that DNA is passed into bacterial cells.

  • Viral DNA contains the template to make more viruses.

  • Bacteria can take up DNA from other bacteria and pass on traits.

Bacterial Transformation Experiment

  • A strain of bacteria with virulence (ability to cause disease) was used.

  • RNA, protein, DNA, lipid, and carbohydrates were collected separately from the virulent bacteria.

  • These components were fed to non-virulent bacteria.

  • Only DNA was able to give the trait of virulence to the other bacteria.

  • DNA contains the information to pass on new traits.

DNA's Role in Heritability

  • DNA carried in living organisms is responsible for heritability (passing on traits).

  • Altering DNA can affect traits.

  • Diseased cells often have changes in their DNA.

  • Cancer cells commonly exhibit alterations in their genome (extra chromosomes, moved DNA pieces).

  • These DNA changes contribute to the disease.

  • Passing on traits occurs via DNA, and disrupting DNA can disrupt traits.

  • Evidence supports DNA as heritable material.

Meiosis vs. Mitosis Review

  • Comparison of meiosis and mitosis.

  • Review of concepts from previous courses (A01 material). DNA Replication

    • Mitosis: DNA is replicated once.

    • Meiosis: DNA is replicated once.
      Cell Division

    • Mitosis: Cell divides once.

    • Meiosis: Cell divides twice.
      Chromosome Separation

    • Mitosis: Sister chromatids are pulled apart.

    • Meiosis: Sister chromatids are pulled apart in meiosis II.

    • Homologous chromosomes are not separated in mitosis.

    • Homologous chromosomes are separated into separate cells during meiosis I.
      Final Cells

    • Mitosis: Daughter cells have identical genetic information to the parent cell.

    • Meiosis: Cells do not have the exact same genetic material, non-identical to the parent cell, and non-identical to each other.

  • Meiosis results in haploid cells (half the amount of DNA; 1n).

  • Mitosis results in diploid cells (2n).

  • Gametes (sperm and egg) are haploid cells that combine to form a diploid organism.

  • Meiosis is the process to make gametes.

Laws of Inheritance

  • Understanding heredity allows us to predict and change our nature and the organisms we work with.

  • Inheritance laws originate from Mendel's work with peas.

  • Pea plants are easy to control for fertilization.

  • Mendel tracked traits over generations by selecting parents and observing offspring.

  • He identified visible phenotypes (seed color, plant height, flower color).

  • Some traits are dominant over others.

Mendel's Lucky Choice

  • Mendel's success was due to his choice of pea plants.

  • Pea plants had traits controlled by one gene with dominant and recessive alleles.

  • Most traits are controlled by multiple genes with complex genetics.

  • Traits often involve risk factors and probabilities.

  • Penetrance = likelihood that a phenotype will show through.

  • Probability = increase or decrease in the likelihood of displaying a trait.

Contest and Review

  • Review of meiosis. Key point: two divisions in meiosis.

  • Haploid-amount of DNA: the amount is denoted by the letter/letters D, N, and A.

Diploid Individuals

  • Each diploid individual has two copies of each gene (one from each parent).

  • Two copies of each chromosome (one from each parent).

  • Genes from mother and father may not be identical.

  • The $A$ gene is depicted by $A$ and $a$ . They are located at the same spot on the same chromosome. They encode the same protein but not might be identical with some slight changes.

Alleles

  • Alleles are different versions of the same gene.

  • Homologous chromosomes have the same set of genes in the same positions.

  • Humans have 23 pairs of homologous chromosomes (including sex chromosomes).

Dominant and Recessive Alleles

  • Alleles can have different effects on the phenotype.

  • A dominant allele is strong enough to cause a particular phenotype.

  • A recessive allele's phenotype is masked by a dominant allele.

  • Phenotype = observable trait; Genotype = genetics.

  • Homozygous: both copies of the gene are the same (e.g., $AA$ or $aa$).

  • Heterozygous: two different alleles (e.g., $Aa$).

  • The recessive phenotype is only observable in the offspring if it's homozygous recessive ($aa$).

Laws of Inheritance: Segregation

  • The Law of Segregation: during meiosis, maternal and paternal chromosomes segregate from one another.

  • Gametes contain one allele for each gene (not both).

  • This occurs during meiosis I when chromosomes are pulled to separate cells.

Laws of Inheritance: Independent Assortment

  • The Law of Independent Assortment: different genes/traits separate into gametes independently.

  • Alleles do not influence each other.

  • Homologous chromosomes separate in meiosis I.

  • Genes on separate chromosomes sort independently into gametes.

Importance of Independent Assortment

  • Independent assortment allows for different combinations of traits.

  • Rescrambling and repackaging create fitter individuals through genetics and evolution.

Punnett Squares

  • Punnett squares help visualize gamete combinations and offspring genotypes.

  • Determine the gametes from each parent. Mom: big $Y$, little $r$; dad: small $y$, big $r$.

  • The way to make your Punnett square is to figure out what your gametes are from each individual, and list your gamete combinations across the top.

Independent Assortment and Crossing Over

  • Crossing over can occur, reshuffling DNA during chromosomes line up into tetrads or bivalent pairs.

  • Genes on the same chromosome can sort independently due to crossing over.

Genetic Recombination

  • Crossing over is also called genetic recombination.

  • Multiple crossing over events can independently sort genetic material to gametes.

Gene Linkage

  • Genes located very close to one another on the same chromosome are inherited as a linkage group.

  • Frequency of independent assortment can map gene proximity on chromosomes.

Mitotic Spindle Apparatus

  • The mitotic spindle apparatus pulls chromosomes to separate poles, organized by centrosomes.

  • Kinetochore structures on kinetochore microtubules connect to chromosomes at the centromere.

  • Cohesins hold homologous chromosomes and sister chromatids together during meiosis I.

  • Cohesins at the centromere hold sister chromatids together until meiosis II.

Cohesin Importance

  • Cohesins are crucial for proper chromosome separation.

  • If cohesins break down at the wrong time, nondisjunction may occur, resulting in an improper number of chromosomes in gametes.

  • Cohesins can break down as women age, leading to a higher likelihood of improper chromosome numbers in daughter cells.

DNA Structure

  • DNA building blocks: deoxyribose sugar, phosphate, and nitrogenous bases.

  • Rosalind Franklin's Photo 51 showed DNA was double helical.

  • DNA Is Double-Stranded: two strands run antiparallel to each other.

  • Each has a sugar-phosphate backbone with bases protruding in the middle.

  • Bases pair via hydrogen bonds.

  • Nucleotide composition: Sugar, phosphate, and nitrogenous base.

  • Directionality runs 5 prime and 3 prime.

  • Directionality is determined by phosphate groups at the 5 prime site versus the 3 prime $OH$ at the other side of the sugar

  • Sugar molecules link at their 5 prime site and 3 prime site, giving directionality.

  • Backbone: Five - prime phosphate.

Purines and Pyrimidines

  • Bases: purines (double ring) and pyrimidines (single ring).

  • Purines: Adenine ($A$) and Guanine ($G$).

  • Pyrimidines: Cytosine ($C$) and Thymine ($T$) in DNA; Uracil ($U$) replaces Thymine in RNA.

DNA Backbone and Directionality

  • The backbone consists of alternating directionality 5 prime to 3 prime sugar and phosphate groups linked by phosphodiester linkages resulting in a non-breanched linear polymer.

  • Bases extend from the backbone and can base pair.

DNA Double Helix

  • Two antiparallel strands form a double helix, with each turn containing 10 bases.

  • The double helix has a minor and major group to give proteins access to a sequence of bases on the helix.

  • These groups all protein machinery to interact with the DNA to carry out cellular proccesses like the beginning of transcription or packaging the DNA for replication.

  • Hydrogen bonds between strands can be broken (denaturation) and reformed (renaturation/annealing/hybridizing). This mean putting the single-stranded DNA molecules back together into their regular formation.

Base Pairing

  • Bases: $A$ pairs with $T$, and $G$ pairs with $C$. Purine and pyrimidine interactions.

  • double-stranded DNA molecule will have the same number of T as A and the same numbers of G as C.

  • The same number of purines and pyrimidines.

Base Composition

  • The proportion of $A+T$ to $G+C$ (GC percentage) varies by organism.

Central Dogma

  • DNA must store genetic information, be replicated/inherited, and express the genetic message.

DNA Structure and Function

  • Storage = Linear sequence of bases (nucleotides).

  • Replication = Base pairing and hydrogen bonds

  • We can open up things connected by hydrogen bonds an use one strand as a template.

  • Hydrogen bonding, complementary base pairing, and nucleotide sequence enables all of this.

  • DNA is the hereditary molecule for all life forms.

  • Evolution is tracked by changes in DNA.