Session #4 AGRI 3409

Introduction to Genetics

• Central Dogma of Molecular Genetics:

  • DNA $\rightarrow$ RNA (transcription) $\rightarrow$ Proteins (translation)

• Father of Genetics:

  • Gregor Mendel, who used pea plants to discover that traits are passed down from parent to offspring by a pair of factors.

• Genes vs. Chromosomes:

  • Genes reside on chromosomes.

  • A gene is a small section of DNA that contains instructions for a specific trait.

• Source of Genetic Variation:

  • Genetic mutations

• Diploid vs. Haploid Cells:

  • Diploid cells have two sets of chromosomes (humans have $2n = 46$).

  • Haploid cells have one set of chromosomes (human gametes have $n = 23$).

• Alien Organism Example:

  • If an alien organism has a diploid number ($2n$) of 100, its haploid number ($n$) is $100/2 = 50$.

• Phenotype vs. Genotype:

  • Genotype: The set of alleles an organism carries for a specific trait.

  • Phenotype: The observable physical trait of an organism.

• Transgenic Organism:

  • An organism with foreign DNA inserted into its genetic code (GMOs).

DNA vs. RNA

• DNA:

  • Double-stranded

  • Contains the nitrogenous bases Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).

• RNA:

  • Single-stranded

  • Contains an -OH group on the C-2' carbon.

• Codons:

  • Nucleotide triplets that code for a specific amino acid.

• Number of Amino Acids:

  • 20 different amino acids can be used to create polypeptides.

• Ribose and 2-Deoxyribose Sugar:

  • Ribose has an OH group on the 2' carbon, while 2-deoxyribose has a H on the 2' carbon

  • Nitrogenous base attaches to the 1' carbon

  • Phosphate group attaches to the 5' carbon

DNA Structure and Analysis

• Criteria for a Molecule to Serve as Genetic Material:

  • Must be able to replicate.

  • Store genetic information.

  • Express information by transcription and translation.

  • Be able to mutate.

• Nitrogenous Bases:

  • Purines: 9-member ring (Adenine and Guanine)

  • Pyrimidines: 6-member ring (Cytosine, Thymine, and Uracil)

• Charge of DNA:

  • Negative, due to the phosphate group.

• Bonds in DNA Backbone:

  • Phosphodiester bonds link phosphate groups to pentose sugars.

• Bonds Between Nitrogenous Bases:

  • Hydrogen bonds link nitrogenous bases.

• Molecular Structure of DNA:

  • Watson and Crick published the paper outlining the molecular structure of DNA.

• Chargaff's Base Composition Rules:

  • $A = T$

  • $C = G$

  • However, $A + T \neq C + G$

• DNA Sequencing Example:

  • If Guanine (G) is 12.5%, then Cytosine (C) is also 12.5%.

  • Therefore, Adenine (A) and Thymine (T) are each $(100 - 25)/2 = 37.5$ .

• Semi-Conservative Model of DNA Replication:

  • Each new DNA molecule consists of one old strand and one new strand.

• Biologically Active Form of DNA:

  • B-DNA

• Left-Handed Double Helix DNA:

  • Z-DNA

• UV Light Absorption:

  • Protein absorbs UV light at 280 nm.

  • Nucleic acids absorb UV light at 254-260 nm.

RNA Functions

• mRNA:

  • Template for protein synthesis.

• rRNA:

  • Structural component of ribosome for protein synthesis.

• tRNA:

  • Carries amino acids for protein synthesis.

• Abundance of RNA Types:

  • Most abundant: rRNA

  • Least abundant: tRNA

• Reverse Transcriptase:

  • An enzyme that converts RNA to DNA.

• DNA Denaturation Factors:

  • Heat or stress.

• Nucleic Acid Electrophoresis:

  • Separates DNA and RNA fragments by size; smaller fragments travel further in the agarose gel.

• Cytosine/Guanine Content and DNA Denaturation:

  • Species with more cytosine/guanine pairs require a higher temperature to denature DNA.

DNA Organization into Chromosomes

• Length of DNA in a Somatic Cell:

  • If stretched out, DNA would be about 2 meters or 6.6 ft long.

• DNA Supercoiling:

  • DNA must supercoil to fit into the nucleus of a cell; this process is facilitated by topoisomerases.

• Chromatin:

  • Made up of DNA and proteins (histones) that is uncoiled and decondensed.

• Chromosomes During Interphase:

  • Chromatin is decondensed and does NOT form visible chromosomes in the cell nucleus during interphase.

• Nucleosomes:

  • Made up of a histone octamer; chromatin.

• Histones Types:

  • H2A, H2B, H3, H4, H1

  • Forms tetramers with H2A, H2B, H3 and H4

• Spacer Histone:

  • H1 acts as a "spacer" between nucleosomes.

Histones and Chemical Modifications

• Histones:

  • Positively charged proteins containing large amounts of lysine and arginine.

• Histone Tails:

  • Are not packed into folded histone domains with nucleosomes and are potential targets for chemical modification.

• Chemical Modifications of Histones:

  • Acetylation:

    • Enzyme: Histone acetyltransferase

    • Adds an acetyl group.

  • Methylation:

    • Enzyme: Methyltransferase

    • Adds a methyl group.

  • Phosphorylation:

    • Enzyme: Kinase

    • Adds a phosphate group.

• Euchromatin vs. Heterochromatin:

  • Euchromatin is typically uncoiled and active.

  • Heterochromatin is typically condensed and inactive.

• Pseudogenes:

  • Non-coding regions of genes that have undergone significant mutational alteration and are not transcribed.

  • Constitute about 2-10% of our genome.