Lecture Note: Genetics, DNA Replication, and the Cell Cycle

BIO 425 9/12/2023 Mutations may be beneficial, neutral, or harmful DNA replication "Semi-conservative replication" Half of the DNA consists of original DNA and the other half of the DNA molecule consists of new DNA DNA replication DNA double helix unwinds (enzyme DNA helicase) New nucleotides are added (Enzyme DNA polymerase), following the base pair rules DNA rewinds/recoils (enzyme DNA ligase) The location where the double helix unwinds is known as a replication fork. DNA replication takes place in a "replication bubble" On one side of the molecule, the nucleotides are added continuously. (leading strand) On the other side, the nucleotides are not added continuously, there is frequent starts and stops (Okazaki fragments). (lagging strand) Gene - A segment of DNA that "codes" for a trait. Our traits are based upon our proteins. Most of our DNA is not used to produce proteins. Some of our DNA is used to produce protein. "Central dogma of biology" DNA -> RNA -> protein DNA -> RNA -> Polypeptide "exon" = DNA that is used to make proteins "intron" = DNA that is not used to make proteins Exon - Expressed Chromatin - Form of DNA. DNA is in long, thin strands Chromosome - DNA is packaged with proteins known as histones. Chromatids = "the arms" of the chromosomes A pair of duplicated chromatids are called "sister chromatids" Centromere - the area where chromatids are joined together. Kinetochore - Attachment point for protein fibers that move the chromosomes during cell division. Types of RNA: tRNA = transfer RNA (has an attachment point for an amino acid) mRNA = messenger RNA rRNA = ribosomal RNA 2 main stages of proteins synthesis: Transcription - Messenger RNA makes a copy of DNA using the base-pair rules. Reason is because DNA is too large to leave the nucleus. Translation - mRNA signals to tRNA to carry amino acids to the ribosome, based upon the genetic code. "open reading frame" Nucleotide bases are "read" in 3's 3 DNA bases = triplet 3 mRNA bases = codon 3 tRNA bases = anticodon Ex. DNA triplet: TCG mRNA codon: AGC tRNA anticodon: UCG Genetic code - "near universal" Code determines which 3 bases are used by mRNA to signal tRNA to bring a specific amino acid to the ribosome. mRNA signals tRNA to bring an amino acid to the ribosome based upon the genetic code. Genetic code is repetitive and degenerate. Some amino acids may be "coded" by more than one sequence. Sometimes the last base doesn't matter. Transcription and translation both have 3 stages: Initiation - Start signal Elongation - RNA gets longer due to new nucleotides. Termination - Stop signal Cell cycle - All the activities of the cell, from one division to the next. Why do cells replicate? Growth Repair of damaged cells Replacement of dead or dying cells Our cells have a limited life span. Stomach lining cells - 3 days Red blood cells - 120 days Skin cells - 30 days Our cells are genetically programmed to die -> apoptosis Before our cells divide, they must go through INTERPHASE Interphase - A time in which cells grow, replicate their DNA, carry out their basic activities, and prepare to divide. Most cells spend most of their time in interphase. Some cells are "arrested" in interphase. Ex. Most brain cells, Skeletal muscle cells After interphase, mitosis occurs. Mitosis - A form of asexual reproduction Results in the production of 2 genetically identical daughter cells that are diploid. Diploid = 2N N = number of types of chromosomes Stages of mitosis Prophase - Chromatin condenses into chromosomes. Spindle fibers attach to the chromosomes. Nucleolus and nuclear envelope disappear. Metaphase - chromosomes align at the cell's equator. Anaphase - chromosomes separate. Each half is considered now to be a separate chromosome. Telophase - Spindle fibers detach. chromosomes elongate. Nuclear envelope and nucleolus reappear. Cytokinesis - The division of the cytoplasm. Starts as early as anaphase Genetics - The study of the patterns of inheritance Genes - segments of DNA that "code" for traits. Locus - The location of a gene on a chromosome. Alleles - Alternate forms of a gene [trait] Typically, there are 2 alleles (for a given gene) Dominant allele - The stronger allele. Recessive allele - The weaker allele (whose effects may be hidden/masked by the dominant allele. Because our cells are diploid, 1 chromosome is maternal and the other is paternal. Genotype = The genetic make up/the combination of alleles. Under normal circumstances (patterns of inheritance) there are 3 potential genotypes: Homozygous dominant = 2 copies of the dominant allele. Homozygous recessive = 2 copies of the recessive allele. Heterozygous = 1 copy of the dominant allele and 1 copy of the recessive allele. Phenotype - A physical characteristic that can be observed, tested for, or measured. Phenotype is based upon the genotype (and the environment). Table: | Genotype | Phenotype | Ex: Hair texture | | :--- | :--- | :--- | | Homozygous dominant | Dominant | Curly hair | | Heterozygous | Dominant | Curly hair | | Homozygous recessive | Recessive | Straight hair |