Chromosomes Quiz

• Basics of Genetics Study Notes from Dr. Sim M. Simolde, Assistant Professor, College of Medicine, Cebu Doctors' University

Chromosomes

• Chromosomes are found in the nucleus of cells
• They are thread-like structures
• Chromosomes carry genetic information from cell to cell
• Genetic information is passed down from parents to their offspring
• Humans have 23 pairs of chromosomes
  • 22 pairs of autosomes
  • 1 pair of sex chromosomes

Chromosome Parts

• P arm: Short arm structure of a chromosome
• Q arm: Long arm structure of a chromosome
• Centromere: Constricted region that controls chromosome segregation during mitosis and meiosis
• Telomere: Region of repetitive DNA sequences at the end of a chromosome
  • Telomeres prevent the ends of chromosomes from tangling

Identical Chromatids

• Each chromosome has 2 identical chromatids
• One chromatid is an exact copy of the other
• Chromatids are formed during the synthesis (S) phase of the cell cycle
• Each chromatid contains one DNA molecule

Nucleosome

• A nucleosome is composed of nucleic acid and histones
• Histones are the most abundant chromatin protein
  • They are rich in arginine and lysine
  • Divided into two types: core histones and linker histones
• Core histones are composed of pairs of H2A, H2B, H3, H4
  • They form a histone octamer
• H1 is seen in the linker region, loosely bound to the nucleosome

Nucleic Acids

• Nucleic acids are composed of:
  • Sugar
  • Nitrogenous base
  • Phosphate
• Sugar (Pentose)
  • C1 attaches to Nitrogenous base
  • C2 attaches to -H or -OH
  • C3 attaches to -OH
  • C4 =
  • C5 attaches to phosphate

Nitrogenous Bases

• Purines (Heterocyclic rings): Adenine, Guanine (PURE As Gold)
• Pyrimidines (Single ring): Cytosine, Uracil (RNA), Thymine (CUT the PY (pie))
• Phosphate Group; Negatively charged, Attached to C5

Nucleosides

• Nucleic acids composed of sugar and nitrogenous base
• Ribose + Adenine = Adenosine
• Ribose + Guanine = Guanosine
• Deoxyribose + Thymine = Deoxythymidine
• Nucleic acid composed of sugar and phosphate
• Adenosine with one phosphate becomes AMP, two phosphates becomes ADP, three phosphates becomes ATP

Complementarity

• Hydrogen bonds exist between nitrogenous bases
• A-T has 2-H bonds
• G-C has 3-H bonds
• Hydrogen bonds are denatured by extremes of heat, pH, and destabilizing agents
  • Stable at pH 4-9
  • Disrupted by formamide
• G-C base pairs require more energy to disrupts that A-T base pairs

Antiparallel Arrangement

• Sugar Phosphate backbone has Phosphodiester bonds
  • This covalent bond is formed by phosphorus and oxygen atoms, creating a stable linkage

Deoxyribonucleic Acid (DNA)

• Length of fully extended eukaryotic molecule is 3 m per genome
• Contains the genetic information of an organism, packaged in chromatin using histones and organized into chromosomes
• Located in the nucleus in eukaryotes and in the nucleoid region of the cytosol in prokaryotes

Ribonucleic Acid (RNA)

• Single stranded molecule
• Ribose sugar containing a hydroxyl group at the 2' position
• Thymine is replaced by the methylated uracil
• Susceptible to alkaline hydrolysis
• Rapidly degraded by RNA - specific enzymes

Chromosomes During Cell Division

• 46 Chromosomes = 46 Chromatin
• 46 Chromosomes = 92 Chromatin
• Chromosomes are visualized during cell division when nuclear envelope breaks down

Cell Structure During Cell Division

• Nuclear membrane
• Nucleoplasm
• Nucleolus
• During this stage
  • Heterochromatin is tightly packed DNA, stained dark, high DNA density, transcriptionally inactive and peripherally located - maintains the structural integrity of the genome
  • Euchromatin - loosely packed DNA is lightly stained, low DNA density, transcriptionally active and centrally located which allows transcription of genes
    • If DNA is inaccessible with the gene inactive, it will bind to HISTONE TAIL
    • if DNA is accessible, it will create a gene active by releasing the HISTONE TAIL

Cell Cycle

• Includes different phases
• Interphase
• Mitosis consists of Prophase, Prometaphase, Metaphase Telophase, Anaphase
• Nondividing cells
• G0, G1, G2

Interphase

• Interval between cell divisions
• Chromatin is relaxed
• Divided into 3 phases: Gap 1 (G1), Synthesis (S), Gap 2 (G2)
• Gap 1 (G1)
  • Cell commits to organelle production, protein synthesis, and DNA repair (Thymidine)
  • G0 phase involves withdrawal from cycle and Quiescence
• Synthesis (S)
  • DNA Replication
  • ↑ Metabolic activity
  • Cell differentiation
  • 2n→ 4n
• Gap 2 (G2)
  • Cell Growth
  • ↑ volume
  • ↑ cytoplasm

Somatic Cells

• Labile cells: Regenerate regularly, cells continuously lost and replaced, include surface epithelium, mucosal epithelium, hematopoietic cells
• Stable cells: Quiescent cells (G0), ↓ proliferative activity, divide in response to injury; include parenchymal calls Liver, kidney, lungs Endothelial cells, Fibroblasts Permanent Cells
• Can not be replaced through the stem cell method non proliferative, no division in postnatal life which includes nerve cells or cardiac muscle cells

Mitosis: Prophase

• Half of mitosis Key events:
• Migration of two centrioles
• Organization of microtubules
• Nuclear envelope breakdown
• Nucleolus disintegrates
• Chromatin fibers condense

Mitosis: Metaphase

• Migration of chromosomes to the equatorial plane/metaphase plate Key events:
• Binding of the spindle fibers to the chromosome kinetochore
• Degradation of cohesin by enzyme, separase
• Sister chromatid arms disjoin, except at the centromere
• Key structures
  • Spindle fiber
  • Kinetochore
    • Is a protein within centromere where spindle fibers attach
  • Cohesin
    • Holds sister chromatin together
  • Microtubule -Shugoshin
    • Prevents cohesin from being degraded by separase

Mitosis: Anaphase

• Shortest stage of mitosis
• Disjunction of sister chromatids Key events:
• Degradation of shugoshin
• Sister chromatids of each chromosome are pulled toward the opposite poles of the cell

Mitosis: Telophase

• Final stage of mitosis Key events:
• Formation of cleavage furrow
• Cytokinesis
• Chromatin loosens

Meiosis

• Reductional division
• Produces gametes
• Involves 2 successive division divisions
• Produces 4 haploid (n) daughter cells
• Each daughter cell is genetically different Crossing over
• Genetic exchange between a homologous pair of chromosomes

DNA Repair

• DNA repair processes take place at different checkpoints in order to address unreplicated material, damaged DNA and chromosome misalignment
• DNA lesions occur because of mismatched strands, pyrimidine dimers double strand breaks etc
• Causes - Errors that escaped proofreading, UV exposure, Deaminating compounds, Ionizing radiation, free radicals Anti tumor drugs REPAIR MECHANISM
• mismatch repair (MMR)
• Nucleotide excision repair (NER)
• Base excision repair (BER)
• Non- homologous end joining (NHEJ)
• Homologous recombination (HR)

Genetic Mutations

• Any permanent change in DNA base sequence
• A physical biochemical change or change as a result of mutagens Potential to change the mRNA sequence
• AA sequence MOST COMMON is point mutations, frameshift mutations frameshift include insertion or deletion OTHERS large segment deletion Splice donor or acceptor Triple repeat expansion

Point Mutation

• May cause change, insertion, or deletion
• Transition
  • Purine to purine/ Pyrimidine to pyrimidine (A to G)
  • A base change that doesn't affect the amino acid
  • Silent mutation sequence
• Transversion
  • Purine to pyrimidine or vice versa (A to C)
  • Missense mutation: A base change that alters a protein's amino acid
  • Nonsense mutation: A base change that causes a premature stop codon

Frameshift Mutation

• When a DNA sequence Is altered by insertion or deletion of one or more nucleotides *NOT a multiple of three

Basic Genetic Terms

• Gene; Functional unit of inheritance that is composed of a sequence of nucleotides and may codes for a product (RNA or protein)
• Locus: Position of a gene in a chromosome
• Allele: Alternative form of a gene in the same locus

Genotype vs Phenotype

• Homozygous same allele T, t
• Heterozygous different allele Tt
• Mendelian Disorders are genetic defects

Autosomal Genetic Inheritance

• Autosomal Dominant requires 1 dominant allele, requires 1 parent affected
• H= Affected Hh and Hh - Mother Unaffected -hh and Affected Father Hh
  • Most common disorders
• Autosomal Recessive requires 2 recessive alleles, 25% express disease, 50% carrier, 25% normal
• Father carrier Ss and Affected carrier - S is the unaffected = 25s% Disease as well as a Carrier

X Linked Dominant (XD)

A. In a cross of heterozygous female with a normal male

• 50% of sons and 50% of daughters with disease B. in a cross of affected male with a normal female/ 100% of daughters with disease 0% of sons with disease
• Most common Metabolic, Skeletal

X Linked Recessive (XR)

• Usually males express phenotype (due to homozygosity for the allele
• Affected males do not transmit the disease to their sons but all of their daughter are carriers In a cross of heterozygous female and affected male SONS 50% (+) disease 50% () disease DAUGHTERS 50% (+) disease 50% carrier
• Most common: Musculoskeletal, Hematopoietic, Metabolic

Test your knowledge about chromosomes. This quiz covers chromosome behavior during cell division, DNA replication, karyotypes, and the function of chromosomes within a cell. Explore the structure and role of chromosomes.