Genes, Genome & Cell Cycle Phases

• A gene is a DNA segment with instructions for making proteins that perform most cell functions, each corresponding to a specific trait.
• A genome is the complete set of DNA in an organism, containing both coding (genes) and non-coding sequences.
• In humans, the genome consists of roughly 3 billion base pairs of DNA, divided into 23 chromosome pairs.
• Histones package DNA within the nucleus, forming nucleosomes and influencing gene regulation by affecting DNA accessibility.

Phases of the Cell Cycle

• The cell cycle involves cell growth, DNA replication, and division, with two main phases: Interphase and M phase (mitotic phase).

Interphase

• Includes G1, S, and G2 phases.
• The cell grows and carries out regular metabolic functions.
• Organelles replicate, and the cell increases in size.
• DNA replication occurs, with each chromosome duplicating into two sister chromatids connected at the centromere.
• By the end of S phase, the cell has twice the initial amount of DNA.
• The cell prepares for mitosis, synthesizing necessary proteins and organelles.
• It also checks for errors in DNA replication.

M Phase (Mitosis)

• Involves cell division through several stages: prophase, metaphase, anaphase, and telophase.

Prophase

• Chromosomes condense and become visible.
• The nuclear envelope breaks down.
• The mitotic spindle forms as centrosomes migrate to opposite poles of the cell.

Metaphase

• Chromosomes align at the cell's equator (metaphase plate).
• Spindle fibers assist the alignment of chromosomes.
• Kinetochores attach to spindle microtubules.

Anaphase

• Sister chromatids are pulled apart to opposite poles of the cell due to protein cleavage.

Telophase

• Chromatids de-condense to their uncoiled state at each pole.
• The nuclear membrane reforms.
• Spindle fibers disassemble.

Cytokinesis

• The cytoplasm divides, forming two distinct daughter cells.
• Animal cells use a cleavage furrow, while plant cells form a cell plate to create a new cell wall.
• Chromatids are identical chromosome copies formed during DNA replication and connected by a centromere.
• A chromosome, made of DNA and protein, carries genetic information; it exists as a single DNA strand pre-replication and as two sister chromatids post-replication.
• Kinetochores are protein structures at the centromere that bind to spindle microtubules, essential for chromosome movement during mitosis.
• Centromeres are constricted chromosome regions where sister chromatids attach and where the kinetochore forms.
• A karyotype is a visual representation of chromosomes arranged by size, shape, and banding patterns, used to detect chromosomal abnormalities.

Major Events in Mitosis

• Prophase involves chromosome condensation, nuclear envelope disintegration, and spindle fiber formation.
• Metaphase involves chromosome alignment at the cell equator and attachment to spindle fibers.
• Anaphase involves centromere splitting and sister chromatid movement to opposite poles.
• Telophase involves chromosome de-condensation and the formation of new nuclear membranes.
• The mitotic spindle, made of microtubules, separates chromosomes during mitosis.
• Centrosomes organize microtubules and contain centrioles, aiding in spindle fiber organization.
• Cell cycle progression is regulated by cyclins and cyclin-dependent kinases (Cdks).
• Cyclins activate Cdks, allowing cell cycle progression; their levels fluctuate to ensure stages proceed when ready.
• Cyclins are proteins regulating the cell cycle by activating Cdks at specific checkpoints.
• Cdks are enzymes activated by cyclins, phosphorylating other proteins to drive the cell cycle.

Cell Cycle Checkpoints

• Checkpoints ensure cell cycle integrity.
• The G1 checkpoint ensures readiness for DNA replication, checking for size, nutrients, and DNA integrity.
• The G2 checkpoint ensures accurate DNA replication completion before proceeding to mitosis.
• The M checkpoint ensures proper chromosome attachment to spindle fibers before anaphase.

Meiosis

• Sexual reproduction introduces genetic variation, enhancing adaptation to changing environments.
• Ploidy and Chromosome Numbers:
• Human somatic cells are diploid (2n) with 46 chromosomes before DNA replication.
• After DNA replication, there are still 46 chromosomes, each with two sister chromatids.
• After Meiosis I, cells are haploid (n) with 23 chromosomes with two chromatids each.
• After Meiosis II, cells remain haploid with 23 chromosomes, each with a single chromatid.
• DNA Molecules:
• Before Meiosis I, there are 92 chromatids (46 chromosomes × 2).
• After Meiosis I, there are 46 chromatids.
• After Meiosis II, there are 23 chromatids.

Phases of Meiosis

• Meiosis I is a reductional division, halving chromosome numbers.
• Prophase I involves chromosome condensation, homologous chromosome pairing (synapsis), and crossing over.
• Metaphase I involves homologous chromosome alignment at the metaphase plate.
• Anaphase I involves homologous chromosome separation to opposite poles.
• Telophase I results in two haploid cells with mixed maternal and paternal chromosomes.
• Meiosis II is an equational division, separating sister chromatids.
• Prophase II involves chromosome condensation in two haploid cells.
• Metaphase II involves chromosome alignment at the metaphase plate in both cells.
• Anaphase II involves sister chromatid separation.
• Telophase II forms four haploid gametes, each with one chromosome copy.
• Crossing over occurs during Prophase I, exchanging DNA segments to create new allele combinations and increase genetic diversity.
• The principle of independent assortment states that genes on different chromosomes assort independently during gamete formation, increasing genetic variation.
• Nondisjunction is when chromosomes or sister chromatids fail to separate during meiosis, causing gametes with abnormal chromosome numbers.

Mendelian Genetics

• Mendel used peas due to their distinct traits, self/cross-pollination ability, and short generation time.
• Dominant alleles are expressed in heterozygotes, while recessive alleles are only expressed in homozygotes.
• A monohybrid cross between two heterozygous individuals (Bb × Bb) yields a 3:1 phenotypic ratio.
• Punnett squares predict genotypic and phenotypic ratios from crossbreeding.
• Tay-Sachs is caused by a recessive allele, leading to the inability to break down certain lipids, resulting in neurodegeneration.
• Huntington’s Disease is caused by a dominant allele, leading to progressive neurodegeneration.
• Achondroplasia is caused by a dominant allele, resulting in a form of dwarfism.
• Carriers are heterozygous individuals with one recessive allele copy; there are no carriers for dominant diseases.

Extending Mendel’s Rules

• Sex-linked genes are located on sex chromosomes, with differing inheritance patterns between males and females.
• Incomplete dominance results in an intermediate heterozygous phenotype (e.g., pink flowers from red and white).
• Codominance results in full expression of both alleles (e.g., AB blood type).
• Mitochondrial DNA is inherited exclusively from the mother.
• Autosomal Dominant Pedigrees: affected individuals typically have affected parents, and the trait is present in every generation.
• Autosomal Recessive Pedigrees: affected individuals may not have affected parents, and the trait may skip generations.
• Polygenic traits are controlled by multiple genes (e.g., skin color, height).
• Multifactorial traits are influenced by both genetics and environment.
• Penetrance is the proportion of individuals with a genotype that expresses the associated phenotype.