A/P Chapter 4 Genetics and Cellular Function

Johann Friedrich Miescher

The scientist who discovered **nuclein** in 1869 from the nuclei of pus cells, which we now know as deoxyribonucleic acid (DNA).

Nucleotides

The building blocks of nucleic acids, each consisting of a **sugar**, a **phosphate group**, and a **nitrogenous base**.

Pyrimidines

Nitrogenous bases containing a **single carbon-nitrogen ring**; includes cytosine (C), thymine (T), and uracil (U).

Purines

Nitrogenous bases containing a **double carbon-nitrogen ring**; includes adenine (A) and guanine (G).

DNA Structure

A **double helix** resembling a spiral staircase, with a backbone of alternating phosphate groups and deoxyribose sugars, and nitrogenous bases forming the steps.

Law of complementary base pairing

The structural rule stating that an organism's DNA bases always pair **adenine with thymine** (via 2 hydrogen bonds) and **guanine with cytosine** (via 3 hydrogen bonds).

Essential function of DNA

To **code for the proteins** that a cell synthesizes.

Chromatin

The fine, thread-like filamentous material composed of DNA complexed with structural proteins, visible in the nucleus during interphase.

Histones

Disc-shaped clusters of **eight proteins** around which DNA winds, acting like spools to orderly shorten and organize the long molecule.

Nucleosome

The fundamental structural unit of chromatin, consisting of a core histone particle wrapped **twice** by a segment of DNA.

Chromosome

A highly condensed, compact rod-like structure of chromatin formed only when a cell is actively preparing to divide.

Sister chromatids

Two identical, parallel nucleoprotein filaments joined together at a centromere, produced during DNA replication.

Centromere

The specialized, constricted region of a chromosome that holds two sister chromatids together.

Kinetochore

A protein plaque located on each side of the centromere that serves as an attachment site for spindle fibers during mitosis.

Ribonucleic acid (RNA)

Smaller, single-stranded nucleotide polymers responsible for interpreting DNA code and guiding cytoplasmic protein synthesis; categorized into **mRNA**, **tRNA**, and **rRNA**.

RNA structural components

Contains the monosaccharide **ribose** instead of deoxyribose, and utilizes the pyrimidine **uracil (U)** instead of thymine (T).

Gene

An information-containing segment of DNA that codes for the production of a molecule of RNA, which usually plays a role in protein synthesis.

Genome

The complete master set of all genetic material contained within one holistic **23-chromosome set** of an individual organism.

Human Genome Project

A historic scientific collaborative effort completed in **2003** that successfully mapped the base sequence of 99% of the human genome.

Genomics

The modern branch of comprehensive biology focused on analyzing the entire genome and its intricate interactions with proteins and the environment.

Genetic code

A specialized system that utilizes **4 distinct nucleotide bases** (A, T, C, G) to code for the correct sequences of all 20 human amino acids.

Base triplet

A sequential sequence of **3 continuous DNA nucleotides** that stands for or yields one specific amino acid.

Codon

A **3-base sequence constructed in mRNA** that directly matches a corresponding DNA triplet.

Stop codons

The three specific mRNA codons (**UAG, UGA, UAA**) that signal the structural end of a genetic message; they do not code for amino acids.

Start codon

The specific mRNA codon (**AUG**) that signals the exact beginning of a genetic reading frame and codes for methionine.

Transcription

The process of copying genetic information from DNA into a complementary strand of messenger RNA (mRNA) inside the nucleus.

RNA polymerase

The principal enzyme that binds to a promoter site on DNA, unzips the helix, and synthesizes a complementary mRNA strand.

Pre-mRNA

The raw, unedited initial RNA transcript produced directly by transcription before any chemical modification.

Exons

The functional, information-carrying segments of a pre-mRNA transcript that are retained and expressed.

Introns

The noncoding, internal segments of a pre-mRNA transcript that must be enzymatically spliced out before translation.

Spliceosome

A specialized molecular machine composed of small nuclear ribonucleoproteins (snRNPs) that chips out introns and pastes exons together.

Alternative splicing

The dynamic regulatory mechanism where a single pre-mRNA transcript can be cut and reconnected in multiple ways to yield different functional proteins.

Translation

The process where a ribosome reads an mRNA message and assembles a corresponding chain of amino acids into a protein.

Anticodon

A 3-nucleotide loop sequence on a tRNA molecule that is perfectly complementary to an mRNA codon.

Ribosome tRNA Binding Sites

The three internal pockets of a ribosome: **A site** (Aminoacyl, accepts incoming tRNA), **P site** (Peptidyl, holds growing peptide chain), and **E site** (Exit, releases spent tRNA).

Initiation (Translation)

The starting phase where mRNA passes through a nuclear pore, binds to a ribosome, and the start codon (AUG) is recognized by an initiator tRNA carrying methionine.

Elongation (Translation)

The phase where sequential tRNAs arrive at the ribosome's A site, match their anticodons to mRNA codons, and move through the P site to form peptide bonds between amino acids.

Termination (Translation)

The final phase where the ribosome hits a stop codon, binds a release factor protein, and detaches the finished protein chain.

Polyribosome

A cluster of multiple ribosomes reading and translating the exact same single mRNA strand simultaneously to speed up protein output.

Chaperone proteins

Specialized intracellular proteins that guide newly synthesized polypeptide chains into their correct three-dimensional shapes.

Leader sequence

The initial segment of a newly forming protein that acts as an address label, often directing the ribosome to dock with the rough endoplasmic reticulum.

Posttranslational modification

The final processing of a protein in the rough ER and Golgi complex, which may include folding, adding carbohydrates, or joining multiple chains.

Secretory vesicles

Membrane-bound sacs that package finished proteins from the Golgi complex and move to the plasma membrane to release contents via exocytosis.

Regulation of gene expression

The precise control mechanism where cells turn specific genes on or off as required, governed by chemical signals or hormone receptors.

Enzymatic synthesis regulation

An indirect genetic process where cells control the production of non-protein molecules (like glycogen or lipids) by regulating the transcription of the enzymes that build them.

DNA Helicase

The replication enzyme that unzips the DNA double helix at the replication fork, exposing the single template strands.

DNA Polymerase

The key replication enzyme that moves along exposed DNA strands, matching free nucleotides to build new complementary strands.

Semiconservative replication

The nature of DNA replication where each final molecule contains one preserved original strand and one brand-new strand.

Leading strand

The newly synthesized strand of DNA that is built continuously in the same direction as the advancing replication fork.

Lagging strand

The newly synthesized strand of DNA that must be built discontinuously in short segments moving away from the replication fork.

Okazaki fragments

The short segments of newly synthesized DNA produced discontinuously along the lagging strand.

DNA Ligase

The specialized enzyme that splices together short segments of newly replicated DNA (such as Okazaki fragments) to form a seamless, continuous strand.

Mutations

Permanent, structural changes or errors introduced into the linear nucleotide sequence of a DNA molecule.

Cell Cycle

The overarching lifecycle of a dividing cell, divided into Interphase ($G_1$, $S$, $G_2$) and the Mitotic Phase ($M$).

First Gap ($G_1$) Phase

The phase of the cell cycle where the cell grows rapidly, synthesizes proteins, and carries out its normal metabolic duties.

Synthesis ($S$) Phase

The specific phase of the cell cycle where the cell creates an exact duplicate copy of its nuclear DNA database.

Second Gap ($G_2$) Phase

The brief interval where the cell finishes replicating its centrioles, synthesizes enzymes needed for division, and checks for DNA errors.

Cyclins and CDKs

Regulatory proteins whose levels rise and fall throughout the cell cycle; their pairing forms structural checkpoints that control a cell's advancement into the next phase.

Mitotic ($M$) Phase

The phase of the cell cycle encompassing mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Mitosis

The formal process of nuclear division that divides replicated chromosomes equally into two identical daughter nuclei.

Prophase

The first stage of mitosis where chromatin condenses into chromosomes, the nuclear envelope breaks down, and the spindle apparatus forms.

Metaphase

The stage of mitosis where chromosomes align perfectly along the midline or equatorial plate of the cell.

Anaphase

The stage of mitosis where sister chromatids are pulled apart by spindle fibers toward opposite poles of the cell.

Telophase

The final stage of mitosis where chromosomes uncoil into chromatin, new nuclear envelopes reform, and mitotic spindles vanish.

Cytokinesis

The division of the cytoplasm, driven by a contracting actin ring that pinches the cell into two separate daughter cells.

Cleavage furrow

The indentation line formed at the cell surface during cytokinesis as the cell pinches in two.

Contact inhibition

The regulatory phenomenon where cells stop dividing once they make physical contact with surrounding cells.

Tumor

An abnormal, uncontrolled mass of tissue growth caused by a failure of proper cell cycle regulation.

Benign tumor

A slow-growing, non-cancerous tumor that remains encapsulated within its original tissue location and does not spread.

Malignant tumor

A fast-growing, cancerous tumor that breaks past tissue boundaries and invades surrounding organs; exhibits metastasis.

Metastasis

The dangerous process where malignant tumor cells break away, travel through the bloodstream or lymph, and establish new tumors elsewhere.

Oncogenes

Mutated genes that cause hyperactive cell division, acting like an accelerator stuck in the "on" position to promote cancer.

Tumor-suppressor genes

Genes that normally produce proteins to halt cell division or fix DNA damage; when mutated or inactivated, cancer can develop unchecked.

Heredity

The biological transmission of genetic traits and physical characteristics from parent to offspring.

Karyotype

An organized graphical layout of an individual's 46 chromosomes, arranged systematically in matching pairs based on physical size.

Homologous chromosomes

A matching pair of chromosomes (one inherited from each parent) that look alike and carry genes for the same traits.

Autosomes

The 22 pairs of human chromosomes that govern general body traits and look alike in both males and females.

Sex chromosomes

The 23rd pair of human chromosomes ($X$ and $Y$) that determine an individual's genetic sex.

Locus

The exact, physical location occupied by a specific gene along the length of a chromosome.

Alleles

Alternative, varied forms of a single gene that produce alternative expressions of a specific trait.

Dominant allele

An allele that completely masks the expression of a matching recessive allele; denoted by an uppercase letter.

Recessive allele

An allele whose phenotypic expression is hidden by a dominant partner; expressed only when homozygous.

Homozygous

Possessing two identical, matching alleles at a specific locus on homologous chromosomes.

Heterozygous

Possessing two completely different, non-matching alleles at a specific genetic locus.

Genotype

The underlying paired allele combination or genetic constitution possessed by an individual.

Phenotype

The observable physical or physiological expression of an individual's traits, produced by the interaction of genotype and environment.

Punnett square

A simple grid diagram used to predict the probability of genotypes and phenotypes resulting from a genetic cross.

Genetic carriers

Individuals who are heterozygous for a recessive trait, showing a normal phenotype but capable of passing the recessive allele to offspring.

Multiple alleles

A pattern of inheritance where a single gene locus has more than two alternative allele possibilities in a population (e.g., ABO blood types).

Codominance

An inheritance pattern where both different alleles are equally dominant, causing both phenotypic traits to be fully expressed simultaneously (e.g., Type AB blood).

Incomplete dominance

An inheritance pattern where a heterozygous genotype yields a blended phenotype halfway between the two homozygous conditions (e.g., hypercholesterolemia).

Polygenic inheritance

An inheritance pattern where multiple separate genes at different loci exert collective control over a single phenotypic trait (e.g., eye color, skin color, height).

Pleiotropy

A genetic phenomenon where a single gene locus influences multiple, seemingly unrelated physical or physiological traits (e.g., Alkaptonuria, Sickle-cell disease).

Alkaptonuria

A rare pleiotropic genetic disorder caused by a defect on chromosome 3 that blocks the breakdown of tyrosine, leading to accumulation of homogentisic acid, darkening of connective tissues and urine, and joint or heart valve damage.

Sex-linked traits

Traits carried on either the X or Y chromosome, resulting in biased inheritance patterns between males and females; men are more commonly affected by X-linked traits because they possess only one X chromosome.

Red-green color blindness

A classic example of an X-linked recessive trait where a mother can pass the defective allele to her son, who will always express it because he lacks a second X chromosome to mask it.

Penetrance

The percentage of a population with a given genotype that actually expresses the predicted phenotypic trait.

Allelic frequency

The relative commonness or rarity of an allele within a population; dominance and recessiveness are entirely unrelated to how common an allele is (e.g., Type O blood is recessive but the most common in North America).

Polydactyly

The presence of extra fingers or toes; a condition caused by a dominant allele that remains exceedingly rare in the human population.

Epigenetics

The fast-growing field of comprehensive biology studying heritable changes in gene expression that are caused by environmental factors without changing the underlying DNA sequence.