Key Concepts in Genetics and Cell Division

gene

A segment of DNA that contains the instructions for making a specific protein or RNA molecule. They are the fundamental units of heredity and play a key role in determining traits in an organism.

allele

A variant form of a gene. Different alleles of a gene can result in variations in the expression of a trait, such as eye color or blood type.

natural selection

Requires:

Variation: Differences in traits among individuals.

Heritability: Traits must be passed from parents to offspring.

Differential survival and reproduction: Some individuals survive and reproduce more than others due to advantageous traits.

Selective pressure: Environmental factors favor certain traits over others.

Law of Segregation

Each individual has two alleles for a trait, and these alleles separate during gamete formation, so each gamete receives only one allele.

Law of Independent Assortment

Genes for different traits assort independently of one another during gamete formation.

Mendel's experiments

Mendel's experiments with pea plants, where he tracked the inheritance of traits across generations, led to these conclusions.

polymerase chain reaction (PCR)

Allows scientists to amplify specific DNA sequences, making it easier to study genetic material for research, medical diagnostics, and forensic analysis.

segregation

Refers to the separation of homologous chromosomes during meiosis, ensuring that each gamete gets only one copy of each gene.

independent assortment

Tthe random distribution of different genes during meiosis, meaning alleles of different genes separate independently, leading to genetic diversity.

locus

Specific location of a gene on a chromosome.

fitness

An organism's ability to survive and reproduce in its environment. It is often measured by the number of offspring an individual contributes to the next generation.

inheritance/heredity

The transmission of genetic traits from parents to offspring through DNA.

preformationism

An early biological theory that suggested that organisms develop from miniature versions of themselves, rather than through gradual development.

blending inheritance

A disproven theory that suggested offspring are a mix of their parents' traits, rather than inheriting discrete genetic units.

genetic model organism

A species extensively studied to understand biological processes, such as fruit flies (Drosophila melanogaster) and mice (Mus musculus).

dominance and recessiveness

Dominant allele: An allele that expresses its trait even if only one copy is present. Recessive allele: An allele that only expresses its trait when two copies are present.

continuous traits

Traits that show a range of variation (e.g., height, skin color).

discrete traits

Traits that have distinct categories (e.g., blood type, flower color).

haploid

Cells with one set of chromosomes (e.g., gametes). (n)

diploid

Cells with two sets of chromosomes (e.g., somatic cells).

Genotype

The genetic makeup of an organism (e.g., BB or Bb for eye color).

Phenotype

The observable traits of an organism (e.g., blue eyes).

Heterozygous

An organism with two different alleles for a gene (e.g., Bb).

Homozygous

An organism with two identical alleles for a gene (e.g., BB or bb).

Griffith Experiment

The 1928 experiment that showed genetic traits could be transferred between bacteria through a process called transformation.

DNA

A double-helix molecule made of nucleotides.

Nucleotide

A nucleoside with one or more phosphate groups attached. It is the building block of DNA and RNA.

Nucleoside

A molecule consisting of a nitrogenous base attached to a sugar (ribose or deoxyribose) but without a phosphate group.

Nitrogenous base

A molecule that forms part of a nucleotide. The four DNA bases are: Adenine (A), Thymine (T), Cytosine (C), Guanine (G). In RNA, uracil (U) replaces thymine.

Phosphate group

Consists of a phosphorus atom bonded to four oxygen atoms. It is a key component of nucleotides, linking them together to form the backbone of DNA and RNA strands.

Ribose

A five-carbon sugar (pentose) that forms the sugar backbone of RNA (ribonucleic acid). It contains a hydroxyl (-OH) group on the 2' carbon, making RNA more reactive and less stable than DNA.

Deoxyribose

A five-carbon sugar found in DNA (deoxyribonucleic acid). It differs from ribose by lacking an oxygen atom at the 2' carbon, which makes DNA more stable than RNA.

Hydrogen bond

A weak electrostatic attraction between a hydrogen atom and an electronegative atom (e.g., nitrogen or oxygen). In DNA, hydrogen bonds hold complementary base pairs (A-T, C-G) together.

Covalent bond

A strong bond where atoms share electrons. In DNA, also links nucleotides together through the sugar-phosphate backbone.

Antiparallel

In DNA, the two strands run in opposite directions: one strand runs 5' to 3', while the complementary strand runs 3' to 5'.

Major groove

The larger space between DNA strands where proteins (like transcription factors) can bind.

Minor groove

The smaller space between strands with limited access for protein interactions.

Transformation

The process by which genetic material is transferred between organisms.

Requirements of genetic material

Genetic material must: Store information, Replicate accurately, Undergo changes (mutations) to allow evolution, Direct the synthesis of proteins.

Transformation

The process where a cell takes up foreign genetic material (usually DNA) from its environment.

RNase

An enzyme that degrades RNA.

DNase

An enzyme that degrades DNA.

Purines

Adenine (A) and Guanine (G), which have a double-ring structure.

Pyrimidines

Cytosine (C), Thymine (T), and Uracil (U, found in RNA), which have a single-ring structure.

Prophase

Chromosomes condense, spindle fibers form.

Metaphase

Chromosomes align at the metaphase plate.

Anaphase

Sister chromatids separate and move to opposite poles.

Telophase

Chromosomes decondense, nuclear envelope reforms.

Cytokinesis

Cytoplasm divides, forming two daughter cells.

Symmetric division

Produces two identical daughter cells (same fate and size).

Asymmetric division

Produces two different daughter cells, often leading to differentiation (e.g., stem cells).

Cohesin

Holds sister chromatids together after DNA replication in S phase.

Spindle assembly checkpoint

Ensures that all chromosomes are attached to spindle microtubules before anaphase begins.

Kinetochore

A protein complex at the centromere that connects chromosomes to spindle microtubules.

Growth factors

Activate signaling pathways (e.g., Ras/MAPK) that increase cyclin D levels, promoting the transition from G1 phase to S phase.

Cell cycle checkpoints

Ensure proper cell cycle progression by detecting DNA damage or incomplete processes.

Tumor suppressors

Prevent uncontrolled cell division.

Cyclins

Bind to CDKs to activate different phases.

CDKs (Cyclin-dependent kinases)

Regulate transitions between phases.

Cyclins

Proteins that regulate CDKs.

Kinetochore

A protein complex at the centromere that attaches chromosomes to spindle fibers during mitosis.

Centromere

The central region of a chromosome where sister chromatids attach and where kinetochores form.

Centrioles

Cylindrical structures made of microtubules that organize the mitotic spindle in animal cells.

Centrosome

The microtubule-organizing center (MTOC) that contains a pair of centrioles and helps form the spindle apparatus.

Spindle Microtubules

Microtubules that extend from the centrosomes and attach to kinetochores to pull chromatids apart during mitosis.

MTOC (Microtubule-Organizing Center)

A structure (e.g., centrosome) that organizes the assembly of microtubules, which are crucial for cell division and intracellular transport.

Ploidy

Refers to the number of complete sets of chromosomes in a cell.

Haploid

One set (e.g., gametes - sperm and egg).

Diploid

Two sets (e.g., most body cells).

Polyploidy

More than two sets (e.g., some plants and amphibians).

Cohesion

The protein complex that holds sister chromatids together after DNA replication until they separate during mitosis or meiosis.

Checkpoints

Control mechanisms that ensure proper division by detecting DNA damage, incomplete replication, or chromosome misalignment.

CDKs

Enzymes that, when bound to cyclins, phosphorylate target proteins to regulate cell cycle progression.

Meiosis

A type of cell division that reduces chromosome number by half, producing four haploid gametes.

Gamete

A haploid reproductive cell (sperm or egg).

Mitosis

A cell division process producing two identical diploid cells for growth and repair.

Interphase

The cell cycle phase where DNA replication (S phase) and cell growth occur before division.

Synapsis

The pairing of homologous chromosomes during prophase I of meiosis.

Cytokinesis

The process of cytoplasmic division that follows mitosis or meiosis.

Synaptonemal Complex

Forms during meiosis to align homologous chromosomes and facilitate crossing-over.

Crossing Over

Occurs in prophase I of meiosis where homologous chromosomes exchange genetic material at chiasmata, leading to recombination.

Recombination Mapping

Measure recombination frequency: 1% recombination = 1 centimorgan (cM) distance apart.

Genetic Cross Ratios

Same chromosome (linked genes): Non-Mendelian ratios due to linkage. Different chromosomes: Mendelian ratios due to independent assortment.

Cytokinesis

The division of the cytoplasm at the end of mitosis or meiosis, forming two daughter cells.

Chromatid

A single copy of a replicated chromosome, connected to its sister chromatid at the centromere.

Autosomes

Non-sex chromosomes; humans have 22 pairs of autosomes.

Centromere

The region where sister chromatids attach and where the kinetochore forms.

Centrosome

The microtubule-organizing center (MTOC) that contains centrioles and directs spindle formation.

Recombination Nodule

A protein complex on the synaptonemal complex that facilitates crossing-over during meiosis.

Linkage

When genes are close together on the same chromosome, they tend to be inherited together.

Recombination

The exchange of genetic material between homologous chromosomes during meiosis.

Homologous Chromosomes

Pairs of chromosomes (one from each parent) that contain the same genes but may have different alleles.

Crossing-over

The exchange of genetic material between homologous chromosomes during prophase I of meiosis.

Spo11

An enzyme that induces double-strand breaks in DNA to initiate crossing-over during meiosis.

Bivalent

A pair of homologous chromosomes physically connected during prophase I.

Nondisjunction

The failure of chromosomes to separate properly during meiosis, leading to aneuploidy.

Trisomy and Monosomy

Trisomy: An extra chromosome (e.g., Down syndrome - Trisomy 21). Monosomy: A missing chromosome (e.g., Turner syndrome - 45,X).

Holliday Junction

A four-stranded DNA structure formed during recombination, crucial for crossing-over.

Karyotype

The complete set of chromosomes in a cell, visually represented for genetic analysis.

Chiasma

The physical crossover point where homologous chromosomes exchange DNA.