Reproduction, Mitosis and Meiosis

Cell cycle

The ongoing set of processes by which a cell grows, prepares for division, and then divides

Interphase

The longer period between cell divisions during which the cell grows, replicates its chromosomes, and produces and assembles the cellular structures needed for cell division

G1 phase

The first gap phase of interphase; a period of rapid growth, metabolic activity, and centriole replication

S phase

The synthesis phase of interphase; the stage during which DNA replication (chromosome synthesis) occurs

G2 phase

The second gap phase of interphase; a period of growth and final preparations for division, including DNA proofreading and repair

M phase

The mitotic phase of the cell cycle; the stage when cell division (mitosis or meiosis) occurs

Mitosis

A cell process that involves nuclear division and results in two genetically identical daughter cells with the same diploid number of chromosomes as the parent cell

Meiosis

A cell process in which the nucleus of a diploid cell divides twice, producing four genetically diverse haploid cells (gametes), each with half the number of chromosomes of the parent cell

Chromosome

A threadlike structure made of DNA that carries genetic information; chromosomes are replicated and passed from one generation of cells to the next

Chromatid

Half of a replicated chromosome; sister chromatids are two identical copies joined at the centromere after DNA replication

Sister chromatids

Two identical copies of a chromosome that remain joined at the centromere after DNA replication; they separate during cell division

Centromere

The region of a chromosome where sister chromatids are connected to each other and where spindle fibers attach during cell division

Diploid (2n)

A cell that contains two complete sets of homologous chromosomes; the number found in most body (somatic) cells

Haploid (1n)

A cell that contains one set of chromosomes; the number found in gametes (sex cells) after meiosis

Homologous chromosomes

A pair of chromosomes that match because they carry the same genes (though possibly different alleles) at the same locations; one chromosome from each parent

Gene

An area of a chromosome that codes for a trait; a gene can come in different forms called alleles

Allele

One of the different forms a gene can take; different alleles of the same gene may produce different versions of a trait

Somatic cell

A diploid body cell that is not involved in reproduction

Sex cell (gamete)

A haploid cell involved in reproduction; formed by meiosis; in animals these are sperm and egg cells

Centriole

A cell structure found in animal cells (but not plant or fungal cells) that is involved in organizing cell division; centrioles occur in pairs at right angles to each other and help form the mitotic spindle

Mitotic spindle

A structure made of microtubules that forms during cell division and separates chromosomes to opposite poles of the cell; extends from centrioles in animal cells

Microtubules

Long protein strands that make up the mitotic spindle and are responsible for pulling chromosomes to opposite ends of the cell during division

Prophase (mitosis)

The first stage of mitosis in which chromosomes condense and become visible, the mitotic spindle forms, centriole pairs move to opposite poles, and the nuclear membrane begins to break down

Metaphase (mitosis)

The second stage of mitosis in which chromosomes line up along the center of the cell (metaphase plate) and each sister chromatid attaches to a microtubule from opposite poles

Metaphase plate

The imaginary plane at the center of the cell along which chromosomes align during metaphase

Anaphase (mitosis)

The third stage of mitosis in which microtubules pull sister chromatids apart at the centromere, moving the resulting daughter chromosomes to opposite poles of the cell

Daughter chromosomes

The individual chromosomes produced when sister chromatids separate at the centromere during anaphase

Telophase (mitosis)

The fourth stage of mitosis in which the cell elongates, nuclear membranes form around each set of chromosomes at the poles, and chromosomes begin to decondense

Cytokinesis

The division of the cytoplasm to produce two separate daughter cells; follows telophase and completes cell division; in animal cells the cell is "pinched" apart

Prophase I (meiosis)

The first stage of meiosis I in which homologous chromosomes pair up to form tetrads, crossing-over occurs between non-sister chromatids, and the spindle begins to form

Tetrad

A grouping of four chromatids formed when two homologous chromosomes (each consisting of two sister chromatids) pair up during prophase I of meiosis; also called a bivalent

Crossing-over

The exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis; generates genetic variation in offspring

Synapsis

The pairing of homologous chromosomes during prophase I of meiosis; the chromosomes are attracted to each other along their entire length

Metaphase I (meiosis)

The stage of meiosis I in which tetrads line up along the metaphase plate and spindle fibers attach to one pair of sister chromatids in each tetrad; homologous pairs are oriented toward opposite poles

Anaphase I (meiosis)

The stage of meiosis I in which spindle fibers separate homologous chromosome pairs, pulling a pair of connected sister chromatids (one homolog) to each pole; chromosome number is reduced

Telophase I and Cytokinesis (meiosis)

The stage of meiosis I in which the homologous chromosomes reach the poles and the cell divides into two daughter cells, each with half the original chromosome number but still containing paired sister chromatids

Meiosis II

The second round of meiotic division, similar to mitosis, in which sister chromatids are separated; produces four haploid daughter cells from the two cells created in meiosis I

Prophase II

The first stage of meiosis II in which sister chromatid pairs move toward the metaphase plate and the spindle forms; no further DNA replication occurs

Metaphase II

The stage of meiosis II in which sister chromatids align along the metaphase plate, with each chromatid oriented toward opposite poles

Anaphase II

The stage of meiosis II in which microtubules pull sister chromatids apart at the centromere; the individual chromatids (now called chromosomes) move to opposite poles

Telophase II and Cytokinesis (meiosis)

The final stage of meiosis II in which nuclear membranes form and the two cells divide, producing four haploid daughter cells total, each with one copy of each chromosome

Spore (plant)

The haploid reproductive cell produced by meiosis in plants; equivalent to gametes in animals; fuses with another spore to form a diploid individual

Fertilization

The fusion of a maternal and paternal gamete (sperm and egg) to form a diploid zygote with a complete set of chromosomes

Independent assortment

The random orientation of homologous chromosome pairs at the metaphase I plate during meiosis, resulting in different combinations of maternal and paternal chromosomes in daughter cells; a major source of genetic variation

Genetic variation

Differences in the genetic makeup of individuals within a population; in meiosis it is generated through crossing-over and independent assortment

Chromosomal aberration

A change in chromosome structure or number; includes deletions, duplications, inversions, and translocations

Deletion (chromosomal)

A chromosomal aberration in which a portion of a chromosome is lost; usually deleterious and often lethal unless small; can cause recessive genes to be expressed (e.g., cat-cry syndrome involves a deletion on chromosome 5)

Duplication (chromosomal)

A chromosomal aberration in which a segment of a chromosome is repeated; can result when a deleted segment is incorporated into another chromatid; example: the Bar mutation in Drosophila results from duplication of an X chromosome segment

Inversion (chromosomal)

A chromosomal aberration in which a segment of a chromosome is reversed in orientation; can change the way or degree that a gene is expressed; pericentric inversions include the centromere, paracentric inversions do not

Pericentric inversion

A chromosomal inversion in which the centromere is located within the inverted segment

Paracentric inversion

A chromosomal inversion in which the centromere is located outside the inverted segment

Cancer

A collection of diseases characterized by uncontrolled growth of cells and their subsequent spread to neighboring tissues; there are more than 100 types, defined by the cell type of origin

Tumor

An abnormal mass of cells produced by uncontrolled cell division; solid tumors form in most cancers; blood-based cancers circulate in the bloodstream

Metastasis

The invasion and spread of cancer cells from a primary tumor to neighboring tissues or other parts of the body; occurs when sufficient mutations accumulate

Cell cycle checkpoint

A control point in the cell cycle where specialized proteins determine whether the cell is ready to proceed to the next phase; checkpoints exist at G1/S, G2/M, and in the middle of mitosis (metaphase)

G1/S checkpoint

The checkpoint at the transition from G1 to S phase that determines whether the cell should divide, delay division, or enter a resting (G0) stage; regulated by cyclin-CDK complexes and growth signals

G0

A resting stage that a cell may enter if conditions are not appropriate for division; decided at the G1/S checkpoint

G2/M checkpoint

The checkpoint at the transition from G2 to M phase that prevents the cell from entering mitosis if DNA was damaged or improperly copied during replication

Metaphase checkpoint (spindle assembly checkpoint)

A checkpoint in the middle of mitosis that ensures all replicated chromosomes have correctly attached to the mitotic spindle before anaphase begins; the cell pauses if a chromosome is unattached or improperly joined

Cyclin-dependent kinase (CDK)

An enzyme that modifies proteins required for cell cycle progression by transferring phosphate groups from ATP; CDKs are present throughout the cell cycle but are only active when bound to a cyclin

Cyclin

A regulatory protein that binds to and activates a CDK; cyclins are produced and destroyed at precise timepoints in the cell cycle; without cyclin, CDKs are inactive

Cyclin-CDK complex

The active form of a cell cycle regulator, formed when a cyclin binds its specific CDK partner; each complex carries out its function only during the specific cell phase when that cyclin is present

Growth factors

Signaling proteins secreted by neighboring cells when conditions are optimal for growth; they bind to surface receptors and trigger a relay of signals that ultimately activate cyclin gene transcription and advance the cell cycle

Proto-oncogene

A normal gene involved in stimulating cells to divide by encoding proteins in the growth-signaling pathway; can be mutated into an oncogene

Oncogene

A mutated proto-oncogene that causes the encoded protein to continually act as if a growth signal is present, driving uncontrolled cell division; a mutation in only one copy is sufficient to cause the growth advantage

Tumor suppressor gene

A gene that produces checkpoint proteins that block cell division unless conditions are appropriate; mutations in tumor suppressors remove these blocks; both copies must be mutated to deregulate the checkpoint (e.g., BRCA1, BRCA2, MSH2)

DNA repair genes

Genes that produce proteins which identify and mend damaged DNA; mutations in repair genes allow errors to accumulate across the genome, increasing the chance of mutations in proto-oncogenes and tumor suppressors

Stepwise progression to cancer

The process by which a normal cell accumulates multiple mutations over time, each tipping the balance toward faster growth, until the collection of mutations allows metastasis; typically requires 2–8 mutations and occurs over many years

Inherited cancer predisposition

When a cancer-causing mutation is present in every cell of an individual because it was inherited through an egg or sperm; dramatically increases lifetime cancer risk but does not guarantee cancer (e.g., BRCA1 mutation raises breast cancer risk to 55–65%)

Acquired (somatic) mutation

A cancer-causing mutation that arises in a body cell during an individual's lifetime from replication errors or mutagen exposure; found only in the subset of descendant cells and is not passed to offspring

HNPCC (Hereditary Nonpolyposis Colorectal Cancer)

A hereditary cancer syndrome caused by mutations in DNA repair genes such as MSH2; raises lifetime colon cancer risk from the typical 4% to 50–80%

MSH2

A DNA repair gene; inherited mutations increase lifetime colon cancer risk to 50–80%; a tumor suppressor-type gene

BRCA1 / BRCA2

Tumor suppressor genes active in DNA repair; inherited mutations raise lifetime breast cancer risk to 55–65% (BRCA1) or 45% (BRCA2) and also increase ovarian cancer risk

Epigenetic silencing (cancer-related)

A mechanism by which cancer-causing changes occur without altering the DNA sequence; an overabundance of methyl groups on cytosine nucleotides alters chromatin conformation, prevents transcription, and effectively silences a gene (e.g., a tumor suppressor)

Transduction

The transfer of genetic material from one bacterial cell to another by a bacteriophage (bacterial virus) and the expression of the transferred gene(s) in the new host

Bacteriophage (phage)

A virus that infects bacteria; can transfer bacterial DNA between cells through transduction; recognizes host cells via molecular interactions between the viral surface and the cell surface

Lytic infection

A type of bacteriophage life cycle in which the phage hijacks the host cell's machinery to replicate its DNA and proteins, assembles new phage particles inside the cell, and then kills the cell (sometimes by lysis/bursting) to release progeny phages

Latent infection (lysogenic cycle)

A type of bacteriophage life cycle in which phage DNA is incorporated into the host chromosome as a prophage and lies dormant; the host (lysogen) reproduces normally until a signal triggers the prophage to enter an active (often lytic) cycle

Prophage

The dormant form of a phage genome integrated into the host bacterium's chromosome during a latent infection

Lysogen

A bacterial cell that has been latently infected by a bacteriophage; carries the phage DNA (prophage) in its chromosome

Generalized transduction

A type of transduction in which a random piece of host bacterial DNA is accidentally packaged into a phage capsid instead of viral DNA; when the phage infects a new cell it injects the bacterial DNA, which may recombine with the new host's genome

Specialized transduction

A type of transduction in which a prophage that pops out of the host chromosome occasionally carries along adjacent host DNA; this bacterial DNA is replicated and transmitted along with the phage DNA to newly infected cells

Capsid

The protein coat of a virus that surrounds and protects the viral genetic material; responsible for recognizing and attaching to host cells; in transduction the capsid packages and delivers bacterial DNA to a new host

Bacterial conjugation

A natural mechanism by which DNA is directly transferred from one bacterial cell to another through physical contact; a form of genetic recombination that increases bacterial adaptability (e.g., transfer of antibiotic resistance genes)

Plasmid

An extrachromosomal, double-stranded, circular piece of DNA found in bacteria that may carry a few to more than 20 genes; can replicate independently of the bacterial chromosome, move between cells, and even transfer between different bacterial species

F factor (fertility factor)

One of the first described plasmids, found in E. coli; contains multiple genes that regulate conjugation; named for its role in "fertility" (ability to conjugate and transfer DNA)

Pili (singular: pilus)

Elongated protein appendages on the surface of a bacterial cell; during conjugation a pilus acts as a bridge between donor and recipient cells, retracting to bring them together and facilitate DNA transfer

Antibiotic resistance transfer

The movement of antibiotic resistance genes from one bacterium to another via conjugation; discovered in 1959; can spread through a bacterial population at an exponential rate, creating major medical challenges