Comprehensive Genetics: Mitosis, Meiosis, Chromosome Mechanics, Linkage, Transposable Elements, and Aneuploidy

Flashcards covering key definitions and vocabulary from genetics lectures on mitosis, meiosis, chromosome structure, sex determination, genetic linkage, transposable elements, and aneuploidy.

Mitosis

Nuclear division that generates two daughter cells containing the exact same number and type of chromosomes as the parent cell.

Meiosis

Nuclear division consisting of two rounds of division following one round of chromosome duplication, generating four haploid ($n$) cells from a diploid ($2n$) germ cell.

Metacentric chromosome

A chromosome whose centromere is located approximately in the middle of the chromosome arms.

Acrocentric chromosome

A chromosome whose centromere is positioned near one end of the chromosome.

Sister chromatids

Two identical copies of a replicated chromosome that are joined together at the centromere and separate during anaphase of mitosis or anaphase II of meiosis.

Homologous chromosomes (Homologs)

Chromosomes that pair during meiosis and carry the same linear sequence of genes, but may harbor different alleles inherited from each parent.

Karyotype

A micrograph of stained metaphase chromosomes arranged systematically in homologous pairs according to size and morphology.

Centrosome

The microtubule organizing center near the nuclear envelope that forms the mitotic spindle; contains a pair of centrioles in animal cells.

Kinetochore

A specialized multiprotein structure located at the centromere of each chromatid that binds spindle microtubules during cell division.

Syncytial embryo

A structure produced when nuclear divisions occur repeatedly without cytokinesis, resulting in thousands of nuclei within a single cell (such as during the first 13 mitotic divisions of Drosophila embryogenesis).

Somatic cells

Non-germline body cells that make up the vast majority of an organism and divide exclusively by mitosis or remain in the $G_0$ stage.

Germ cells

Specialized precursor cells set aside during embryogenesis that migrate to the reproductive organs and undergo meiosis to produce haploid gametes.

Synaptonemal complex

A proteinaceous zipper-like structure that physically aligns and holds homologous chromosomes together during zygotene and pachytene of prophase I.

Prophase I substages

The five successive cytological stages of meiotic prophase I: leptotene (chromosomes condense), zygotene (synapsis begins), pachytene (crossing over occurs), diplotene (synaptonemal complex dissolves, chiasmata visible), and diakinesis (chromatids in tetrads fully visible).

Tetrad

A structure formed in prophase I consisting of two paired homologous chromosomes, comprising four chromatids in total.

Ploidy

The number of complete sets of chromosomes in a cell, determined by the total number of chromosomes present rather than the count of sister chromatids.

Primary oocyte arrest

The extended pauses during human female oogenesis: meiosis I arrests in the diplotene stage during fetal development until puberty, and meiosis II arrests at metaphase II until fertilization.

Polar body

A small, nonfunctional haploid cell produced during asymmetric meiotic divisions of oogenesis that contains discarded genetic material with minimal cytoplasm.

Spermatogenesis

The symmetric process in male testes beginning at puberty wherein one diploid primary spermatocyte undergoes two meiotic divisions to produce four equal haploid spermatids that differentiate into mature sperm.

SRY

The sex-determining region of the Y chromosome; the single primary master switch gene in humans responsible for initiating male testis development.

Pseudoautosomal regions (PARs)

Homologous regions located at both ends of the human X and Y chromosomes that allow sex chromosomes to pair and segregate properly during male meiosis I.

Barr body

A condensed, transcriptionally inactivated X chromosome found in the somatic cell nuclei of mammalian females, serving to achieve dosage compensation.

Complete androgen insensitivity syndrome (CAIS)

An intersex condition in $XY$ individuals caused by a nonfunctional X-linked androgen receptor gene, resulting in internal testes, lack of internal male or female tracts, and female external genitalia.

Heterogametic sex

The sex that produces two genetically distinct types of gametes with respect to sex chromosomes (e.g., $XY$ human males or $ZW$ avian females).

Homogametic sex

The sex that produces only one type of gamete with respect to sex chromosomes (e.g., $XX$ human females or $ZZ$ avian males).

Hemizygous

The genotype of a diploid organism describing a gene present in only a single copy, such as X-linked genes in human males.

Crisscross inheritance

The inheritance pattern characteristic of X-linked recessive traits wherein phenotypic traits pass from affected fathers to carrier daughters, and subsequently from carrier mothers to affected sons.

Syntenic genes

Genes that are located on the same physical chromosome, whether linked or sufficiently separated to assort independently.

Linked genes

Syntenic genes positioned closely together on a chromosome that do not assort independently, yielding a recombinant frequency ($\text{RF}$) of less than $50\%$ in test crosses.

Null hypothesis (Linkage analysis)

The statistical baseline assumption stating that two examined genes are not linked, predicting independent assortment with a $1:1:1:1$ segregation ratio among test cross progeny.

Chi-square test ($\chi^2$)

A statistical goodness-of-fit test formulated as $\chi^2 = \sum \frac{(O - E)^2}{E}$ used to evaluate whether deviations between observed and expected progeny counts are due to random chance or signify genetic linkage (using $p \le 0.05$ as the rejection cutoff).

Phosphodiester bond

The strong covalent linkage connecting the $3'$ hydroxyl group of one nucleotide's deoxyribose to the $5'$ phosphate group of the adjacent nucleotide in a DNA polymer.

Semiconservative replication

The mechanism of DNA synthesis in which each replicated double helix contains one intact template parental strand and one newly synthesized daughter strand.

Topoisomerase

An enzyme that relieves the positive supercoiling and torsional tension generated ahead of the replication fork by transiently cleaving and swiveling the DNA strands.

Okazaki fragments

Short stretches of newly synthesized DNA produced discontinuously in the $5'$ to $3'$ direction on the lagging strand template during DNA replication.

Ames test

A biological screening assay utilizing histidine-requiring Salmonella strains to detect whether a chemical substance induces DNA mutations and carries potential carcinogenicity.

Transposable element (TE)

A mobile segment of DNA ranging in size from $50\text{ bp}$ to $10\text{ kb}$ that has the ability to move from one position to another within the host genome.

Barbara McClintock

The geneticist who discovered transposable elements through her cytogenetic studies of kernel mottling in maize, earning the Nobel Prize in 1983.

Retrotransposon

A class of transposable elements that mobilizes via an RNA intermediate copied into cDNA by reverse transcriptase through a 'copy and paste' mechanism.

DNA transposon

A class of transposable elements that mobilizes directly as DNA without an RNA intermediate, using a transposase-catalyzed 'cut and paste' mechanism.

LINEs (Long interspersed elements)

Autonomous poly-A type retrotransposons ($6-8\text{ kb}$ long) in the human genome that encode ORF1 (an RNA-binding protein) and pol (reverse transcriptase/endonuclease), accounting for approximately $20\%$ of human DNA across $1{,}000{,}000$ copies.

SINEs (Short interspersed elements)

Nonautonomous poly-A type retrotransposons ($< 0.3\text{ kb}$ long) derived from RNA polymerase III transcripts that depend on LINE machinery for mobility, accounting for $13\%$ of the human genome across $2{,}000{,}000$ copies.

HERVs (Human endogenous retroviruses)

LTR-type retrotransposons ($1-11\text{ kb}$ long) with long terminal repeats that carry genes homologous to retroviral gag, pol, and env; make up approximately $8\%$ of the human genome as immobile evolutionary relics.

Autonomous transposable element

An intact mobile element that encodes all the functional proteins (such as transposase or reverse transcriptase) necessary to catalyze its own movement, exemplified by the Ac element in maize.

Nonautonomous transposable element

A defective transposable element lacking intact coding sequences that requires the enzymes generated by an autonomous element elsewhere in the genome to transpose, exemplified by the Ds element in maize.

Composite transposon

A mobilized genetic unit generated when two neighboring transposons on the same chromosome have their outermost inverted repeats recognized by transposase, translocating intervening host genes (such as drug-resistance markers).

Aneuploidy

A condition in which an individual possesses a chromosome number that is not an exact multiple of the haploid set, resulting in the loss or gain of one or more specific chromosomes.

Nullisomy

A form of aneuploidy resulting from the loss of both members of a homologous chromosome pair ($2n - 2$).

Monosomy

A form of aneuploidy characterized by the loss of a single chromosome from a diploid set ($2n - 1$); typically lethal for human autosomes.

Trisomy

A form of aneuploidy characterized by the gain of a single extra chromosome to a diploid set ($2n + 1$), exemplified by Down syndrome (Trisomy 21).

Down syndrome

An autosomal aneuploidy condition characterized by trisomy 21 ($2n + 1$) that occurs with a birth frequency of approximately $1/700$.

Turner syndrome

A sex-chromosome aneuploidy in phenotypic females who are monosomic with an $XO$ karyotype ($2n - 1$), characterized by short stature, webbed neck, sterility, and a birth frequency of $1/5000$.

Klinefelter syndrome

A sex-chromosome aneuploidy in tall, sterile phenotypic males caused by an $XXY$ or related poly-X karyotype ($2n + 1$) with a birth frequency of $1/500$.

Meiotic nondisjunction

The failure of homologous chromosomes to segregate during meiosis I (generating $100\%$ aneuploid gametes: two $n + 1$ and two $n - 1$) or of sister chromatids to separate during meiosis II (generating $50\%$ aneuploid gametes: one $n + 1$, one $n - 1$, and two normal $n$).

Gynandromorph

A sexual mosaic individual possessing both male and female body sectors, caused by mitotic chromosome loss of an X chromosome in early development of an $XX$ zygote in Drosophila.

Colchicine

A spindle-inhibiting chemical agent used experimentally in genetics to prevent spindle fiber formation and block chromosome movement during mitosis, inducing genome doubling.

Autopolyploid

A polyploid organism in which all duplicate chromosome sets originate entirely from a single parental species.

Allopolyploid

A hybrid polyploid organism whose chromosome sets are derived from two distinct, but related, parental species.

Amphidiploid

An allopolyploid organism containing two complete diploid chromosome sets, one derived from each ancestral species (e.g., Raphanobrassica possessing 18 chromosomes).