Chromosome

A chromosome carries genes and other genetic material. A haplotype records the alleles carried on one chromosome at specified genetic loci.

In a typical eukaryotic cell nucleus, an unreplicated chromosome comprises one long double-stranded DNA molecule with associated proteins; a replicated chromosome comprises two associated sister chromatids. It need not be X-shaped: that familiar outline depicts a condensed, replicated state.^[ref]

For these ordinary centromere-bearing chromosomes, count by centromere units before separation: the paired sister centromere regions count together as one replicated chromosome. Once sister chromatids separate in anaphase of mitosis or meiosis II, each becomes a daughter chromosome. Thus the undivided anaphase biological cell in the $2n=6$ mitotic example transiently contains twelve daughter chromosomes, but each pole receives six and each completed daughter cell nucleus remains diploid. State the counting compartment and stage; summing both poles is not the ploidy of one daughter.

A locus prone to abnormal gaps, undercondensation or breakage under specified cellular conditions. A cytological gap is not by itself proof of a DNA double-strand break.

Examination of chromosome number and morphology. Resolution is insufficient for many small sequence changes, so a normal karyotype does not establish an unchanged genome.

A chromosome-number state differing by individual chromosomes from an exact whole-set complement. It is distinct from a uniform change in the number of complete chromosome sets.

A sex chromosome whose non-recombining regions provide a predominantly paternal human genealogy. That genealogy does not trace all ancestors of an individual.

A joined chromosome structure produced by some recombination events and requiring resolution before successful bacterial chromosome allocation.

The specialised end region of a linear chromosome, with DNA and proteins that support chromosome-end protection and maintenance.

The compaction and organisation of chromosomal material during division. Condensation helps chromosome handling but is distinct from the removal of sister-chromatid cohesion.

A chromosome involved in genetic sex determination. Its inheritance and copy number can expose incompatibilities differently in the two sexes of hybrids.

A change in the organization of chromosome segments. A balanced parental arrangement can preserve sequence dosage while causing abnormal segregation in a heterozygous hybrid.

The number and structural organization of a biological cell's chromosomes. A difference between parental karyotypes can affect pairing and segregation in hybrids, but its consequences require direct tests rather than inference from the chromosome count alone.

Chromosome segregation allocates chromosomes to the products of division. Mitosis separates sister chromatids, preserving ploidy in completed daughter cell nuclei. Meiosis I separates homologous chromosomes with their sister chromatids still associated, reducing diploid to haploid ploidy; meiosis II separates the sister chromatids. Only the first of the two divisions reduces the number of complete sets.^[ref]

For example, correct chromosome segregation of the stipulated three homologous chromosome pairs gives each meiosis I product one $A$, one $B$ and one $D$, each replicated. Giving one product both $A$ members and the other none would preserve the total DNA across products while failing to give each a complete set. Conservation of material alone does not prove correct chromosome segregation.

Physical association of replicated sister chromatids before their regulated separation. It supports orderly biorientation and segregation.

Attachment of one kinetochore to microtubules connected to both spindle poles. It can evade an unattached-kinetochore checkpoint signal and produce lagging chromosomes.

Attachment of sister kinetochores toward opposite spindle poles, supporting accurate allocation of sister chromatids.

A centromere is the chromosome region where a kinetochore assembles.^[ref] Following DNA replication, both sister chromatids have their own copied centromere regions; the counting convention groups the attached sisters as one replicated chromosome. Cohesin maintains sister association: a centromere is not an extra DNA bridge joining two separate DNA molecules.^[ref]

Propagation or re-establishment of a functional centromeric chromatin state across divisions without specification by a unique DNA sequence alone. Inheritance of chromosome function and invariance of the precise occupied base pairs are different claims.

A functional centromere established at a location different from the ancestral centromere. Its formation can demonstrate that particular repeat arrays are not obligatory for centromeric function.

A kinetochore is a protein assembly at a centromere that attaches a chromosome to microtubules of the spindle apparatus. Each replicated chromosome has sister kinetochores; their attachment and subsequent separation help distribute the copied material. A kinetochore is therefore the attachment machinery, while the centromere is its chromosome site.^[ref]

The kinetochore ablation experiment tests this mechanical role directly. Counting kinetochores as independent chromosomes before sister separation would confuse duplicated attachment machinery with ploidy.

Homologous chromosomes carry corresponding genetic loci, although their alleles may differ. In the diploid $AB/ab$ example, the two homologous chromosomes carry the parental haplotypes $AB$ and $ab$.

A chromatid is one of the two copies of a replicated chromosome. Each homologous chromosome contributes two sister chromatids before separation in meiosis; chromosomal crossovers can exchange segments between nonsister chromatids.

// Target: biology.bigb

Sister chromatids are the paired chromatids originating from DNA replication of the same chromosome.