Chromosomes

Arcocentric

A chromosome that has its centromere close to one end - chromosomes 13,14,15,21 and 22 in humans

Autosome

Any chromosome that is not X or Y sex chromosome

Centromere

The position on a chromosome where the sister chromatids are joined; the location of the kinetochore where the spindle fibers attach during cell division to pull the chromatids apart

Kinetochore

Protein complex that assembles on the centromere

Chromatid

In a dividing cell, a chromosome consists of 2 identical sister chromatids joined at the centromere. After cell division, and until the DNA is next replicated, a chromosome consists of a single chromatid

Chromatin

A general term for the DNA complex that makes up chromosomes

Euchromatin 

Chromatin with a relatively open structure in which genes can be active; the opposite of heterochromatin 

G-banding

A standard procedure in which chromosomes are treated so that they stain in a characteristic and reproducible pattern of dark and pale bands

Heterochromatin

Chromatin that is highly condensed and genetically inactive. Found mainly at centromeres

Homologous chromosomes

The 2 no. 1s or 2 no. 2s, etc in a person. Note that unlike sister chromatids, homologous chromosomes are not copies of one another, and they may differ in small ways (minor DNA sequence differences) or sometimes in large ways (because of translocations, etc).

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

Inversion

A structural abnormality in which part of a chromosome is in the wrong orientation to the rest

Karyotype

A person’s chromosome constitution - also used loosely to describe a display of a person’s chromosomes (visual representation showing number, size, and shape)

Metacentric

A chromosome that has its centromere in the middle (e.g. human chromosomes 3 and 20)

Monosomy

Having one copy of one particular chromosome, but 2 of all the others (i.e. 45 in total for an autosomal monosomy)

Robertsonian translocation

A special type of translocation in which 2 arcocentric chromosomes are joined close to their centromeres 

Sister chromatids

The 2 chromatids of a chromosome as seen in a dividing cell. Sister chromatids are copies of each other, made during the preceding round of DNA replication

Submetacentric

A chromosome that has a long arm and a short arm, e.g. most human chromosomes

Tetomere

The special structure at the end of each chromosome arm 

Translocation

A structural abnormality in which 2 chromosomes swap non-homologous segments

Triploidy

Having 3 complete sets of chromosomes (i.e. 69 in total - XXX, XXY, XYY). Common at conception but triploid embryos and fetuses never survive to term and those that do so do not live for long

Trisomy

Having 3 copies of 1 particular chromosome, but 2 of all the others (i.e. 47 in total)

Autosomal trisomies

All possible autosomal trisomies can be found among early miscarriages, but only trisomies 13.18 and 21 generally survive to term. Chromosomes 13,18 and 21 have lowest density of genes of any chromosomes in our genome, so that fewer genes are present in abnormal numbers in these trisomies compared to trisomies of other similar sized chromosomes