chromosome variation

chromosome mutations

• rearrangements - alters structure of chromosome

• aneuploids - alters number of chromosomes

• polyploids - one or more complete set of chromosomes are added

chromosome rearrangement

4 forms: - all happen in prophase I

• duplication

• inversion

• translocation

• deletion

duplication

tandem duplication - duplicated segment immediately adjacent to original segment

displaced duplication - duplicated segment located some distance from the original segment

reverse duplication - when the duplication is inverted

terminal duplication - at the end of a chromosome arm

interstitial duplication - within a chromosome arm

homozygous duplication - carries duplication on both chromosomes

heterozygous duplication - carries duplication on one chromosomes so chromosomes no longer homologous and don’t align properly, leads to ‘loop’ structure forming 

inversion

• section of the gene flipped the wrong way around

• paracentric inversion - do not include centromere

• pericentric inversion - includes centromere

effects of inversions

• gene broken into 2 parts - one part moves to a new location and destroys function of gene

• position effect - position of the gene altered by an inversion

effects of inversions on meiosis

homozygous - no effect

heterozygous - gene order of 2 homologs differs, homologous sequences can only align and pair up if the 2 chromosomes form an inversion loop:

crossing over leads to production of abnormal gametes for paracentric (centromere not involved) inversions:

• in prophase I, inversion loop forms so homologous sequences pair up - synapsis, swaps orientation of inverted chromosome

• single cross over within inversion loop results in unusual structure as 2 outer chromatids are not involved in loop and 2 inner chromatids are

• one chromatid has 2 centromeres (dicentric) and other has none (acentric) - acentric chromosome non viable and is lost

• in anaphase I, centromeres are pulled to opposite poles and homologous chromosomes separate

• dicentric chromatid stretched across centre of nucleus so a dicentric bridge is formed - this breaks as centromeres are pulled further apart

• 2nd division chromatids separate and 4 gametes produced:

1. 2 contain original chromosomes

2. 2 contain recombinant genes so are not viable - portions of genes lost

• also disrupts recombination by reducing crossing over within inverted regions and increasing it elsewhere in the genome

translocation

interaction between 2 non-homologous chromosomes and segment from each chromosome exchanged

non-reciprocal - one direction only

reciprocal - 2 bits exchanged

Robertsonian

effects of translocation

• homologous sections of chromosomes interact to form cross-like shapes, end up with gametes with 3 different combinations: alternate, adjacent 1 (not viable), adjacent 2 (not viable)

Robertsonian translocation

• short arm of acrocentric chromosome exchanged with the long arm of another chromosome to create one large metacentric chromosome

• small fragment lost as it does not have enough mass to separate properly in meiosis/mitosis

• 2 chromosomes fused so that person has 45 chromosomes instead of 46

• can result in trisomy in gametes - leads to down syndrome

fragile sites

chromosomal regions susceptible to damage

aneuploidy

• increase/decrease in number of chromosomes

• variations in copy number

• can result in:

deletion of centromere - prevents spindle microtubules from attaching in anaphase so cell division cannot occur

Robertsonian translocation

non-disjunction - both chromosomes go into one cell and not other, end up with trisomy

types of aneuploidy

nullisomy - loss of both members of a pair of homologous chromosomes, 2n-2

monosomy - loss of a single chromosome, 2n-1

trisomy - gain of a single chromosome, 2n+1

tetrasomy - gain of 2 homologous chromosomes 2n+2

polyploidy

presence of more than 2 sets of chromosomes

autopolyploidy - single species, arises through non-disjunction in mitosis/meiosis

allopolyploidy - 2 species, extra chromosome sets come from a different species through hybridisation of species with different numbers of chromosomes

significance:

• increase in cell size - bigger nucleus due to increased DNA content, cell grows to maintain nucleus:cytoplasm ratio

• larger plants - bigger cells = bigger organs = bigger plant

• evolution - may give rise to new species