Polidy (Lucy) Flashcards
What is ploidy?
The chromosome number within a cell or within the cells of an organism
What is euploidy?
Variation in the no. complete sets of chromosome
What is aneuploidy?
Variation in the no. particular chromosomes within a set
Do all cells of an organism always have same amt of DNA?
No, different varieties of a single species may cont different bumbers of copies of the same genome
Why do changes is ploidy matter?
- Change in no chromosome sets in the germ line can give rise to a new variety or species
- May also occur somatically by chance & be pathological/be programmed & be part of developmental program of an organism
- Give rise to –> endopolyploidy & polytenization
- Entire chromosme may also be ‘eliminated’ during development
- Specific seq may be removed; called ‘chromosome dimunition’
Give an example of why polyploidy matters:
Many agriculturally important plants are polyploidy
Strawberries = octoploid strawberries are sold lots in supermarkets over diploid which are smaller
Give an example of polyploidy in amphibians:
African clawed toad = Xenopus laevis (4N) is larger & Xenopus tropicalis (2N)
How is the no. chromosomes represented in a basic ancestral set?
Represented as x - this is the monoploid number
n, the haploid number is no. chromosomes in gamete
What are the names for the number of chromosome sets in somatic cells of the following;
2x =
3x =
4x =
6x =
2x = diploid
3x = triploid
4x = tetraploid
6x = hexaploid
What are the 2 ways polyploids can form?
- Autopolyploids
- Allopolyploids
What are autopolyploids?
All chromosomes from same species; 4x = AAAA
By spontaneous premiotic endoreduplication
What are allopolyploids?
Chromosome sets from different species; 4x = AA BB
By interspecific hybridisation & endoreduplication (we need this to get 2 copies of each)
A and B sets often partially homologous - homeologous e.g. bread wheat AA BB DD
What is one rule of polyploidy that is generally followed?
Species rarely have odd numbers of x number (monoploid number) e.g. 3x
Why is accurate segregation in meiosis important for polyploidy?
Accurate segregation at the first meiotic division req chromosome pairing & so req a PAIRING PARTENER
How efficient is meiosis in diploids & tetraploids?
Meiosis yields viable gametes
How efficient is meiosis in triploids?
Very, very inefficient due to random segregation of unpaired homologue
What advantages over diploid relatives can polyploidy cause?
- Inc cell size = larger fruits, flowers & organism
- Advantageous characteristics from multiple species (allopolyploids) (immediate hybrid vigour)
- Whole genome duplications allow long term evolution of genetic novelty
What are the possible disadvatages of polyploidy?
Sterility (problems forming “balanced” gametes during meiosis), especially for odd-numbered polyploids
What can even numbered allopolyploids produce?
Can produce balanced gametes at high freq & exist as fully fertile polyploid specie propagated by seed
(i.e. all the chromosmes can pair up as bivalents & segregation in orderly)
What happens if we have 3x pairing?
Makes a pairing between 2 homologous chromosomes but leaves an extra chromsomes that doesn’t pair
Third extra one segregates randomly
What happens with a triploid organism who’s karyotype should be x=2?
3 copies of chromosmes –> 2 accurately segregate; but 3rd segregates randomly
Makes aneuploid gametes
Got to have same no. chromosomes to be viable
What is an aneuploid gamete?
A gamete with an abnormal amount of chromosomes
Why do gametes have to have the same number of chromosomes to be viable?
Zygote has different no. copies of chromsomes
Proteins produced will not be balanced = devleopmental issues
Zygote will not be viable
Why is triploidy in chromsome 22 not equivalent 23?
22 has many more genes than 23 (even tho 23 is larger it is gene sparse)
Therefore 23 isn’t affected too much when there is an extra chromsome present however 22 being gene dense this will have signifiacnt effect (same for 15)
= embryo inviable as it’s triploid
Give an example of a triploid organism:
Bananas are triploid
They are all therefore clones of each other
All have same DNA so are at risk of being destroyed due to disease they’re not resistant to
Could all die
What ploidy do most crops have?
Most crops are polyploid
Are changes in chromosome structure/number always random?
NO
In some species changes can occur as part of normal devleopment; they are not pathological or accidental but are programmed
What are some examples of programmed changes in chromosome number/structure?
- Endopolyploidy
- Polytenization
- Chromosome elimination
- Chromosome diminution
- Gene amplification
(Programmed changes in genome structure)
What is endoreduplication?
A process where a cell duplicates its DNA w/o undergoing cell division
Results in cells w multiple copies of the same chromosome
Umbrella term for endocycling & endomitosis
What is endomitosis?
Cell undergoes some aspects of mitosis but fails to complete the process
Leads to cells w multiple copies of chromosomes
What is endocycling?
Where cell undergoes multiple rounds of DNA replication w/o dividing into separate cells
= multiple copies of their genetic material
Multiple rounds of endomitosis
What does endocycling often result in?
The production of polytene chromosomes cont many sister chromatids bound together
What are the 2 different types of endomitosis?
- If mitosis aborts between anaphase A & anaphase B –> sister chromatids seperate but are ecapsulated into the same nucleus
- If mitosis proceeds further = multiple nuclei form in the endoreduplication
A breast cancer cell line can show evidence of tetraploidy
Shows links between tetraploidy & cancer
Name 3 types of programmed changes in chromosome/genome sets:
- Endopolyploidy
- Polytenization
- Multinucleate cells
What are programmed changes in the number of chromosomes/genome sets per cell usually for?
Usually to provide increased RNA synthesis rates
Cell; usually incompatible w normal chromosome segregation & maintenance of intact genomes
What is endopolyploidy?
Repeated chromosome replication w/o cell division
(e.g. nurse cells of drosophila egg chamber; DNA levels up to 4000 X C)
What is polytenization?
A kind of end-polyploidy with in-register chromatid pairing
What are multinucleate cells?
Temporary endopolyploidy
Nuclear multiplication w/o cell division
Germ line genome conserved e.g. oocytes
What are some programmed changes in the number of individual chromosomes and/or genes per cell?
- Chromosome diminution
- Chromosome elimination
- Gene amplification
What is chromosome diminution?
Mitosis happens in germ line (where gametes are produced) = 2 sister chromatids
In somatic cells certain sequences get deleted & gives rise to altered chromosome structures
What happens during chromosome diminution?
(In somatic cell lineages)
Chromatin dimunition (certian seq deleted)
Eliminated DNA is enriched for germline-expressed genes
Cell division
Results in altered somatic chromosomes
Give an example of chromosome elimination:
Classic example are sciarid flies where paternal chromosomes are selectively eliminated at 3 stages of the life cycle
Guve 3 examples of gene amplification in whole organisms
1 - Ribosomal DNA amplification in Xenopus oocytes involves an extrachromosomal rolling circle
2 - Maturation of micronucleus in cilliates protozoa e.g. Tetraphymena
3 - Chorion gene amplification in Drosophila nurse cell
What are the 2 types of programmed gene amplification?
1 - Extrachromosomal
2 - Intrachromosomal
What is extrachromasomal gene amplification?
Does not permanently affect genome of dividing cells or germ line
What is intrachromosomal gene amplification?
Alters genome; incompatible with chromsome segregation
i.e. these are “dead-end” cells that do not divide (Drosophila follicle cells are also endopolyploid) but are specialised for high rates of chroion RNA/protein production
Specialised for high rate of protein production
Watch video back on gene amplification rDNA in Xenopus oocytes
The rolling circle thing (or megan if you have notes on this)
Watch video back on gene amplification slide II
Maturation of macronucleus
Megan if oyu have notes pls xx
Watch video back on gene amplification III
Amplification of chorion genes in Drosophila
Megan pls if u have notes xx
What is gene amplification useful for
Very useful for protein production;
Bc gene amplification can occur in cells in culture & in cancer cells; amplified genes exist as either small extrachromasomal regions (double minutes) or integrated as (homogenously staining regions)
What are the 2 ways which amplified genes can exist?
Either:
Double minute chromosomes extra-chromasomally –> unstable
OR
As integrated homogeneously staining regions –> stable
Where does oncogene amplification often occur?
In solid tumours
(Oncogenes are amplified, produces more copies of amplified oncogenes = triggers production of tumour)
Give 3 examples of amplified oncogenes:
- MYCN oncogene amplified 20% neuroblastomas; visible as HSRs
- EGFR: 40% gliomas; as double minutes
- ERBB2 in great, ovarian and gastric cancers
Summary:
What are the 5 types of changes in ploidy & chromosome structure?
1 - Different types of polyploids
2 - Developmental changes in policy
3 - Programmed changes in chromosome structure
4 - Gene amplification in development
5 - Gene amplification in tumours & biotechnology
Give 3 examples of a polyploid organism:
Wheat, strawberries & Xenopus
