Numerical Chromosomal Abnormalities Flashcards
What is the purpose of the cell cycle
Triggered when more cells are needed.
What happens in the G1 phase of the cell cycle
Cell makes a variety of proteins needed for DNA replication and grow
What happens in the S phase of the cell cycle
Chromosomes are replicated so that each chromosome now consists of two sister, identical chromatids
What happens in the G2 phase of the cell cycle
Synthesis of proteins especially microtubules and finish growing in size
What are homologous chromosomes
Chromosomes with the same gene sequence, loci, chromosomal length, and centromere location
Why are chromosomes sometimes shown with a single chromatid and some shown with two sister chromatids?
- Single chromatids are shown before the S phase
- sister chromatids are shown after the S phase
Describe the steps in mitosis
- Prophase - Nuclear membrane breaks down and spindle fibres form
- Metaphase - Sister chromatids align at the centre and the spindles attach at the centromere
- Anaphase - Chromatids are pulled apart to either pole
- Telophase - Nuclear membrane reforms and the spindle fibres break down
- Cytokinesis - The cell splits into 2 identical daughter cells
Summarise what happens in Mitosis
- Normal form of cell division
- Sister chromatids identical
- Each daughter cell receive 1 chromatid of each chromosome
- Daughter cells genetically identical to parent
- Each chromosome behaves independently
- Homologues do not interact
- Align as 46 separate chromosomes
- All somatic cells
Describe the steps in Meiosis I
- Prophase I - Nuclear membrane breaks down and spindle fibres form
- Metaphase I - Sister chromatids align at the centre and the spindles attach at the centromere, Crossing over occurs
- Anaphase I - Chromatids are pulled to either pole
- Telophase I - Nuclear membrane reforms and the spindle fibres break down
- Cytokinesis - The cell splits into 2 cells that will undergo meiosis II
Describe the steps in Meiosis II
- Normal mitosis steps but no DNA replication takes place.
- Forms 4 haploid gametes
How does crossing over occur in metaphase I
- Homologous chromosomes align
- Form a bivalent structure via the synaptonemal complex
- Exchange genetic material (recombine)
- New allele combinations and haplotypes are formed
Summarise what happens in meiosis
Two phases: meiosis I and meiosis II
Meiosis I:
- Align as 23 bivalents
- Allows for chiasma formation (i.e. recombination)
- Pulls apart homologues from one another
- Daughter cells have 23 chromosomes (each with 2 chromatids)
Meiosis II:
- Align as independent chromosomes
- Sister chromatids pulled apart
- Daughter cells have 23 chromosomes (each of 1 chromatid)
Very important to introduce natural variation via independent assortment of chromosomes and recombination
Daughter cells genetically unique
Describe the human Karyotype
Humans have 23 pairs of chromosomes:
- 22 pairs autosomes, 1 pair sex chromosomes XX or XY
Metacentric:
- p & q arms even length
- 1-3, 16-18
Submetacentric:
- p arm shorter than q
- 4-12, 19-20, X
Acrocentric;
- Long q, small p
- p contains no unique DNA
- 13-15, 21-22, Y
What are some chromosomal numerical abnormalities
HAPLOID:
- one set of chromosomes (n=23) as in a normal gamete.
DIPLOID:
- cell contains two sets of chromosomes (2n=46; normal in human)
POLYPLOID:
- multiple of the haploid number (e.g. 4n=92)
ANEUPLOID:
- chromosome number which is not an exact multiple of haploid number - due to extra or missing chromosome(s) (e.g. 2n+1=47) (trisomy, monosomy)
How does aneuploidy arise
- error in chromosome segregation in either meiosis I or II
What are other ways we get genetic differences in gametes
- Crossing over
- Independent assortment of maternal and paternal homologs
What are some Autosomal aneuploidies?
- Trisomy 13 - Patau’s
- Trisomy 18 - Edward’s
- Trisomy 21 - Down’s
What are some Sex Chromosomes Aneuploidies
- Turner’s - 45, X in women
- Triple X syndrome - 47, XXX in women
- Klinefelter’s - 47,XXY in men
- 47, XYY in men
What happens in mitotic non-disjunction
- Majority of cell are 2n, some are 2n + 1 which
- cells which have 2n - 1 die off whereas 2n + 1 lives on
Define what is meant by mosaicism
- The presence of two or more genetically different cell lines derived from a single zygote
Describe what happens in trisomy rescue lag
- During anaphase lag the cell gets rid of an extra chromosome to return to disomic.
- This only happens in a proportion of cells leading to mosaicism
What is the biological relevance to mosaicism
- Mosaic phenotype throughout to be less severe
Difficult to assess:
- What proportion of different cell types
- Which tissue/organs are affected
What are partial trisomy and monosomy
- Full monosomy arises by non-disjunction
- Partial monosomy/trisomy (microdeletion/duplication syndromes) far more common - mechanism different
