Numerical Chromosomal Abnormalities Flashcards

1
Q

What is the purpose of the cell cycle

A

Triggered when more cells are needed.

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2
Q

What happens in the G1 phase of the cell cycle

A

Cell makes a variety of proteins needed for DNA replication and grow

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3
Q

What happens in the S phase of the cell cycle

A

Chromosomes are replicated so that each chromosome now consists of two sister, identical chromatids

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4
Q

What happens in the G2 phase of the cell cycle

A

Synthesis of proteins especially microtubules and finish growing in size

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5
Q

What are homologous chromosomes

A

Chromosomes with the same gene sequence, loci, chromosomal length, and centromere location

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6
Q

Why are chromosomes sometimes shown with a single chromatid and some shown with two sister chromatids?

A
  • Single chromatids are shown before the S phase
  • sister chromatids are shown after the S phase
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7
Q

Describe the steps in mitosis

A
  • 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
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8
Q

Summarise what happens in Mitosis

A
  • 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
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9
Q

Describe the steps in Meiosis I

A
  • 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
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10
Q

Describe the steps in Meiosis II

A
  • Normal mitosis steps but no DNA replication takes place.
  • Forms 4 haploid gametes
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11
Q

How does crossing over occur in metaphase I

A
  • Homologous chromosomes align
  • Form a bivalent structure via the synaptonemal complex
  • Exchange genetic material (recombine)
  • New allele combinations and haplotypes are formed
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12
Q

Summarise what happens in meiosis

A

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

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13
Q

Describe the human Karyotype

A

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
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14
Q

What are some chromosomal numerical abnormalities

A

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)
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15
Q

How does aneuploidy arise

A
  • error in chromosome segregation in either meiosis I or II
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16
Q

What are other ways we get genetic differences in gametes

A
  • Crossing over
  • Independent assortment of maternal and paternal homologs
17
Q

What are some Autosomal aneuploidies?

A
  • Trisomy 13 - Patau’s
  • Trisomy 18 - Edward’s
  • Trisomy 21 - Down’s
18
Q

What are some Sex Chromosomes Aneuploidies

A
  • Turner’s - 45, X in women
  • Triple X syndrome - 47, XXX in women
  • Klinefelter’s - 47,XXY in men
  • 47, XYY in men
19
Q

What happens in mitotic non-disjunction

A
  • Majority of cell are 2n, some are 2n + 1 which
  • cells which have 2n - 1 die off whereas 2n + 1 lives on
20
Q

Define what is meant by mosaicism

A
  • The presence of two or more genetically different cell lines derived from a single zygote
21
Q

Describe what happens in trisomy rescue lag

A
  • 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
22
Q

What is the biological relevance to mosaicism

A
  • Mosaic phenotype throughout to be less severe

Difficult to assess:
- What proportion of different cell types
- Which tissue/organs are affected

23
Q

What are partial trisomy and monosomy

A
  • Full monosomy arises by non-disjunction
  • Partial monosomy/trisomy (microdeletion/duplication syndromes) far more common - mechanism different