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Principles of Disease 16 > Chromosomes and Cell division > Flashcards

Chromosomes and Cell division Flashcards

1
Q

Describe the basic structure and packaging of chromosomes.

A

The tips of the arms are called Telomeres. During replication, parts of these tips are lost.

The DNA is expressed in sections called Euchromatin, where it is more open, and the denser Heterochromatin contains the DNA that the chromosome is not currently using.

The DNA is packaged by wrapping itself around histone proteins, like beads on a string, to form Nucelosomes. These form the chromatin, which then wraps itself into a fibre, which ultimately becomes the chromosome.

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2
Q

Describe the laboratory diagnosis of genetic disease due to abnormalities in chromosome number. 


A

Chromosome analysis using karyotyping to find the number of chromosomes and if translocation has occurred

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3
Q

Describe the types of structural chromosomal abnormalities. 


A

Robertsonian Translocation - This occurs in Acrocentric chromosomes (i.e. One arm is shorter than the other). This gives one chromosome with both long arms, while the other has both the short ones.

Deletions - This occurs when sections of chromosomes just seem to disappear from one of the arms, if not both.

Inversions - can be Paracentric (when a section of the DNA in the arm of the chromosome is inverted) or Pericentric ( when the DNA around the centromere of the chromosome is inverted)

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4
Q

Define a telomere

A

A telomere is a region of repetitive nucleotide sequences at each end of a chromosome, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes

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5
Q

How long is a telomere usually in human chromosomes?

A

10-15kb

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6
Q

Define telomerase

A

Telomerase - is a ribonucleoprotein that adds a species-dependent telomere repeat sequence to the 3’ end of telomeres, which

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7
Q

What is the hayflick limit?

A

The Hayflick limit is the number of times a normal human cell population will divide until cell division stops. Empirical evidence shows that the telomeres associated with each cell’s DNA will get slightly shorter with each new cell division until they shorten to a critical length.

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8
Q

When does the cell reach its Hayflick limit?

A

Usually after 50-70 divisions

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9
Q

What are the repeats seen at telomeres?

A

5’-TTAGGG-3’ repeats

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10
Q

What links the okazaki fragments on the lagging strand in DNA replication?

A

DNA ligase

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11
Q

What are centromeres?

A
  • Constricted region joining sister chromatids
  • Repetitive satellite DNA sequences
  • Site of kinetochore
  • Protein complex that binds to microtubules
  • Required for chromosome separation during cell division
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12
Q

What is satellite DNA?

A

Satellite DNA consists of very large arrays of tandemly repeating, non-coding DNA. Satellite DNA is the main component of functional centromeres, and form the main structural constituent of heterochromatin.

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13
Q

Whats the difference between heterochromatin and euchromatin?

A 
Heterochromatin
–	Condensed structure
–	Silenced genes
 Euchromatin 
–	Open structure
–	Active genes
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14
Q

What charge do histones have?

A

Positive

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15
Q

What is the purpose of packaging DNA?

A
  • Negatively charged DNA neutralised by positive charged histone proteins
  • DNA takes up less space
  • Inactive DNA can be folded into inaccessible locations until required
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16
Q

What is a karyotype?

A

A karyotype is the number and appearance of chromosomes in the nucleus of a eukaryotic cell. The term is also used for the complete set of chromosomes in a species or in an individual organism and for a test that detects this complement or measures the number.

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17
Q

What is Giemsa staining?

A

The giema stain is specific for the phosphate groups of DNA and attaches itself to regions of DNA where there are high amounts of adenine-thymine bonding (heterochromatin). Giemsa stain is used in Giemsa banding, commonly called G-banding, to stain chromosomes and often used to create a karyogram (chromosome map). It can identify chromosomal aberrations such as translocations and rearrangements.

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18
Q

Which arms are the p arms of a chromosome?

A

short

19
Q

Which arms are the q arms of a chromosome?

A

long

20
Q

What are metacentric chromosomes?

A

Metacentric - These are X-shaped chromosomes, with the centromere in the middle so that the two arms of the chromosomes are almost equal.

21
Q

Which chromosomes of a normal human karyotype are metacentric?

A

In a normal human karyotype, five chromosomes are considered metacentric: chromosomes 1, 3, 16, 19, and 20.

22
Q

What can result in the formation of metacentric chromosomes?

A

Balanced translocation: the fusion of two acrocentric chromosomes to form one metacentric chromosome.

23
Q

What are submetacentric chromosomes?

A

Submetacentric- if arms’ lengths are unequal, the chromosome is said to be submetacentric.

24
Q

What are acrocentric chromosomes?

A

Acrocentric- if the p (short) arm is so short that it is hard to observe, but still present, then the chromosome is acrocentric

25
Q

Which chromosomes in the human karyotype are acrocentric?

A

The human genome includes six acrocentric chromosomes: 13, 14, 15, 21, 22 and the Y chromosome.

26
Q

What is Fluorescence in situ hybridization (FISH) used for?

A

Fluorescence in situ hybridization (FISH) is a cytogenetic technique that uses fluorescent probes that bind to only those parts of the chromosome with a high degree of sequence complementarity.
It is used to detect and localize the presence or absence of specific DNA sequences on chromosomes.
FISH is often used for finding specific features in DNA for use in genetic counseling, medicine, and species identification.

27
Q

How can different FISH probes be used for different investigations?

A

Centromeric probes
– Useful for determining chromosome number
Telomeric probes
– Useful for detecting subtelomeric rearrangements
Whole chromosome probes
– Cocktail of probes
– Spectral karyotype
– Useful for detecting translocations and rearrangements

28
Q

Describe meiosis

A

• Cell division in germ cells
• Diploid cells (in ovaries and testes) divide to form haploid cells
• Chromosomes are passed on as re-arranged (recombined) copies
– Creates genetic diversity

29
Q

What are the two processes of gametogenesis?

A
  • Oogenesis = process of egg formation

* Spermatogenesis = process of sperm formation

30
Q

What is X inactivation?

A
  • X-inactivation is a process by which one of the copies of the X chromosome present in female mammals is inactivated.
  • The inactive X chromosome is silenced by its being packaged in such a way that it has a transcriptionally inactive structure called heterochromatin.
  • As nearly all female mammals have two X chromosomes, X-inactivation prevents them from having twice as many X chromosome gene products as males, who only possess a single copy of the X chromosome.
  • The choice of which X chromosome will be inactivated is random in placental mammals such as humans, but once an X chromosome is inactivated it will remain inactive throughout the lifetime of the cell and its descendants in the organism.
31
Q

What occurrence in nature is a common example of x inactivation?

A

The coloration of tortoiseshell and calico cats is a visible manifestation of X-inactivation. The black and orange alleles of a fur coloration gene reside on the X chromosome. For any given patch of fur, the inactivation of an X chromosome that carries one gene results in the fur color of the other, active gene.

32
Q

Describe the process of mitosis

A 
Interphase
Prophase
•	Chromosomes condense
•	Nuclear Membrane disappears
•	Spindle fibres form from centrioles
Metaphase
•	Chromosomes align at equator of cell
•	Attached by microfilaments to each centriole
•	Maximum condensation of chromosomes
Anaphase
•	Sister chromatids separate at centromere
•	Separate longitudinally
•	Move to opposite ends of cell
Telophase
•	New nuclear membranes form
•	Each cell has 46 chromosomes (diploid)
Cytokinesis
•	Cytoplasm separates
•	2 new daughter cells
33
Q

Contrast the processes of spermatogenesis and oogenesis

A 
o	Spermatogenesis (Commences in Puberty)
Four sperm cells are formed per meiotic cycle, over a course of around 60 days. They undergo many more divisions than eggs, meaning there are more chances for mutations to occur.
o	Oogenesis (Occurs in early embryonic life)
Each meiotic cycle produces one ovum and 3 polar bodies and this process occurs over 10-50 years as these polar bodies mature into ova.
34
Q

What is Robertsonian translocation?

A

This occurs in Acrocentric chromosomes (i.e. one arm is shorter than the other)
This gives one chromosome with both long arms, while the other has both the short ones.

35
Q

What is chromosomal deletion?

A

This occurs when sections of chromosomes just seem to disappear from one of the arms, if not both.

36
Q

Give examples of chromosomal inversions

A

Inversions
• Paracentric - This occurs when a section of the DNA in the arm of the chromosome is inverted.
• Pericentric - This is when the DNA around the centromere of the chromosome is inverted.

37
Q

Describe the diagnosis of disease due to abnormalities in chromosome number (Aneuploidy).

A 
o	Trisomy 21 (Down’s Syndrome)
o	Trisomy 13 (Patau Syndrome)
o	Trisomy 18 (Edwards Syndrome)
o	X – (Turner Syndrome)
o	XXY (Klinefelter Syndrome)
38
Q

Describe the origin and diagnosis of Downs syndrome

A

Trisomy 21 (Down’s Syndrome)
• 90% caused by non-disjunction (Chromatid sisters don’t split, and both pairs are carried into one gamete)
• Chances increase with advancing maternal age
• Distinct facial characteristic
• IQ less than 50

39
Q

Describe the origin and diagnosis of Patau syndrome

A

Trisomy 13 (Patau Syndrome)
• Usually caused by non-disjunction (90%)
• Multiple dysmorphic features
• Very few survive beyond first year of life

40
Q

Describe the origin and diagnosis of disease due to abnormalities in chromosome number (Aneuploidy).

A

Trisomy 18 (Edwards Syndrome)
• 90% Non-disjunction
• Most die within first year or month of life

41
Q

Describe the origin and diagnosis of Turner syndrome

A 
X – (Turner Syndrome)
•	Short stature and infertile
•	Neck webbing and widely spaced nipples
•	Normal intelligence and lifespan
•	97% zygotes terminate at conception
42
Q

Describe the origin and diagnosis of Klinefelter syndrome

A 
XXY (Klinefelter Syndrome)
•	Tall stature and long limbs
•	Infertile, small testes
•	50% develop breasts (gynaecomastia)
•	Mild learning difficulties
43
Q

What is aneuploidy?

A

Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human cell having 45 or 47 chromosomes instead of the usual 46.

44
Q

What is the anogram for the steps in meiosis?

A

PMAT PMAT

o	Prophase I
o	Metaphase I
o	Anaphase I
o	Telophase I
o	Prophase II
o	Metaphase II
o	Anaphase II
o	Telophase II

Principles of Disease 16 (37 decks)

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