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FunMed Week 3 > The Human Genome and Chromosomes > Flashcards

The Human Genome and Chromosomes Flashcards

1
Q

the nuclear genome is made up of..

  • how many base pairs?
  • how many protein coding regions?
  • how many chromosomes?
A

base pairs: 3.2 x 109 base pairs

protein coding regions: 25 000

chromosomes: 46

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2
Q

which stage of cell division can chromosomes most clearly be seen in?

A

metaphase (where chromosomes line up before seperated or divided)

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3
Q

describe and explain the structure of a chromosome

A

telomeres:

- at each end

  • protective DNA cap.
  • contains repetitive DNA

Centromere:

  • found in centre
  • keeps sister chromatids together and attaches them to microtubules
  • repetitive DNA
  • organised into short arm - p (petite) and long arm - q
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4
Q

what are the bands on chromosomes called? x2

what do they contain?

A

G-light band:

- Gene rich

  • GC rich
  • early replicating

G-dark band:

- gene poor

AT rich

Late replicating

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5
Q

which chromosomes are autosomes and which are sex chromosomes?

A

autosomes: 1-22

sex chr: 23 - XX - female, XY male

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6
Q

what is aneuploidy?

A

abnormal number of chromosomes

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7
Q

how are genes organised in a chromosome?

A

arranged in a linear fashion along each chromosome

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8
Q

describe the structure of a gene / what areas are found in a gene?

A
  • exons: regions of genes that encode proteins
  • introns: non-coding sequences.
    structure: exon followed by intron (exon, intron, exon etc)

- control elements: at the start of gene

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9
Q

describe v basic overview of DNA replication

A
  • transcription of DNA
  • creates: pre-mRNA
  • splicing of pre-mRNA
  • creates: mRNA
  • translation of mRNA
  • creates: protein
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10
Q

what are the different components of non sequencing bits of genes?

A

non coding sequences:

intergenic regions (regions between the exon/intron components), introns

control elements (e.g. promoters and enhancers - switch transcription on / off)

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11
Q

what are the following?

  1. aneuploidy
  2. polyploidy
  3. disomy?
  4. trisomy?
  5. monosomy?
  6. tetrasomy?
A
  1. aneuploidy: abnormal chr number
  2. polyploidy: gain of one or more haploid sets
  3. disomy: two copies of a chromosome
  4. trisomy: three copies of a chromosome
  5. monosome: one copy of a chromosome
  6. tetrasomy: four copies of a chromosome
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12
Q

what are the ways you can have abornmal chromsome structure?

A

​chromomes can under go:

  • ​deletion
  • duplication
  • inversion
  • translocation (one part of a chr joins another part of a chr)
  • Robertsonian translocation (two long arms of acrocentric chr (13, 14, 15 21, 22) join at their centromeres
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13
Q

give an example of where translocations can occur

A

in brain tumours - (e.g. glioblastoma multiforme)

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14
Q

from whom are autosome aneuploidy syndromes usually derived from?

A

maternally derived

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15
Q

name:
- three autosome aneuploidy syndromes

  • two sex chromsomes aneuploidy syndromes
  • one all chromosomes aneuploidy syndrome
A
  • three autosome aneuploidy syndromes:

Down, Edwards and Patau syndromes

  • two sex chromsomes aneuploidy syndromes:

Turner and Klinefelter syndrome

  • one all chromosomes aneuploidy syndrome

Triploidy

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16
Q

name distinguishing features of Downs Syndrome and likely comorbidities

how many live births are DS?

What % of DS are spontanously aborted and still born?

A

stats: 1/700 births, >60% spontanously aborted, 20% still born

Features: Upward slanting eyes, marked muscle hypotonia (floppy), may have single palmar crease as child, learning difficulty, congenitial heart malformations (40%), Alzheimers (70% chance) - gene for APP

17
Q

explain the three different patterns that can cause Down Syndrome

A
  1. trisomy 21: 95% DS - three copies of chr 21
  2. Robertsonian translocation (chrs 14 and 21): 4%. have an extra copy of chr. 21 due to aforementioned translocation
  3. Mosiacism: 1%. have normal and trisomy 21 cell linearges. occurs postzygotically. milder features (some cells have normal genome, some cells have trisomy 21 genome)
18
Q

what are trisomies (particularly Downs Syndrome) associated with?

why does this risk factor cause DS ?

A

age (2.67/1000 @ 34 versus 71/1000 @ 48)

why?: eggs are held at corssing-over stage (prophase 1) in meiosis from approx. 6 months of gestation of the oocyte. wear and tear occurs at meiosis process with increasing maternal age

19
Q

describe the symptoms and prognosis of Edwards and Patau syndrome

A

Edwards Syndrome - trisomy 18: multiple malformations (esp. heart). clenched hands with overlapping fingers)

Patau syndrome - trisomy 13: multiple malformations. Particularly: incomplete lobation of brain, cleft lip

In both: most babies die in first few weeks of life. if not - severe mental retardation

20
Q

describe Klinefelter syndrome?

A

Klinefelter syndrome: (47, XXY). Extra X chromosome. Infertile - no sperm produced. Poorly developed secondary sexual characteristics, tall. 1/1000 males

21
Q
A
22
Q

describe Turner syndrome?

A

Turner syndrome -(45, X) Loss of X chr: 1/5000 females. 99% lost spon. in preg. short. abscence of menstruation (amenorrhea). congenital heart d: 20%. webbed neck

23
Q

describe 3 prenatal diagnoses that can be undertaken to test for aneuploidy

A
  1. amniocentesis: genetic testing of amniotic fluid. using needle to extract transabdominal. 15-18 weeks of preg (risk to miscarriage: 1/100). ultrasound guidance used. t

2. chorionic villus sampling: genetic testing of tissue from placenta (choroinic villi), ultrasound guidance used transabdominal or transcervical. 12-14 weeks

3. non invasise technqiues: ultrasound imaging of back of neck of embryo at 11-14 weeks. if depth of fluid at back of neck is 3.5-4.4 mm = 70% chance of delivering baby with no major abnormalities.

24
Q

what is X-inactivation?

A

early on in embryonic development, one of the two X-chr in each cells is randomly inactivated = X-inactivation or lyonization

The level of gene activity produced by a single X chromosome is the normal “dosage” for a human. Men have this dosage because, well, they only have one X chromosome! Women have the same dosage for a different reason: they shut down one of their two X chromosomes in a process called X-inactivation.

In X-inactivation, an X chromosome is compacted (or, as my intro bio professor liked to say, “crumpled up into a ball”), to make a small, dense structure called a Barr body. Most of the genes on the Barr body are inactive, meaning that they are not transcribed.

A woman has two X chromosomes, one from each parent. Which one will she inactivate? X-inactivation is a random process that happens separately in individual cells during embryonic development. One cell might shut down the paternal X, while its next-door neighbor might shut down the maternal X instead.

25
Q

what does x-inactivation cause in the adult female cells?

A

all adult female cells are a clone of either one active X chromosome or one inactive chromosome

26
Q

how can you see the X-inactive region of female somatic cell when a person has undergone X-inactivation?

A

female somatic cells that have X-inactivation have Barr Bodies. represents inactive chromsome in female somatic cell. this type of chromatin is called heterochromatin

27
Q

do pseudoautosomal regions of x-chromsome undergo inactivation?

A

No.

Pseudoautsomal regions contain genes that pair up with Y-chromosome during meiosis

28
Q

what is Xist?

A
  • X-inactivation is assisted / coded by non coding RNA: XIST (X inactivion centre)
  • Xist assocates closely with the X-chr from which it is expressed -> leads to chromatin changes and spatial reorganisation of chr. chr will condense into Bar > results in transcriptional inactivation in that chr.
29
Q

how can you reverse X-inactivation?

A

only reversed in germ cells, where there are 2 fully activated X chromosomes

30
Q

What is the SRY region on the Y chromosome?

A

SRY = Sex Determining Region of Y-chr.

SRY encodes for proteins that is a member of the HMG (high mobility group transcription factorrs) group -> which in turn are responsible for switching on genes that determine male sex determination

31
Q
A

FunMed Week 3 (10 decks)

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