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Cell Biology > 7. Chromosomes > Flashcards

7. Chromosomes Flashcards

1
Q

Chromosomes

A
  • term coined in 1880’s: coloured bodies
  • carriers of genetic info
  • during S phase, chromosomes duplicate and form dyads
  • the attached duplicated chromosomes are called sister chromatids
  • ploidy indicates the number of sets of chromosomes
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2
Q

Ploidy

A
  • most animals are diploid- have two copies of each chromosome, one from the mother and one from father
  • gametes are haploid- have one set- arise through meiosis
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3
Q

diploid number

A

2n

number of chromosomes per somatic (body) cell

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4
Q

Haploid number

A 
n
number of chromosomes in gametes
-humans 2n=46 (22 autosomes and 2 sex chromosomes)
-mouse 2n=40
-drosophila 2n=8
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5
Q

aneuploidy

A

abnormal number of chromosomes

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6
Q

examples of aneuploidy?

A

trisomy= one extra chromosome

monosomy=missing chromosome

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7
Q

nondisjunction

A
  • meiosis I starts normally; tetrads align at metaphase
  • one set of homologs fails to separate
  • meiosis II occurs normally
  • all gametes have abnormal number of chromosomes
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8
Q

Chromosomes are NOT indestructible!

A

-can be broken and when repaired they may get scrambled.

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9
Q

translocations

A

movement to new chromosome

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10
Q

inversions

A

portion of chromosome is flipped

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11
Q

deletions

A

section of DNA excised

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12
Q

insertions

A

section of DNA inserted

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13
Q

translocations are the transfer of a piece of one chromosome to a _____________ chromosome

A

nonhomologous

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14
Q

translocations can result in change in phenotype by:

A
  1. breaking a gene
  2. moving gene to a region where it can be controlled by another regulatory sequence
  3. creating hybrid gene
    (can elongate one side of chromosome)
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15
Q

Some cancers are due to translocations?

A

CML chronic myelogenous leukemia
-the c-abl gene encodes a kinase that regulate cell proliferation; the translocation alters the controls of this gene, placing it under the control of another gene’s bcr promoter

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16
Q

Translocation distinguishes human and ape karyotypes

A
  • apes=48 chromosomes
  • humans = 46 chromosomes
  • genes in humans and chimps differ by
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17
Q

centromeres

A

the constricted central portion of each chromosome

  • the DNA contains alpha satellite DNA; made of non-transcribed 171 base repetitive sequences
  • repeated 1000s of times
  • attach to the kinetochores during M phase
18
Q

telomeres

A

-non coding regions at the ends of the chromosomes
-short repeated sequence: repeated 500-5000X
-include specialized proteins
-form a capped end structure
TTAGGG in humans

19
Q

telomere functions

A
  • protect ends of chromosome from nucleases
  • allow cells to distinguish chromosome ends from broken DNA
  • prevent chromosomes from fusing with each other
  • attachment to nuclear scaffold
20
Q

problem with telomeres

A

when chromosomes are replicated in most cells, the telomeres get progressively shorter

21
Q

What is the end-replication problem?

A

the shortening of DNA molecules with each cell division

22
Q

DNA polymerase

A

an enzyme that replicates DNA strands

23
Q

DNA polymerase steps

A
  • DNA polymerase needs and RNA primer to get started
  • builds in a 5’ to 3’ direction to make new DNA strand
  • RNA primer is removed, leaving small overhang
  • overhang folds back to form a loop (safer from nucleases)
  • every time DNA is replicated it gets a bit shorter - eventually this would be a problem
24
Q

each round of DNA replication leaves 50-200 bp DNA _______ at the 3’ end

A

unreplicated

25
Q

cells with telomeres that are 10-12 kb in length (average) divide

A

50-60 X

26
Q

cellular senescence is triggered when?

A

when telomeres are about 4-6kb long

27
Q

telomerase

A

solution to the problems

an RNA containing enzyme that adds more nucleotides to the 3’ end of the telomere DNA strands

28
Q

Steps of telomerase

A
  1. telomerase RNA binds to complementary sequence
  2. nucleotides added to DNA 3’ terminus (elongation)
  3. telomerase slides over and its RNA binds to another complementary sequence (translocation)
  4. more nts added to DNA 3’end (elongations
  5. other strand filled in using DNA polymerase
29
Q

the steady loss of telomeres results in a limited life-span for most cells

A
  • in culture, most of our cells cycle a set number of times, them die
  • same seems to happen in eukaryotic organisms
  • due to lack of telomerase enzyme in our cells
30
Q

telomerase is not present in all cells: present in:

A
  • many tumor cells-unlimited divisions
  • one celled organisms- unlimited divisions
  • primordial germ cells - unlimited divisions
  • tissue stem cells
31
Q

telomerase is absent in ___

A

cultured cells from normal tissue - 50 divisions

32
Q

what happened with telomerase was experimentally added to normal cells?

A

more cycles observed

33
Q

some factors in cell aging:

A

telomere shortening
accumulated errors
chronic risk exposures such as oxidants, UV
glycation- sugar binding DNA, proteins, lipids

34
Q

Could loss of telomeres cause the aging process?

A
  • yes but there are other factors too

- loss of telomeres certainly provides a maximal limit to the number of cell divisions

35
Q

could we provide telomerase therapy to restore telomere length ?

A
  • many cancer cells have reactivated their telomerase
  • some anticancer therapies are directed at inhibiting telomerase
  • if you think you could extend life by adding telomerase, you could increase the risk of cancer
36
Q

Dolly

A

-1996
-1st cloned mammal from adult cell
-took nucleus from the udder of an adult ewe, and transplanted it into a de-nucleated embryo
-died at age 6 (most sheep live till 12)
dolly’s telomeres were 20% shorter than normal sheep by age 3, she started developing serious arthritis at age 5, premature aging occurred

37
Q

genome

A

the entire DNA content of an organism / cell

38
Q

Types of DNA sequences

A
  • highly repeated sequences
  • moderately repeated sequences
  • non-repeated, single copy sequences
39
Q

Highly repeated sequences

A

> 100,000 copies/genome;

40
Q

moderately repeated DNA

A
  • 20-80% of genomes
  • few copies to ten of thousand of copies/genome
  • some sequences code for proteins needed in large quantities (e.g ribosomal proteins)
  • most do no code for any protein
    (diagram. red denotes regions rich in moderately repetitive sequence son a chromosome)
41
Q

Non-repeated DNA

A

in humans only about 2% of the genome codes for proteins!

blue denotes genes encoding proteins

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