7. Chromosomes

Question 1

Chromosomes

Answer 1

• 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

Question 2

Ploidy

Answer 2

• 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

Question 3

diploid number

Answer 3

2n

number of chromosomes per somatic (body) cell

Question 4

Haploid number

Answer 4

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

Question 5

aneuploidy

Answer 5

abnormal number of chromosomes

Question 6

examples of aneuploidy?

Answer 6

trisomy= one extra chromosome

monosomy=missing chromosome

Question 7

nondisjunction

Answer 7

• 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

Question 8

Chromosomes are NOT indestructible!

Answer 8

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

Question 9

translocations

Answer 9

movement to new chromosome

Question 10

inversions

Answer 10

portion of chromosome is flipped

Question 11

deletions

Answer 11

section of DNA excised

Question 12

insertions

Answer 12

section of DNA inserted

Question 13

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

Answer 13

nonhomologous

Question 14

translocations can result in change in phenotype by:

Answer 14

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)

Question 15

Some cancers are due to translocations?

Answer 15

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

Question 16

Translocation distinguishes human and ape karyotypes

Answer 16

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

Question 17

centromeres

Answer 17

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

Question 18

telomeres

Answer 18

-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

Question 19

telomere functions

Answer 19

• 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

Question 20

problem with telomeres

Answer 20

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

Question 21

What is the end-replication problem?

Answer 21

the shortening of DNA molecules with each cell division

Question 22

DNA polymerase

Answer 22

an enzyme that replicates DNA strands

Question 23

DNA polymerase steps

Answer 23

• 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

Question 24

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

Answer 24

unreplicated

Question 25

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

Answer 25

50-60 X

Question 26

cellular senescence is triggered when?

Answer 26

when telomeres are about 4-6kb long

Question 27

telomerase

Answer 27

solution to the problems | an RNA containing enzyme that adds more nucleotides to the 3' end of the telomere DNA strands

Question 28

Steps of telomerase

Answer 28

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

Question 29

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

Answer 29

- 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

Question 30

telomerase is not present in all cells: present in:

Answer 30

- many tumor cells-unlimited divisions - one celled organisms- unlimited divisions - primordial germ cells - unlimited divisions - tissue stem cells

Question 31

telomerase is absent in ___

Answer 31

cultured cells from normal tissue - 50 divisions

Question 32

what happened with telomerase was experimentally added to normal cells?

Answer 32

more cycles observed

Question 33

some factors in cell aging:

Answer 33

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

Question 34

Could loss of telomeres cause the aging process?

Answer 34

- yes but there are other factors too | - loss of telomeres certainly provides a maximal limit to the number of cell divisions

Question 35

could we provide telomerase therapy to restore telomere length ?

Answer 35

- 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

Question 36

Dolly

Answer 36

-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

Question 37

genome

Answer 37

the entire DNA content of an organism / cell

Question 38

Types of DNA sequences

Answer 38

- highly repeated sequences - moderately repeated sequences - non-repeated, single copy sequences

Question 39

Highly repeated sequences

Answer 39

>100,000 copies/genome;

Question 40

moderately repeated DNA

Answer 40

- 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)

Question 41

Non-repeated DNA

Answer 41

in humans only about 2% of the genome codes for proteins! | blue denotes genes encoding proteins