Chromosomes

Question 1

What is chromatin?

Answer 1

Chromatin is a DNA/protein complex in which
chromosomes exist.

Question 2

What is the key feature of chromatin organization?

Answer 2

condensation or compaction

Question 3

How long is the DNA molecule in the average
human chromosome when stretched out to its full
length?

Answer 3

5 cm long

Question 4

What is the difference between metaphase and interphase?

Answer 4

Metaphase is highly condensed for transmission to daughter cells, while interphase is the real
functional chromosome undergoing replication and transcription.

Question 5

What is a nucleosome?

Answer 5

DNA wrapped around a histone
octamer.

Question 6

__________ are folded loops of chromatin separated by _______.

Answer 6

Topological domains
Boundary element

Question 7

What is the mechanism of giant interphase chromosomes formation?

Answer 7

Cellular giantism” is driven by “DNA” and involves approximately 10 cycles of DNA replication without cell division, resulting in all approximately 1024 daughter chromatids being in perfect alignment to create a giant chromosome with “dark bands” (topological domains) and “light bands” (boundary elements).

Question 8

What are dark bands and light bands in giant interphase chromosomes?

Answer 8

Dark bands are condensed chromatin (topological domains), and light bands are boundary elements.

Question 9

What are polytene chromosomes?

Answer 9

Polytene chromosomes are giant chromosomes with many parallel identical chromatids formed by repeated replication without separation (10 cycles -> 1024 copies).

Question 10

What do polytene chromosome puffs show?

Answer 10

chromatin decondensation with transcriptional activation.

Question 11

What are puffs associated with?

Answer 11

They are associated with the active form of RNA polymerase II, which means active transcription.

Question 12

What are metaphase sister chromatids?

Answer 12

They are the identical products of the previous semiconservative replication of a single chromosomal DNA molecule.

Question 13

What does the karyotype of metaphase chromosomes show?

Answer 13

It shows the chromosomal complement of the species.

Question 14

What is chromosome painting?

Answer 14

It is a technique that reveals the karyotype by fluorescence in situ hybridization (FISH) with a panel of probes representing sequences differentially distributed among the chromosomes.

Question 15

Can chromosomes break and rejoin?

Answer 15

Yes, they can.

Question 16

What are translocations?

Answer 16

Chromosomes can break and rejoin giving translocations.

Question 17

What can cause disease in an organism during a somatic cell division cycle?

Answer 17

Mutations caused by chromosome rearrangements can cause disease, such as cancer.

Question 18

What is the effect of germ-line chromosome rearrangements on offspring?

Answer 18

reduced fertility.

Question 19

Why is the karyotype so consistent across a species?

Answer 19

Germ-line chromosome rearrangements are usually a dead end, so the karyotype is consistent across a species.

Question 20

What are the elements required for replication and stable inheritance of linear chromosomes?

Answer 20

1) Origin of replication
2) Centromere
3) 2 telomeres (ends)

Question 21

Why does the introduction of a circular bacterial plasmid with the wild-type yeast LEU gene into leu- yeast cells matter in the experiment?

Answer 21

The introduction of the circular bacterial plasmid containing the wild-type yeast LEU gene into leu- yeast cells is significant because it allows researchers to assess whether the plasmid can “rescue” leucine-independent growth in these cells, which would indicate that the LEU gene in the plasmid can function in yeast cells.

Question 21

In a yeast experiment, what is the significance of having the LEU gene inactivated by mutation in leu- yeast cells?

Answer 21

In leu- yeast cells, inactivating the LEU gene by mutation means that these cells require external (exogenous) leucine for growth because they can’t produce it themselves.

Question 22

Why can’t the LEU plasmid replicate in yeast cells, even though it replicates well in bacteria?

Answer 22

The LEU plasmid cannot replicate in yeast cells because bacterial origins of DNA replication do not function in eukaryotes like yeast. Yeast and bacteria have different replication mechanisms.

Question 23

How does the introduction of a random piece of yeast DNA containing a yeast origin of replication into the plasmid change its ability to support leu- yeast growth in the absence of leucine?

Answer 23

When a random piece of yeast DNA containing a yeast origin of replication is inserted into the plasmid, it enables the plasmid to support leu- yeast growth in the absence of leucine. This is because the yeast origin of replication allows the plasmid to replicate within yeast cells, thus providing the necessary LEU gene function for growth.

Question 24

What is an autonomously replicating sequence (ARS)?

Answer 24

A yeast origin of DNA replication.

Question 25

What is required for the replication of plasmid from normal yeast? (but …)

Answer 25

ARS
but mitotic segregation is faulty

Question 26

What is CEN?

Answer 26

DNA sequence from a yeast chromosome centromere

Question 27

What is required for good segregation?

Answer 27

CEN

Question 28

What is the relationship between the centromere
and spindle microtubules?

Answer 28

The centromere links to spindle microtubules

Question 29

How are microtubules attached? (3)

Answer 29

CENP-A is a centromeric protein that recruits the
CBF3 complex, which recruits the Ndc80 complex.
The Ndc80 complex attaches to microtubules in the centromere.

Question 30

What is the structural link between a chromosome and the spindle microtubules that ensures proper chromosome segregation during cell division?

Answer 30

The structural link between a chromosome and the mitotic spindle fibers is the “kinetochore,” which forms at the centromere of the chromosome.

Question 31

How can circular plasmids be converted to linear DNA molecules?

Answer 31

By cutting at a single site with a restriction endonuclease.

Question 32

Does a plasmid with ARS and CEN that works well as an experimental circular chromosome in yeast also work well as a linear chromosome?

Answer 32

no
(do not function as efficiently for linear chromosomes due to differences in DNA replication and segregation mechanisms.)

Question 33

What is the function of telomeres?

Answer 33

• protect from exonuclease
• prevent end-to-end fusion
• solve a replication problem faced by linear DNA

Question 34

How can you make linear plasmids containing ARS and CEN behave like normal chromosomes in yeast?

Answer 34

To make linear plasmids containing ARS and CEN behave like normal chromosomes in yeast, you need to add a genomic fragment with “TEL” sequences to both ends of the plasmid. This addition provides the necessary telomeric sequences for stability and proper replication.

Question 35

What is the solution to the problem of chromosome
shortening due to lagging strand end-shortening?

Answer 35

Telomerase: Telomerase is a reverse transcriptase that carries its own template RNA complementary to the
telomeric DNA repeat.
that can extend telomeres, restoring chromosome length.

Question 36

What do telomeres contain?

Answer 36

Simple repeat DNA sequences.

Question 37

How does telomerase give primase more template
DNA to prime on?

Answer 37

By extending the template strand, telomerase gives primase more template DNA to prime on.

Question 38

Where is telomerase only active?

Answer 38

in germ cells and stem cells.

Question 39

Why are cancer cells interested in telomerase?

Answer 39

Telomerase is often re-activated in cancer cells and
is a target for cancer therapy.

Question 40

What is the unusual gene expression mechanism in
Tetrahymena?

Answer 40

Tetrahymena does not express genes from its “main” genome, which is maintained in a non-transcribed form in the micronucleus. Transcription occurs in the macronucleus from millions of gene-sized DNA pieces.