Lab 7 - Chromosomes

Question 1

Why are histone’s positively charged?

Answer 1

Histones have many amino acids that are basic; very positively charged and therefore becomes overall positively charged itself.

Question 2

What role does the histone being positively charged play in chromosome packing?

Answer 2

Because the histone is positively charged, it strongly attracts the negatively charged DNA molecule and DNA wraps itself tightly around the histone creating a nucleosome; the first level of DNA packing.

Question 3

What is the purpose of the histone core?

Answer 3

Promotes and guides DNA coiling

Question 4

What is the purpose of H1?

Answer 4

Act to stabilize the structure

Question 5

What is a Solenoid?

Answer 5

A cylindrical fibre that is made up of nucleosome units

Question 6

What are the 5 levels of chromosome packing?

Answer 6

1. Bead-on-string: A double loop of DNA wrapped around a histone core of 8 histones; nucleosome
2. Solenoid: Made up of 6 nucleosomes, H1s, and DNA spacers
3. Chromatin fibre: A cylindrical fibre made up of solenoids
4. Chromatid: coiling of chromatin fibre
5. Chromosome: supercoiling of chromatid

Question 7

What is Heterochromatin?

Answer 7

Highly condensed portions of the chromatin; DNA in these sections are expressed less or never

Question 8

What is Euchromatin?

Answer 8

Sections of chromatin that is less condensed than heterochromatin; only condensed during cell division which means it’s highly expressed

Question 9

What is the staining like for heterochromatin vs euchromatin?

Answer 9

Heterochromatin are highly condensed so they hold onto the die more&raquo_space; darkly stained

Euchromatic are not condensed so they do not hold onto the die as well&raquo_space; lightly stained

Question 10

Where is the centromere located on a chromosome that’s “Metacentric”?

Answer 10

In the middle

Question 11

Where is the centromere located on a chromosome known as “submetacentric”?

Answer 11

Somewhere between the middle and the end

Question 12

Where is the centromere located on a chromosome that is known is “Acrocentric?”

Answer 12

Close to the end

Question 13

Where is the centromere located on a chromosome that is known as “Telocentric”?

Answer 13

At the end

Question 14

What is a telomere?

Answer 14

Specialized repeating segment of DNA that stabilizes the end of the chromosome and prevents other chromosome fragments from fusing to it, and protects the chromosome from enzymatic degradation.

Question 15

What are the 3 main things that happen during prophase of mitosis and prophase II of meiosis?

Answer 15

• Nucleolus and nuclear membrane breakdown
• Chromosomes condense
• Spindle apparatus begins to form

Question 16

What is the main thing that happens during metaphase during mitosis and metaphase II during meiosis and how does this differ from metaphase I?

Answer 16

Chromosomes line up at the metaplate. This is is different in metaphase I because it’s homologous chromosomes that line up rather than sister chromatids as in in metaphase and metaphase II.

Question 17

What is the key thing that happens during Anaphase in mitosis and Anaphase I? How does Anaphase II differ from those 2 phases?

Answer 17

Sister chromatids are pulled to opposite poles by spindle fibres. The difference with Anaphase I is that it’s homologous pairs that split apart, not sister chromatids

Question 18

What are the 2 key things that happen during Telophase and Cytokinesis?

Answer 18

• Nuclear membrane and nucleolus reforms

- Cleavage furrow forms or cell plate begins to form

Question 19

What is the purpose of Indole-3-Acetic Acid?

Answer 19

Acts as a growth hormone that encourages cell division in root tips

Question 20

What is the purpose of adding root tips to HCl and then incubating?

Answer 20

Fixation; kills the tissue without distorting the nuclear material.
Hydrolyzes the root tip; so that it can stain better, and softens the root tip so it can be properly squashed.

Question 21

What stain is used on the root tip?

Answer 21

Aceto-orcein

Question 22

What in the stain; aceto-orcein, causes cytoplasm to swell, and combined with the pressure of thumb, cause the chromosome to spread apart?

Answer 22

Acetic acid

Question 23

Do all the cells appear to begin and undergo mitosis at the same time in the root tip?

Answer 23

No, they are in many different phases. This is because the amount of time spent in each phase can vary from cell to cell

Question 24

What area of the chromosome squash had a higher concentration of mitotic figures?

Answer 24

In the apical meristem, near the bottom of the root, since that’s where the most mitotic activity occurs

Question 25

What are chromosome bands?

Answer 25

Patterns that appear after staining that are specific to each type of chromosome

Question 26

What is Giemsa?

Answer 26

A stain that is used to reveal chromosome bands; specifically called G-bands

Question 27

What 3 factors influence banding patterns?

Answer 27

• proteins that are present (interact with dye)
• relative proportions of AT and GC pairs
• repetitiveness of codons in DNA sequence

Question 28

What is a Karyotype?

Answer 28

The particular array of chromosomes that an individual has

Question 29

Explain the 7 steps of how a Karyotype is prepared?

Answer 29

1. Add blood to a vial of culture medium w/ stimulator for mitosis
2. Centrifuge and discard supernatant
3. Add saline, fixative and resuspend pellet
4. Prepare and stain cells for microscopy (Geimsa - 짐사)
5. Observe under microscope
6. Photograph and enlarge chromosomes
7. Cutout and arrange

Question 30

What is the purpose of dying chromosomes?

Answer 30

Allow us to observe the chromosome and via banding patterns we’re able to distinguish chromosomes from each other

Question 31

What are polytene chromosomes? Why are they useful to scientists?

Answer 31

Chromosomes that are large and can be viewed microscopically without dying. This is useful because the process of dying can be eliminated and are therefore easier to work with.

Question 32

Why do the “puff” regions in polytene chromosomes appear the way that they do?

Answer 32

Because the area where the puff is, is being transcribed, thus the DNA there is uncoiled taking up more room and creating a puff.

Question 33

What molecule is being created at a puff in polytene chromosomes?

Answer 33

mRNA

Question 34

What is the purpose of polymerase chain reactions?

Answer 34

It allows us to take a small amount of DNA and amplify it. This allows us to be able to increase the number of tests we can run on it.

Question 35

What do polymerase chain reactions require?

Answer 35

Polymerase chain reactions amplify DNA using, DNA polymerase and oligonucleotide primers, the 4 deoxynucleotides; A, G, T, C

Question 36

What are the 3 steps to polymerase chain reactions?

Answer 36

1. Denaturation: Double stranded DNA denatured into single strands via heat
2. Primer Annealing: Primers annealed to the single stranded DNA
3. Extension: DNA polymerase in the reaction mixture extends the oligonucleotide primers in the 5’ to 3’ direction using single stranded DNA as a template

Question 37

What is a Jumping gene?

Answer 37

A gene that “reproduces” by copying itself and inserting into new chromosome locations

Question 38

What is the purpose of Chelex during polymerase chain reactions?

Answer 38

When samples are boiled to lyse the cell, metal ions are released which would inhibit polymerase chain reaction. Fortunately by adding chelex, it binds to the metal ions, allowing polymerase chain reactions to continue undisturbed.