Unit 2: Genes and Heredity

Question 1

What are chromosomes ?

Answer 1

Chromosomes are highly organized and compacted chromatin.

Question 2

What is chromatin ?

Answer 2

Chromatin is DNA molecule wrapped around with some proteins (histones)

Question 3

What are the parts of a chromosome ?

Answer 3

—-*————
Both ends are telomeres
The center is a centromere
The short arm is referred to as ‘p’ (from “petit”) and the long one as ‘q’

Question 4

How many molecules of DNA are in a chromosome ?

Answer 4

It depends. One single-stranded chromosome has 1 molecule of DNA, whereas a double-stranded chromosome has 2 molecules of DNA

Question 5

What is a chromatid ?

Answer 5

It is one part of a double stranded chromosome (1/2). It corresponds to 1 molecule of DNA.

Question 6

What is ploidy ?

Answer 6

The ploidy refers to the number of sets of chromosomes.

Question 7

What is the difference between haploid cells and diploid cells ? Can you give examples ?

Answer 7

Haploid cells have 1 set of chromosome (that is, one replica of each) and are used from sexual reproduction (gametes). Diploid cells have 2 sets of chromosomes (2 copies of each). Somatic cells (body cells) are diploid.

Question 8

What is the ploidy ?

• —.———-
• —.———-

Answer 8

2n (2 identical chromosomes)

Question 9

What is the n number ?

Answer 9

It is the number of chromosomes in 1 set (eg: humans have n=23).

Question 10

What is aneuploidy ? Give 4 examples.

Answer 10

It is having an abnormal number of chromosomes.
Monosomy: 2n-1
Trisomy: 2n+1
Nullisomy: 2n-2
Tetrasomy: 2n+2

Question 11

What are the possible changes in chromosome structure ?

Answer 11

Inversion (chunk “flips”)
Translocation (extra piece added from other chr)
Deletion (a piece is removed)
Duplication (1 gene is duplicated)

Question 12

What could cause chromosoms to break ?

Answer 12

X-rays, chemicals

Question 13

What are the stages of the cell cycle (with their description) ? What changes with each stage ?

Answer 13

Interphase:

• G1 (growth 1), most of the life cycle of the cell. Normal cell stuff happens, such as growth and protein synthesis. The cell size and content (mitochondria, chloroplast) increase. The cell use genes to synthetize proteins (gene expression)
• S phase, starts when the cell receives a signal for duplication. S stands for “synthesis, where DNA will be synthesized.
• G2, the cell prepares for division
• Mitosis, the process of cell division

Question 14

What happens to the chromosomes at each stage of mitosis ?

Answer 14

Prophase: Chromosomes start to condense in DS
Prometaphase: Chromosomes finish to condense and are captured by spindle fibers
Metaphase: All chromosomes are captured, they align at cell plane
Anaphase: The chromosomes are pulled apart
Telophase: The chromosomes start to decondense

Question 15

Explain how a chromosome forms and condenses for mitosis.

Answer 15

The DNA molecules wrap around round proteins called histones to form chromatin fibre. Chromatin itself folds over itself to form a bigger fiber, which will then fold again in circles to form the ‘X’ shape we are familiar with.

Question 16

Describe cytokinesis.

Answer 16

For animals, this process starts in ANAPHASE. This is the process in which the cell will be separated in 2 new cells. 2 things occur. First, a contractile ring encircles the cell at the cell plate and starts to compress it (made of akin and motor proteins), forming a hourglass shape. Second, spindle fibers inside the cells push on each other to further elongate the hourglass.
For plants, the process is different. The new cells are delimited by the creation of a new cell wall at the cell plate.

Question 17

What are the conditions that have to be met at G1/S checkpoint ?

Answer 17

• The cell has to be the right size (2x mass of a normal cell)
• It needs to have all it needs to grow, along with an adequate growth rate
• Specific growth factors (molecules) have to be present

Question 18

If all conditions are met, what happens at G1/S ? What happens if this compound is not released or the conditions are not met ?

Answer 18

If all correct, SPF (S-phase Promoting Factor) will be produced.
If conditions are not met, SPF is not released. The cell cycle stops and the cell moves to G0, which is when the cell activities are carried but the cell will not divide. Mitosis is suppressed.

Question 19

Can you give an example of cells that do not respond to SPF ? What happens ?

Answer 19

Cancer cells. They divide, wether SPF is released or not, resulting in uncontrolled growth.

Question 20

What happens when SPF is activated ?

Answer 20

The S phase is initiated and the DNA starts to replicate

Question 21

How is DNA replicated ?

Answer 21

The DNA double helix is split in 2 and processed by proteins. Other proteins are in charge of adding the corresponding bases to complete the double helix and replace what is missing. In the end, there are 2 double helices, each made of 1/2 old DNA and 1/2 new DNA.

Question 22

When is the G2/M Checkpoint ?

Answer 22

It happens after the S-phase, when the DNA is replicated.

Question 23

What is verified during the G2/M checkpoint ?

Answer 23

• The cell mass is verified again
• Some cells measure the time elapsed
• Some cells measure growth rate again
• /!\ Complete, ACCURATE, DNA replication

Question 24

What is intiated if the G2/M checkpoint is complete ?

Answer 24

MPF, Mitosis Promoting Factor

Question 25

What happens at the release of MPF ?

Answer 25

• Chromosomes condense
• Nuclear membrane breaks down
• Mitotic spindles start to form
• Centrosomes start to move

Question 26

What is required at the end of metaphase for anaphase to proceed ?

Answer 26

The Mitotic Spindle Checkpoint

Question 27

In what consist the mitotic spindle checkpoint ? What is verified, and what is released ?

Answer 27

Check the attachement of spindle fibers to kinetochores, and if there is equal pressure on each side. If everything is ok, APC is activated (Anaphase Promoting Complex)

Question 28

What happens when APC is released ? What do the kinetochores do ?

Answer 28

APC allows chromatids to separate. The kinetochores send signals along the microtubules with molecular motors. APC inactivates MPF at Telophase, so everything can go back to G1.