Chromosomes and Cell Division

Question 1

binary fission in prokaryotes

Answer 1

• cell with DNA in the middle
• Circular DNA is attached to inside of membrane- replicates itself and there are 2 of them
• When cell is ready to divide- senses attachment sites, and starts to pinch in to make a new cell between attachment sites
• -> 1 chromosome in each daughter cell
• replicate DNA from single origin of replication (bi-directional)
• Mitochondria and chloroplast also divide by binary fission

Question 2

bidirectional replication

Answer 2

• in prokaryotes
• from left to right
• replication fork

Question 3

Eukaryotic cell division

Answer 3

• Multiple linear pieces of DNA (instead of circular pieces)

- linear pieces = chromosomes

Question 4

chromosomes

Answer 4

• exist in pairs
• Need 2 because sexually reproducing organisms- get 1 copy from mother 1 from father
• humans have 23 pairs of chromosomes
• enclosed in the nucleus
  centromere- pinched part in middle of chromosome

Question 5

mitosis

Answer 5

• cell division

- Process that ensures the proper distribution of chromosomes

Question 6

chromosomes during cell division

Answer 6

• Chromosomes become condensed
• Cell separates chromosomes and split cell in half
• Cytokinesis- splitting of cytoplasm

Question 7

Chromosome (DNA) Packing- histones

Answer 7

• In each cell, we have about 1 meter of DNA
• Packed into 30 micron cell
• Histones (protein in chromatin)- are positively charged and neutralize negatively charged DNA
• Nucleosomes- Octamers of histone “core” proteins wrap DNA around themselves, giving it the appearance of “beads on a string”
  First level- histone packing and nucleosomes
  Second level- folding of the30 nm fibers
  –> When the chromosome is going to package even DENSER→ folds the 30nm fibers into big loops and keeps condensing and keeps folding… gives chromosomes its final shape

Question 8

interphase

Answer 8

G1+S+G2 = Interphase

“G1” and “G2” stand for “gaps” between “M” and “S” phases

Question 9

G0

Answer 9

Some cells spend much of their time in “G0”, not involved in preparing for or carrying out cell division (e.g. neurons)

Question 10

epithelial cells

Answer 10

• often growing and reproducing constantly (skin, lining of intestines), outer layer desquamation

Question 11

S Phase

Answer 11

• Chromosomes get completely replicated
• Region of double stranded DNA–> show some bubbles = replication bubbles
• bidirectional replication
• DNA replicates in a semi-conservative manner
• every new strand of DNA has one parental strand, and one new strand

Question 12

Where do we see chromosomes?

Answer 12

• ONLY see chromosomes in process of mitosis

- can’t see in interphase b/c not yet condensed

Question 13

Kinetochore

Answer 13

• Kinetochore structures (made of proteins) are the attachment sites for microtubules at the centromere regions

Question 14

centromeres

Answer 14

• pinched area of chromosomes
• The number of chromosomes is defined by the number of centromeres
• sister chromatid= double arms

Question 15

Microtubules

Answer 15

• involved in moving chromosomes

Question 16

telomeres

Answer 16

• Ends of each sister chromatid

Question 17

banding pattern

Answer 17

• visual picture of chromosomes= karyotypes
• banding pattern- characteristic of chromosome
• 23 kinds of chromosomes, 46 all together
• Sex chromosomes at the end (X and Y)
• Metacentric- middle, acrocentric- to the side, telocentric- on the end

Question 18

chromosome painting

Answer 18

• Molecular trick- sequences known in each chromosome

- Attach fluorescent markers that stick only on certain chromosomes

Question 19

Scleroderma (kinetochore antibody staining)

Answer 19

• Scleroderma- hard skin
• Antibodies- isolated from patients with an auto immune disease
• Make antibodies that attack skin and skin not as flexible, also organ damage
• patients make antibodies against centromere proteins

Question 20

“activated” lymphocyte

Answer 20

• Lymphocyte- inactive- nucleus has lots of heterochromatin (condensed and inactive genetic material)
• heterochromatin converts to euchromatin
• When immune system responds to infection and it is activated–> now DNA needs to be expressed so cell can turn into an antibody

Question 21

Mitosis- prophase

Answer 21

• before phase
• Chromosomes condense
• Gene activity stops
• Nucleolus disappears
• Centrioles- have replicated and are moving to opposite poles–> forming of mitotic spindle

Question 22

mitosis- pro-metaphase

Answer 22

• Nuclear envelope breaks down
• chromosomes are now a little more visible and nuclear membrane has disappeared
• Chromosomes are moving around (and attached to microtubules that pull them back and forth to see if they have them all)

Question 23

mitosis- metaphase

Answer 23

• Microtubules attached to ALL chromosomes, and chromosomes line up along metaphase plate
• fully formed spindle

Question 24

Spindle Microtubules (3)

Answer 24

1. Polar microtubules- pole to pole microtubules
   move with microtubules motors (black dots)
2. Aster fibers- star like
   Out in all directions near pole
3. Kinetochore microtubules- from pole to kinetochore at centrosome on chromosomes

Question 25

mitosis- anaphase (A and B)

Answer 25

• Sister chromatids split at the centromere and start to move toward their respective poles
• Poles move apart from each other
• FROM 4 to 8 chromosomes in cell–> to separate–> NO chromatids
• find major motors at kinetochore
  A. chromosomes are pulled towards poles
  B. poles move apart (motors work and microtubules move to opposite sides)

Question 26

Microtubule motor experiment

Answer 26

• Have cell on a slide, spindle is there, laser bleaches area out of microtubules
• Microtubules are stained, take laser an cut some of immunofluorescence
• See that bleached area remains in place but chromosomes get closer to pole

Question 27

Kinetochore Pac-Man mechanism

Answer 27

• Kinetochore motors walk along the microtubules at the same time that they depolymerize the ends
• This pulls chromosome off as the end shrinks “chewing up microtubule?”
• ** source of major motion that is happening when chromosomes are moving towards pole

Question 28

mitosis- telophase

Answer 28

• Chromosomes de-condense
• spindle fibers fade
• Gene activity resumes
• Nucleolus reappears
• Nuclear envelope reforms
• chromosome close to poles

Question 29

cytokinesis (animal cells)

Answer 29

• happens at the same time as telophase, but not the same thing
• When in telophase, start to form contractile ring of actin filaments (myosin- motors)
• Form ring that tightens up and eventually pinches cells in half–> cleavage furrow (contractile ring)
• NO sister chromatids, but there are chromosomes

Question 30

plant mitosis and cytokinesis (plant cells)

Answer 30

• plant cells do mitosis within the cell wall
• plant cells do have mitotic spindles, they do NOT have aster fibers or centrioles at the poles
• Golgi- produces new components of cell wall and starts lining up in spate in center of cell
• Use microtubules to put into position (vesicles walk along microtubules)= cell plate is made of vesicles
• cell formation of a plant plate (instead of cleavage furrow)