Cell division

Question 1

What is Chromatin?

Answer 1

combination of DNA and proteins that make up chromosomes

Question 2

What is a chromosome?

Answer 2

» In cell cycle, chromosome is a tightly coiled form
of the DNA-protein complex
» In molecular biology, “chromosome” simply
means “DNA strand and associated proteins”

Question 3

What are Chromatids

Answer 3

Identical copies of the same chromosome (“sister chromatids”), joined by the centromere

Question 4

What is cell division

Answer 4

How cells reproduce (ie, 1 parent cell divides into 2
daughter cells)
» In unicellular organisms (prokaryotes, some eukaryotes) cell division = reproduction
» In multicellular organisms, simple cell division enables growth (eg, to replace damaged tissue)

Question 5

What is a daughter cell?

Answer 5

genetically identical to parent cell and each other

Question 6

How do different cells divide?

Answer 6

Prokaryotes divide by binary fission.

Eukaryotes divide by mitosis or meiosis

Question 7

What is Binary fission and how does it work?

Answer 7

It is the way a prokaryote divides
1. Copy genome
• From one origin of replication, replicate DNA in both
directions to produce 2 large circular chromosomes
• Tightly pack chromosomes w/ special proteins + “supercoiling”
2. Increase cell size
3. Split into two daughter cells

Question 8

What is Mitosis?

Answer 8

Typical cell division, growth and repair

Question 9

What is Meiosis

Answer 9

» Necessary for sexual reproduction
» Additional steps to avoid excess DNA and increase
genetic variability of offspring

Question 10

Mitosis is only a small part of the cell cycle, what is the estimated time for the cycle?

Answer 10

» In normal human cell cycle (~24h):

interphase ~23h and mitosis <1h

Question 11

How is DNA copied & packaged in Eukaryotic cell division?

Answer 11

• During S phase
  » Copy all DNA (ie, multiple chromosomes) starting from many points of origin
• In early stages of mitosis
  » Condense into tightly packed chromatin fibres wrapped around special proteins called “histones”

Question 12

When was the first record of mitosis?

Answer 12

1882: Flemming records process of mitosis

Question 13

What happens in the interphase?

Answer 13

» G1: cell growth, normal gene expression
» S: copy DNA
» G2: cell growth, duplicate organelles

Question 14

What happens in Mitosis?

Answer 14

PPMAT

Question 15

What happens in Cytokinesis?

Answer 15

2 new cells divide

Question 16

What does the first P in PPMAT stand for?

Answer 16

P - prophase is where the chromosomes become visible as paired chromatids and the nuclear envelope disappears.

Question 17

What does the second P in PPMAT stand for?

Answer 17

P - prometaphase is the process that separates the duplicated genetic material carried in the nucleus of a parent cell into two identical daughter cells

Question 18

What does the M stand for in PPMAT?

Answer 18

M - metaphase is where the chromosomes become attached to the spindle fibres.

Question 19

What does the A stand for in PPMAT?

Answer 19

A - anaphase is where the chromosomes move away from one another to opposite poles of the spindle.

Question 20

What does the T stand for in PPMAT?

Answer 20

T - telophase is where the chromatids or chromosomes move to opposite ends of the cell and two nuclei are formed.

Question 21

What is Cytokinesis?

Answer 21

The cytoplasmic division of a cell at the end of mitosis or meiosis, bringing about the separation into two daughter cells.

Question 22

How does Cytokinesis work?

Answer 22

• Actin microfilaments form a ring on inside of cell membrane, in middle of dividing cells (at location of metaphase plate) → forms a narrow groove (“cleavage furrow”)
• Filaments are drawn together, pinching the cell membrane together → separates cytoplasm into 2 identical daughter cells

Question 23

What happens if there are errors in the cell cycle?

Answer 23

• Errors in cell cycle can lead to uncontrolled cell growth, which can
  lead to cancer
• Essentially, cancer is a disease of mitosis

Question 24

How many sets of chromasomes do most eukaryotes have?

Answer 24

Most eukaryotes have two sets of homologous chromosomes
» Referred to as “diploid (2n)”
» eg, humans have 46 chromosomes, but in fact
23 pairs (n = 23, 2n = 46)

Question 25

What are organisms with only one set of chromosomes called?

Answer 25

Organisms that have only 1 set of chromosomes (1n) are called “haploid”

Question 26

what is sexual reproduction

Answer 26

Fusion of two cells from different individuals

Question 27

How many chromosomes are needed for sexual reproduction to be possible?

Answer 27

For sexual reproduction to be possible, organisms need to produce cells with half the number of chromosomes as the parental cell = “gametes”

Question 28

What does mitosis produce?

Answer 28

• Mitosis

» Produces 2 genetically identical diploid cells (ie, each has 2n chromosomes)

Question 29

What does Meiosis produce?

Answer 29

• Meiosis

» Produces 4 haploid cells with half the number of chromosomes as parent cell (ie, each has 1n chromosome)

Question 30

What are the three types of sexual life cycles?

Answer 30

• Animals
• plants and some algea
• Most fungi and some protists

Question 31

What is the first main stage of meiosis?

Answer 31

- Meiosis I: separate homologous chromosomes and reduce number by half (ie, 2n → n)
» Prophase I and Prometaphase I
» Metaphase I
» Anaphase I
» Telophase I and cytokinesis
- No DNA replication

Question 32

What is the second main stage of meiosis?

Answer 32

Meiosis II: separate sister chromatids (≈ mitosis)
» Prophase II and Prometaphase II
» Metaphase II
» Anaphase II
» Telophase II and cytokinesis

Question 33

Meiosis stages diagram

Answer 33

1 diploid cell
2n chromosomes
(each w/ 2 chromatids)
          |
2 haploid cells
1n chromosome
(each w/ 2 chromatids)
          |
4 haploid cells
1n chromosome
(each w/ 1 chromatid)

Question 34

What are the two special events in Meiosis?

Answer 34

• Synapsis (during prophase I)
  » Homologous pairs of chromosomes bind together (forming “tetrads”)
• Crossing over
  » Large sections of chromosomes are exchanged during synapsis

Question 35

What is Asexual reproduction?

Answer 35

Asexual reproduction (by mitosis)
» Produces 2 identical daughter cells (clones)
» Allows reproduction from a single cell

Question 36

which form of reproducion is Meiosis?

Answer 36

Sexual reproduction (by meiosis)
» Produces 4 haploid daughter cells (“gametes”), genetically different from parent and from each other
» Requires 2 organisms, each providing 1 haploid (male provides sperm, female provides egg)

Question 37

What is the advantage of Meiosis?

Answer 37

Meiosis produces much greater genetic diversity than mitosis
» Independent assortment → new combinations
» Crossing over → mixing chromosomes

Question 38

What are the advantages of genetic variability?

Answer 38

Genetic diversity provides greater flexibility to adapt to changes in the environment
» Asexual reproduction creates exact clones. Good if species is well adapted to its environment but if environment changes it takes time to adapt
» Sexual reproduction creates different “versions”. Some may be better adapted to various environments, so if environment changes, there are already well-suited “versions”

Question 39

How do mitochondria and chloroplasts divide?

Answer 39

mitochondria (and chloroplasts) have their own DNA,

and divide by binary fission

Question 40

Where are the mitochondria inherited from?

Answer 40

In sexual reproduction, mitochondria are inherited exclusively from the mother (mitochondria in sperm are destroyed by egg after fertilisation)
» Analysis of mitochondrial DNA (mtDNA) can help establishing maternal ancestry (and evolutionary history)