The Cell Cycle, Mitosis, Meiosis

Question 1

What are the 2 Cell Types based on Chromosome Content?

Answer 1

1. Gametes
2. Somatic Cells

Question 2

Gametes

Answer 2

• 23 different chromosomes = haploid (n)
• Includes ova (eggs) and sperm

Question 3

Somatic Cells

Answer 3

• Includes all cells of body except cells undergoing/resulting from meiosis
  
  • Muscle cell, nerve cell, etc
• 46 chromosomes = diploid (2n)
  
  • = 23 Homologous pairs

Question 4

What is Homologous Chromosomes?

Answer 4

• A chromosome pair (1 from each parent) that are highly similar in length, centromere position, and have genes for the same trait (e.g. eye colour) in the same location (locus)
• But the homologous chromosome may have different versions of that gene (alleles) that code for proteins that produce alternate versions of that trait
  
  • e.g blue eyes (chromosome from one parent), brown eyes (chromosome from the second parent)

Question 5

What are the 23 chromosome pairs that each somatic cell has?

Answer 5

• 23 chromosomes from the ovum paired with 23 chromosomes from a sperm = 46 chromosomes (diploid/2n somatic cell)
• Chromosomes and chromosome pairs are numbered 1-23:
  
  • 1 to 22 = autosomal chromosomes (autosomes)
  • 23 = sex chromosome

Question 6

1 to 22 = autosomal chromosomes (autosomes)

Answer 6

• contain genes for somatic characteristics
  
  • e.g. hair and eye colour, height
• Each autosomal chromosome pair is not identical, but equivalent = homologous chromosomes

Question 7

23 = sex chromosome

Answer 7

• Contain genes that determine biological sex, XX = female & XY = Male
• X or Y from Sperm
• X from ovum

Question 8

Cell Cycle

Answer 8

Diploid somatic cell (2n) → 2 genetically identical somatic diploid cells (2n)

Question 9

What are the two stages of Cell Cycle?

Answer 9

1. Interphase
2. Mitotic (M)/ Cell Division phase

Question 10

Interphase

Answer 10

• Chromosomes are present as long thread-like chromatin strands
• Normal cellular metabolic activities occuring e.g. protein synthesis
• Phases of Interphase:
  
  • G1 Phase
  • S Phase
  • G2 Phase

Question 11

What happens in G1 Phase?

Answer 11

• Growth, metabolism
• Centrosome replication begins
• NOTE: cells that do not divide again once mature, (i.e. remain in G1) are said to be in the G0 Phase
  
  • e.g. nerve and some muscle cells

Question 12

What happens in S Phase?

Answer 12

• Growth & DNA synthesis
• Chromosomes replicate, but are still present as long thread-like chromatin strands (not individually visible)
• ALWAYS occurs before cell division (mitosis and meiosis)
• Replicates are called sister chromatids (Identical chromatids)
  
  • Attached to each other at an area of DNA called the centromere
  • Kinetochores (proteins) form on each centromere (1/chromatid)

Question 13

What Happens in G2 Phase?

Answer 13

• Growth, metabolism
• Production of enzymes & other proteins needed for cell division
• Centrosome replication is completed

Question 14

What is Involved in the Mitotic Phase (M Phase)?

Answer 14

• Mitosis
• Cytokinesis

Question 15

What is Mitosis?

Answer 15

• Division of nuclear material (chromosomes)
  
  • 4 phases, but continuous cycle
    
    1. Prophase
    2. Metaphase
    3. Anaphase
    4. Telophase

Question 16

What is Cytokinesis?

Answer 16

Division of the cytoplasm

Question 17

When does Mitotic Phase ends?

Answer 17

• Mitosis & cytokinesis complete
• Resulting diploid cells go into interphase (G1)
• Cycle starts over

Question 18

What happens in Prophase?
(All over the place)

Answer 18

• Chromosomes condense (thicken, coil) - become individually visible
• Nucleoli disappear and nuclear envelope breaks up
• Centrosomes move to opposites sides (poles) of the cell
• Spindle microtubules grow out of centrosomes and attach to kinetochoreproteins of each sister chromatid, forming the spindle apparatus
  
  • Spindle begins to move chromosomes toward cell equator

Question 19

What happens in Metaphase?

Answer 19

46 replicated chromosomes line up on cell equator

Question 20

What happens in Anaphase?

Answer 20

• Spindle microtubules shorten and pull the kinetochores away from each other - causes separation of centromeres, which results in separation of the sister chromatids into 92 individual chromosomes
• 46 chromosomes migrate to each pole
• Cytokinesis (division of cytoplasm) begins

Question 21

What happens in Telophase?
(Prophase in Reverse)

Answer 21

• Chromosomes uncoil to long thread-like strands of chromatin
• nucleoli, nuclear envelope reappear
• Spindle disassembles
• Cytokinesis completes (or may end after telophase ends)

Question 22

Meiosis

Answer 22

• Produces gametes (ovum and sperm)
  
  • 1 cell (diploid, 2n) produces 4 genetically unique gametes (each haploid, n)

Question 23

What is involved in Gamete Formation?

Answer 23

• Interphase (G1, S, G2)
  ↓
• 2 cell division
  
  1. Meiosis I
  2. Meiosis II

Question 24

What happens in Meiosis I?

Answer 24

• Reduction division in which chromosome number changes from 2n to n
• events same as in mitosis except:
  
  1. Prophase I
  2. Metaphase I
  3. Anaphase I
  4. Telophase I

Question 25

Prophase I

Answer 25

Homologous chromosomes attach together to form tetrads (4 chromatids in a row)

Question 26

Metaphase I

Answer 26

- 23 tetrads line up along cell equator

Question 27

Anaphase I

Answer 27

- Tetrads (homologous chromosome pairs0 separate and migrate to opposite poles i.e. 1 homologous chromosome (with 2 sister chromatids) goes to each pole - Sister chromatids DON'T separate

Question 28

Telophase I

Answer 28

Identical steps to Telophase of mitosis but with 23 replicated chromosomes instead of 46

Question 29

What happens after Meiosis and cytokinesis?

Answer 29

- Each new cell has 23 different chromosomes (half the number of chromosomes) = haploid (n) - These cells DO NOT have homologous chromosomes - Each cell has a single copy of each autosomal chromosome and a single sex chromosome

Question 30

Meiosis II

Answer 30

Is the same as mitosis except it starts with 23 chromosomes instead of 46

Question 31

What are the events in Meiosis II?

Answer 31

1. Prophase II 2. Metaphase II 3. Anaphase II 4. Telophase II 5. Cytokinesis

Question 32

Metaphase II

Answer 32

23 replicated chromosomes line up at equator

Question 33

Anaphase II

Answer 33

Sister chromatids split and 23 chromosomes migrate to each pole

Question 34

Cytokinesis

Answer 34

Is the same as for mitosis/meiosis I (begins in anaphase/anaphase I/ anaphase II, and continues through telophase/ telophase I/ telophase II)

Question 35

Metaphase (Mitosis)

Answer 35

46 chromosomes line up at equator

Question 36

Anaphase (Mitosis)

Answer 36

Kinetochores split & 46 chromosomes migrate to each pole

Question 37

After Cytokinesis (Mitosis)

Answer 37

- = interphase - 2 identical diploid cells - 23 pairs of chromosomes in each = 46 in total

Question 38

Prophase I (Meiosis I)

Answer 38

Tetrads form

Question 39

Metaphase I (Meiosis I)

Answer 39

23 tetrads line up at equator

Question 40

Anaphase I (Meiosis I)

Answer 40

Tetrads split & 23 replicated chromosomes migrate to each pole

Question 41

After Cytokinesis (Meiosis I)

Answer 41

- 2 haploid cells - 2 nearly identical copies of each of 23 chromosomes with sister chromatids

Question 42

Metaphase II (Meiosis II)

Answer 42

23 chromosomes line up at equator

Question 43

Anaphase II (Meiosis II)

Answer 43

Kinetochores split, sister chromatids separate and 23 chromosomes migrate

Question 44

After Cytokinesis (Meiosis II)

Answer 44

- = gametes - 4 haploid cells - 1 copy of each of 23 chromosomes

Question 45

Why Meiosis?

Answer 45

- Need to turn diploid germ cells into haploid gametes (ova/sperm) - 23 chromosomes from haploid (n) sperm cell combine with 23 chromosomes from haploid ovum (n) to form a single 46 chromosome cell called a zygote (2n)