Lab 7 (cell division) Study Guide
1. Define homologous chromosomes:
a. Matching pairs of chromosomes (one from mom and one from dad).
b. They carry identical genes arranged in the same order; the genes may be of the same form
or may be different (alleles); size, shape and location of the centromere (constriction point)
are the same.
c. Humans have 23 homologous pairs of chromosomes (46 total chromosomes).
2. Describe the human homologous chromosome pairs.
a. Autosomal chromosome pairs #1-#22
b. Sex chromosome pair #23 determines sex (in mammals XX=female & XY = male)
3. Define diploid and haploid and identify what cell types they represent.
a. Diploid cells (2n) have both members of each homologous chromosome pair = 46
chromosomes; human body (somatic) cells are diploid.
b. Haploid cells (n) have one member of each homologous chromosome pair =23
chromosomes; sex cells (egg and sperm) are haploid.
4. What is the purpose of mitosis? It produces genetically identical cells.
a. Asexual reproduction: organisms that reproduce by dividing in half and offspring are
genetically identical to the parent. It occurs in various unicellular and multicellular
organisms.
b. Permits growth of multicellular organisms (from a single celled- zygote to a multicellular, mature adult)
c. Replaces lost or damaged cell/ tissues. Some examples are: (1) cells that undergo
mitosis in the bone marrow for the renewal of blood cells; (2) replacement of dead and
sloughed off epidermal cells of the skin or of cells lining the stomach and small intestine;
and (4) the repair of damaged tissues.
5. What is the purpose of meiosis?
a. Meiosis produces haploid gametes (egg and sperm) for sexual reproduction.
b. Cells that undergo meiosis are the gamete-producing cells in the testes or ovaries.
6. Describe the steps of the cell cycle.
a. Mitotic phase: mitosis (division of the nucleus) & cytokinesis (division of the cytoplasm)
b. Interphase (G1, S, G2): period of cell growth; DNA replication (in S phase) and centriole
pair replication (G2); cell growth occurs throughout, with the cytoplasm and everything in
it doubling (i.e. organelles); period in which cell performs all of its functions, such as
protein synthesis; 90% of the total time of the cell cycle is spent in interphase.
7. What is the mitotic spindle?
a. A football-shaped structure of microtubules (long protein fibers of tubulin) that guides the
separation of sister chromatids of a replicated chromosome.
b. 2 centriole pairs (of 2 centrosomes) function to organize the mitotic spindle
c. It forms as centrosomes move to opposite poles of a cell.
d. Some of the microtubule fibers from opposite poles attach to sites within each sister
chromatid called kinetochores.
8.
For mitosis know the general steps. Prior to start, DNA & centriole pairs are replicated.
a. Prophase: replicated chromosomes condense, nuclear envelope breaks down, spindle
forms and grabs on to chromosomes.
b. Metaphase: centromeres of all replicated chromosomes are lined up at the metaphase
plate (in a single-file line)
c. Anaphase: sister chromatids of replicated chromosomes separate and move to opposite
poles
9.
Telophase & Cytokinesis (usually occur at same time): Chromosomes are at opposite poles,
nuclear envelope forms around each set, the mitotic spindle disappears, DNA relaxes, and
cytokinesis divides the cytoplasm in half. Two diploid cells are produced.
10. Summary of mitosis in humans:
a. Starts with: One diploid cell with 46 replicated chromosomes (2 sister chromatids joined
at the centromere).
b. Ends with: 2 diploid cells with chromosomes made of one sister chromatid; these cells are
genetically identical to the parent cell.
11. For meiosis, know the general steps: Prior to start, DNA and centriole pairs are replicated.
a. Prophase I: replicated chromosomes condense, nuclear envelope breaks down,
homologous chromosomes pair up and exchange segments (crossing- over), the spindle
forms and grabs onto replicated chromosomes.
b. Metaphase I: replicated homologous chromosome pairs randomly line up on either side
of the metaphase plate (in a double-file line)
c. Anaphase I: replicated homologous chromosome pairs separate & move to opposite poles
d. Telophase I/ cytokinesis: (usually occur at same time): replicated homologous
chromosome pairs are at opposite poles, +/- nuclear envelope forms around each set, the
mitotic spindle disappears, DNA +/- relaxes and cytokinesis divides the cytoplasm in half.
Two haploid cells with replicated chromosomes are produced.
e. Centrioles pairs duplicate, but not DNA
f. Prophase II: replicated chromosomes condense, nuclear envelope breaks down, spindle
forms and grabs on to chromosomes.
g. Metaphase II: centromeres of replicated chromosomes are lined up at the metaphase plate
(in a single-file line)
h. Anaphase II: sister chromatids of replicated chromosomes separate and move to opposite
poles
i. Telophase & Cytokinesis (usually occur at same time): producing 4 haploid cells with
chromosomes made up of single chromatids
12. Summary of meiosis:
a. Starts with: One diploid cell with 46 replicated chromosomes (2 sister chromatids)
b. Meiosis I ends with: 2 haploid cells with 23 replicated chromosomes each
c. Meiosis II ends with: 4 haploid cells that have 23 chromosomes that are made of single
chromatids (cells are genetically unique gametes)
13. How does meiosis produce genetic variability?
a. In prophase I: genetic recombination (new combination of alleles) created by crossing
over between homologous chromosomes pairs
b. In metaphase I: there is random alignment of the homologous chromosomes one either
side of the metaphase plate of the cell (allows for segregation and independent assortment
of the paired alleles of genes)
14. How are mitosis and meiosis different?
a. They both start with diploid cells that have chromosomes that are duplicated, but mitosis
ends with two cells that are diploid & identical, while meiosis ends with 4 haploid cells that
are genetically unique.
b. Prophase of mitosis versus prophase I of meiosis I:
i. In prophase I of meiosis homologous chromosomes pair up and cross over; this does
not occur in prophase of mitosis.
c. Anaphase of mitosis versus anaphase of meiosis I:
i. Anaphase of mitosis – sister chromatids of replicated chromosomes separate
ii. Anaphase I of meiosis – homologous chromosomes separate
15. How is meiosis II similar to mitosis? Though cells at the start of meiosis II are haploid and cells
at the start of mitosis are diploid, in both sister chromatids of replicated chromosomes separate.