Biology 3201
Unit 2 – Reproduction and Development
Ch. 14 – Cellular Reproduction (pp. 458-483)
14.1 – How Body Cells Reproduce
The Cell Cycle (see fig 14.1, p. 460)
Somatic cell: refers to a body cell; anything not a germ (sex) cell
Cell cycle: a continuous sequence of cell growth and division
The cell cycle consists of two main stages
1. Interphase – growth phase; includes G1, S phase, and G2
G1 (gap 1): cell carries out metabolic activities and prepares for cell division
S phase: DNA is replicated
G2 (gap 2): centrioles replicate and cell prepares for division
2. division stage – includes mitosis and cytokinesis; shortest stage
Different cells have different timing for their cells cycles; some take longer than others to
go thorough their cycle, and they also spend different amounts of time in each stage.
Mitosis – division of the cell’s nucleus where the daughter cells receive the exact number
of chromosomes and genetic makeup as the parent cell
Cytokinesis – separation of the cytoplasm and the formation of two new daughter cells;
cytokinesis occurs after telophase of mitosis
Parent cell – the original cell that divides during mitosis to form two new daughter cells
Daughter cells – the cells produced during mitosis of a parent cell
In order for an organism to grow, repair, and maintain its function new cells are needed to
replace old ones. Each cell that undergoes mitosis produces 2 new cells. Mitosis allows
the regeneration of damaged tissue (like cuts) and to replace worn out cells (like red
blood cells)
Mitosis ensures that the same amount of genetic information in each type of cell.
Chromatin: the long fibers that form chromosomes and contain DNA, RNA and various
proteins. Found in the nucleus of cells.
Chromosome: condensed chromatin structure formed when cells replicate (divide) (see
fig. 14.6, p. 462)
Chromatid: one half of a chromosome. Two sister chromatids are joined by a centromere
to form a chromosome
Human somatic cells have 46 chromosomes (22 pairs plus the sex chromosomes). Before
mitosis, the parent cell has 46 chromosomes. After mitosis, the daughter cells each have
46 chromosomes.
Stages of Mitosis
Before mitosis begins, DNA is replicated during interphase
1. Prophase
→ chromatin coils up to form chromosomes
→ the nuclear membrane and nucleolus disappear
→ centrioles produced during interphase migrate to opposite poles of the cell
→ spindle fibers (microtubules) start to form between the two centrioles
2. Metaphase
→ spindle fibers attach to centromeres of chromosomes and the spindle fibers guide the
chromosomes to the equator of the cell
→ chromosomes are lined up at the equator and each chromatid is attached to a spindle
fiber
3. Anaphase
→ centromeres split apart and chromatids of chromosomes are pulled towards opposite
ends of the cell (the microtubules shorter)
4. Telophase
→ chromatids reach opposite ends of the cell
→ chromatids unravel to form chromatin again
→ spindle fibers break down and disappear
→ nucleolus and nuclear membrane begin to reform
→ cytokinesis occurs resulting in the formation of two new daughter cells, identical to
the parent
Mutations affecting cell division
Mutation
-
permanent change in the DNA of an organism
can occur spontaneously or by certain compounds, radiation, etc.
mutations that occur in parent cells are passed on to daughter cells
most mutations I somatic cells are not important because those
daughter cells can be replaced by normal cells
however, if a mutation affects a gene (DNA) which controls cell
division, cancer can result (uncontrolled rapid growth of cells)
14.2 – How Reproductive Cells are Produced
germ cell: a sex cell that produces sperm or egg
Unlike somatic cells, germ cells are produced by meiosis
Meiosis: type of cell division that occurs only in reproductive organs producing
reproductive cells called gametes (sperm and egg). Meiosis is a form of reductive
division.
Reductive division: division of cells that reduces the chromosome number. For example,
in meiosis the chromosome number is reduced from diploid to haploid
Diploid (2n): cells that contain two copies of every chromosome
Haploid (n): cells that contain only one copy of every chromosome, half the number of a
diploid cell
In humans, we have 22 pairs of autosomes (chromosomes not involved in determining
sex) and 1 pair of sex chromosomes (XX or XY). Males have XY while female are XX.
Sperm cells carry either an X chromosome or a Y chromosome and 22 autosomes, while
eggs can only carry an X chromosome and 22 autosomes.
Meiosis occurs in two sequential phases; both of which are very similar to mitosis. As
well, DNA is replicated in the cell during interphase just like in mitosis. Unlike mitosis,
however, meiosis involves 2 rounds of cell division producing 4 daughter cells and each
daughter cell contains half of the DNA as the parent cell.
Stages of Meiosis (see fig. 14.14, p. 472)
Meiosis I
1. Prophase I
→ chromatin condenses to form chromosomes
→ pairs of homologous chromosomes align to form tetrads. Sometimes
chromatids from homologous chromosomes intertwine and crossing over occurs
- homologous chromosomes: chromosomes that contain the same gene
sequences but may not be made up of the same allele
- tetrads: a homologous pair found in prophase I
- alleles: alternate form of a gene
- crossing over: the process where non-sister chromatids exchange genes
during prophase I of meiosis (see fig. 14.15, p. 473). Allows for
genetic variety
→ nuclear membrane and nucleolus diasappear
→ centrioles produced during interphase migrate towards opposite
ends of the cell
→ spindle fibers begin to form between the two centrioles
2. Metaphase I
→ spindle fibers attach to pairs of sister chromatids of tetrads and pull them towards the
equator
→ chromosomes line up at equator in their homologous pairs; one on one side of the
equator, the other on the other side of the equator
3. Anaphase I
→ homologous chromosomes separate and are pulled towards opposite ends of the cell
(sister chromatids are held together)
4. Telophase I
→ does not occur in all cells. If it does not, the cells go directly to meiosis II
→ if telophase I occurs, homologous chromosomes begin to uncoil and spindle fibers
disappear, cytoplasm divides, nucleolus and nuclear membrane reforms and two cells are
formed
Meiosis II
5. Prophase II
→ spindle fibers reform
→ centrioles are at opposite ends of the cells
→ chromosomes form
6. Metaphase II
→ spindle fibers attach to centromeres of chromosomes and guid them to equator of cell
where they line up
7. Anaphase II
→ centromeres split apart and chromatids move to opposite poles of the cell
8. Telophase II
→ chromatids unravel; spindle disappears; cytokinesis occurs; nuclear membrane and
nucleolus reappear.
Gamete Formation
Gametogenesis – gamete formation where daughter cells, or gametes, are produced at the
end of meiosis II resulting in the production of sperm and egg.
Spermatogenesis – the process of male gamete production in animals
Oogenesis – the process of female gamete production in animals
(see fig. 14.18, p. 477)
Spermatogenesis begins with diploid germ cells called spermatogonia. After meiosis I,
one spermatigonia divides into two cells called primary spermatocytes. Primary
spermatocytes undergo meiosis II to form secondary spermatocytes, which develop into
sperm. In the end, from 1 parent spermatigonia 4 sperm cells are produced.
Oogenesis begins with a diploid cell called an oogonia. After meiosis I, one oogonia
forms one primary oocyte and one polar body (due to unequal division of the cytoplasm).
The primary oocyte and polar body undergo meiosis II to form one secondary oocyte and
three polar bodies. The secondary oocyte develops into an egg cell, while all the polar
bodies die. Only one functional egg cell comes from this process, as the unequal division
of the cytoplasm makes the egg cell big (needs extra nutrients).
Sperm cells vs. egg cells (see table 14.2, p. 478)
Sperm
Egg
- Small
- Large
- mobile
- not mobile
- have a cap called an acrosome which
- covered by a thick outer coating. After
contains enzymes used to enter the egg cell one sperm penetrates the egg, no more can
enter
- millions produced continuously (300
- one egg matures per month from puberty
million-500 million)
to menopause
- 50-100 mitochondria per cell
- about 140, 000 mitochondia per cell
- before ejaculation: uses fat for energy
- can only live for about a day or so with its
- after ejaculation: uses sugar (fructose) for food supply
energy
Technologies Based on Cell Division
(1) Cloning
→ make an exact copy of an organism, either an entire organism (reproductive) or parts
(therapeutic)
→ animal cloning techniques involve bypassing the meiosis step of animal reproduction
(2) Stem Cell Research
→ stem cells: undifferentiated (nonspecialized) cells that can give rise to any other type
of cell
→ found in primarily the bone marrow but also found in blood, muscle tissue, brain, etc.
→ sources include aborted fetuses, unused embryos from in-vitro fertilization treatments,
cord blood (from placenta)