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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
- Small
- mobile
- have a cap called an acrosome which
contains enzymes used to enter the egg cell
- millions produced continuously (300
million-500 million)
- 50-100 mitochondria per cell
- before ejaculation: uses fat for energy
- after ejaculation: uses sugar (fructose) for
energy
Egg
- Large
- not mobile
- covered by a thick outer coating. After
one sperm penetrates the egg, no more can
enter
- one egg matures per month from puberty
to menopause
- about 140, 000 mitochondia per cell
- can only live for about a day or so with its
food supply
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)