Biology
CHAPTER 2: REPRODUCTION
The Cell Cycle and Mitosis
➢ The Cell Cycle and Mitosis
➢ Diploid (2n) cells have two copies of each chromosome; haploid (n) cells have one copy.
○ The cell cycle contains five stages. The G1, S, and G2 stages are collectively called
interphase, during which the DNA is uncoiled in the form of chromatin.
○ In the G1 stage (presynthetic gap), cells create organelles for energy and protein
production, and increase their size. The restriction point, during which the DNA is checked
for quality, must be passed for the cell to move into the S stage.
○ In the S stage (synthesis), DNA is replicated. The strands of DNA, called chromatids, are
held together at the centromere.
○ In the G2 stage (postsynthetic gap), there is further cell growth and replication of
organelles in preparation for mitosis. Another quality checkpoint must be passed for the
cell to enter into mitosis.
○ In the M stage (mitosis), mitosis and cytokinesis occur.
○ In the G0 stage, the cell performs its functions without preparing for division.
➢ p53 plays a role in the two major checkpoints of the cell cycle (G1 to S, and G2 to M).
➢ Cyclins and cyclin-dependent kinases (CDK) rise and fall during the cell cycle. Cyclins bind to
CDKs, phosphorylating and activating transcription factors for the next stage of the cell cycle.
➢ Cancer occurs when cell cycle control becomes deranged, allowing damaged cells to undergo
mitosis without regard to quality or quantity of the new cells produced. Cancerous cells may
begin to produce factors that allow them to delocalize and invade adjacent tissues or metastasize
elsewhere.
➢ Mitosis produces two genetically identical diploid daughter cells from a single cell and occurs in
somatic cells.
➢ Mitosis has four phases:
○ In prophase, the chromosomes condense, the nuclear membrane dissolves, nucleoli
disappear, centrioles migrate to opposite sides of the cell, and the spindle apparatus
begins to form. The kinetochore of each chromosome is contacted by a spindle fiber.
○ In metaphase, chromosomes line up along the metaphase plate (equatorial plate).
○ In anaphase, sister chromatids are separated and pulled to opposite poles.
○ In telophase, the nuclear membrane reforms, spindle apparatus disappears, and cytosol
and organelles are split between the two daughter cells through cytokinesis.
Meiosis
➢ Meiosis occurs in gametocytes (germ cells) and produces up to four nonidentical haploid sex
cells (gametes).
➢ Meiosis has one round of replication and two rounds of division (the reductional and equational
divisions).
➢ In meiosis I, homologous pairs of chromosomes (homologues) are separated from each other.
Homologues are chromosomes that are given the same number, but are of opposite parental
origin.
○ In prophase I, the same events occur as in prophase of mitosis, except that homologues
come together and intertwine in a process called synapsis. The four chromatids are
referred to as a tetrad, and crossing over exchanges genetic material between one
chromatid and material from a chromatid in the homologous chromosome. This accounts
for Mendel’s second law (of independent assortment).
○ In metaphase I, homologous chromosomes line up on opposite sides of the metaphase
plate.
, ○ In anaphase I, homologous chromosomes are pulled to opposite poles of the cell. This
accounts for Mendel’s first law (of segregation).
○ In telophase I, the chromosomes may or may not fully decondense, and the cell may
enter interkinesis aer cytokinesis.
➢ In meiosis II, sister chromatids are separated from each other in a process that is functionally
identical to mitosis. Sister chromatids are copies of the same DNA held together at the
centromere.
The Reproductive System
➢ Biological sex is determined by the 23rd pair of chromosomes in humans, with XX being female
and XY being male.
➢ The X chromosome carries a sizeable amount of genetic information; mutations of X-linked
genes can cause sex-linked disorders. Males are hemizygous with respect to the unpaired genes
on the X chromosome, so they will express sex-linked disorders, even if they only have one
recessive disease-carrying allele. Women with one copy of the affected allele are called carriers.
➢ The Y chromosome carries little genetic information, but does contain the SRY (sex-determining
region Y) gene, which causes the gonads to differentiate into testes.
➢ The male reproductive system contains both internal and external structures.
○ Sperm develop in the seminiferous tubules in the testes. They are nourished by Sertoli
cells.
○ Interstitial cells of Leydig, in the testes, secrete testosterone and other male sex
hormones (androgens).
○ The testes are located in the scrotum, which hangs outside of the abdominal cavity and
has a temperature 2° to 4°C lower than the rest of the body.
○ Once formed, sperm gain motility in the epididymis and are stored there until ejaculation.
○ During ejaculation, sperm travel through the vas deferens to the ejaculatory duct, and
then to the urethra and out through the penis. The seminal vesicles contribute fructose to
nourish sperm and produce alkaline fluid.
○ The prostate gland also produces alkaline fluid.
○ The bulbourethral glands produce a clear viscous fluid that cleans out any remnants of
urine and lubricates the urethra during sexual
○ arousal.
○ Semen is composed of sperm and seminal fluid from the glands above.
➢ In spermatogenesis, four haploid sperm are produced from a spermatogonium.
○ After S stage, the germ cells are called primary spermatocytes.
○ After meiosis I, the germ cells are called secondary spermatocytes.
○ After meiosis II, the germ cells are called spermatids.
○ After maturation, the germ cells are called spermatozoa.
➢ Sperm contain a head, midpiece, and flagellum.
○ The head contains the genetic material and is covered with an acrosome—a modified
Golgi apparatus that contains enzymes that help the sperm fuse with and penetrate the
ovum.
○ The midpiece generates ATP from fructose and contains many mitochondria.
○ The flagellum promotes motility.
➢ The female reproductive system only contains internal structures.
○ Ova (eggs) are produced in follicles in the ovaries.
CHAPTER 2: REPRODUCTION
The Cell Cycle and Mitosis
➢ The Cell Cycle and Mitosis
➢ Diploid (2n) cells have two copies of each chromosome; haploid (n) cells have one copy.
○ The cell cycle contains five stages. The G1, S, and G2 stages are collectively called
interphase, during which the DNA is uncoiled in the form of chromatin.
○ In the G1 stage (presynthetic gap), cells create organelles for energy and protein
production, and increase their size. The restriction point, during which the DNA is checked
for quality, must be passed for the cell to move into the S stage.
○ In the S stage (synthesis), DNA is replicated. The strands of DNA, called chromatids, are
held together at the centromere.
○ In the G2 stage (postsynthetic gap), there is further cell growth and replication of
organelles in preparation for mitosis. Another quality checkpoint must be passed for the
cell to enter into mitosis.
○ In the M stage (mitosis), mitosis and cytokinesis occur.
○ In the G0 stage, the cell performs its functions without preparing for division.
➢ p53 plays a role in the two major checkpoints of the cell cycle (G1 to S, and G2 to M).
➢ Cyclins and cyclin-dependent kinases (CDK) rise and fall during the cell cycle. Cyclins bind to
CDKs, phosphorylating and activating transcription factors for the next stage of the cell cycle.
➢ Cancer occurs when cell cycle control becomes deranged, allowing damaged cells to undergo
mitosis without regard to quality or quantity of the new cells produced. Cancerous cells may
begin to produce factors that allow them to delocalize and invade adjacent tissues or metastasize
elsewhere.
➢ Mitosis produces two genetically identical diploid daughter cells from a single cell and occurs in
somatic cells.
➢ Mitosis has four phases:
○ In prophase, the chromosomes condense, the nuclear membrane dissolves, nucleoli
disappear, centrioles migrate to opposite sides of the cell, and the spindle apparatus
begins to form. The kinetochore of each chromosome is contacted by a spindle fiber.
○ In metaphase, chromosomes line up along the metaphase plate (equatorial plate).
○ In anaphase, sister chromatids are separated and pulled to opposite poles.
○ In telophase, the nuclear membrane reforms, spindle apparatus disappears, and cytosol
and organelles are split between the two daughter cells through cytokinesis.
Meiosis
➢ Meiosis occurs in gametocytes (germ cells) and produces up to four nonidentical haploid sex
cells (gametes).
➢ Meiosis has one round of replication and two rounds of division (the reductional and equational
divisions).
➢ In meiosis I, homologous pairs of chromosomes (homologues) are separated from each other.
Homologues are chromosomes that are given the same number, but are of opposite parental
origin.
○ In prophase I, the same events occur as in prophase of mitosis, except that homologues
come together and intertwine in a process called synapsis. The four chromatids are
referred to as a tetrad, and crossing over exchanges genetic material between one
chromatid and material from a chromatid in the homologous chromosome. This accounts
for Mendel’s second law (of independent assortment).
○ In metaphase I, homologous chromosomes line up on opposite sides of the metaphase
plate.
, ○ In anaphase I, homologous chromosomes are pulled to opposite poles of the cell. This
accounts for Mendel’s first law (of segregation).
○ In telophase I, the chromosomes may or may not fully decondense, and the cell may
enter interkinesis aer cytokinesis.
➢ In meiosis II, sister chromatids are separated from each other in a process that is functionally
identical to mitosis. Sister chromatids are copies of the same DNA held together at the
centromere.
The Reproductive System
➢ Biological sex is determined by the 23rd pair of chromosomes in humans, with XX being female
and XY being male.
➢ The X chromosome carries a sizeable amount of genetic information; mutations of X-linked
genes can cause sex-linked disorders. Males are hemizygous with respect to the unpaired genes
on the X chromosome, so they will express sex-linked disorders, even if they only have one
recessive disease-carrying allele. Women with one copy of the affected allele are called carriers.
➢ The Y chromosome carries little genetic information, but does contain the SRY (sex-determining
region Y) gene, which causes the gonads to differentiate into testes.
➢ The male reproductive system contains both internal and external structures.
○ Sperm develop in the seminiferous tubules in the testes. They are nourished by Sertoli
cells.
○ Interstitial cells of Leydig, in the testes, secrete testosterone and other male sex
hormones (androgens).
○ The testes are located in the scrotum, which hangs outside of the abdominal cavity and
has a temperature 2° to 4°C lower than the rest of the body.
○ Once formed, sperm gain motility in the epididymis and are stored there until ejaculation.
○ During ejaculation, sperm travel through the vas deferens to the ejaculatory duct, and
then to the urethra and out through the penis. The seminal vesicles contribute fructose to
nourish sperm and produce alkaline fluid.
○ The prostate gland also produces alkaline fluid.
○ The bulbourethral glands produce a clear viscous fluid that cleans out any remnants of
urine and lubricates the urethra during sexual
○ arousal.
○ Semen is composed of sperm and seminal fluid from the glands above.
➢ In spermatogenesis, four haploid sperm are produced from a spermatogonium.
○ After S stage, the germ cells are called primary spermatocytes.
○ After meiosis I, the germ cells are called secondary spermatocytes.
○ After meiosis II, the germ cells are called spermatids.
○ After maturation, the germ cells are called spermatozoa.
➢ Sperm contain a head, midpiece, and flagellum.
○ The head contains the genetic material and is covered with an acrosome—a modified
Golgi apparatus that contains enzymes that help the sperm fuse with and penetrate the
ovum.
○ The midpiece generates ATP from fructose and contains many mitochondria.
○ The flagellum promotes motility.
➢ The female reproductive system only contains internal structures.
○ Ova (eggs) are produced in follicles in the ovaries.
