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Biology MCAT

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Biology And Biochemistry MCAT
Cell Cycle and Mitosis
In animals, autosomal cells are said tp be diploid 2n, which means that they contain two copies
of each chromosome. Germ cells are haploid continuing only one copy of each chromosome
INTERPHASE
(G1, S, G2)
• Interphase is the longest part of the cell cycle, even actively diving cells spend about 90
percent of their time in interphase.
• Cells that do not divide are in an offshoot of G1 called G0
• G0 stage, the cell is simply living and carrying out its functions, without any preparation for
division
• During interphase individual chromosomes are not visible with light microscopy because
they arena less condensed form known as chromatin
• Chromatin, is the less condensed form of chromosomes
• DNA is less condensed making RNA polymerase activity possible so that Genes can be
transcribed
• SIDNOTE: During mitosis it is preferred to condense DNA into tightly coiled chromosomes to
avoid losing any genetic material during division
G1
• During G1 stage the cell creates organelles for energy and protein production(mitochondria,
ribosomes, and endoplasmic reticulum) while also increasing in size.
• Passage to the S phase is is covered by a restriction point
• Criteria such as proper complement of DNA must be met for the cell to pass the restriction
point and enter the synthesis stage
S Stage: Synthesis of DNA
• During the S stage the cell replicates its genetic material so that each daughter cell will have
identical copies
• After replication, each chromosome consist of two identical chromatids, that are bound
together at a specialized region known as the centromere
• Note that the ploidy of the cell does not change even though the number of chromatids
has doubled
• Human cell in this stage has 46 chromosomes even though 92 chromatids are present
• Cells entering G2 have twice as much DNA as cells in G1
G2
- The cell passes through another quality control checkpoint. DNA has already been duplicated
and the cell checks to ensure that there are enough organelles and cytoplasm for two daughter
cells.
- The cells check to make sure the DNA replication proceeded correctly to avoid passing on an
error to daughter cells that may further pass on the error to their progeny
Control of the Cell cycle
• Checkpoints are located between G1 and S, and S phase and G2 and M phase
• G1/S checkpint, the cell determines I the condition of the DNA is good enough for synthesis
• The main protein in control of this is p53
• At G2/M the cell is mainly concerned with ensuring that is has achieved adequate size and the
organelles have been properly relocated to support two daughter cells, p53 is also important
here
• Molecules responsible for the cell cycle are known as cyclins and cyclin dependent
kinases(CDK)
• CDKs bind to the cyclins creating complex that can then be phosphorylate transcription factors
• Transcription factors then promote transcription of genes required for the next stage of the cell
cycle
Homologous chromosomes- two homogenous chromosomes(one chromosome from mother,
one from father)
In meiosis, during prophase I, the chromatin condenses into chromosomes, the spindle apparatus
forms, and the nucleoli and nuclear membrane disappear. The first major difference between
meiosis and mitosis occurs at this point: homologous chromosomes come together and intertwine
in a process called synapsis. At this point, each chromosome consists of two sister
chromatids, so each synaptic pair contains four chromatids and is referred to as a tetrad.
Chromatids of homologous chromosomes may break at the point of synapsis, called the chiasma
(plural: chiasmata) and exchange equivalent pieces of DNA. This process is called crossing over.
Note that crossing over occurs between homologous chromosomes and not between sister
chromatids of the same chromosome(paternal and maternal chromosomes cross over, not
paternal sister chromatids because this would cause no change)—the latter are identical, so
crossing over would not produce any change. Those chromatids involved are left with an altered
but structurally complete set of genes. Such genetic recombination can unlink linked genes,
thereby increasing the variety of genetic combinations that can be produced via gametogenesis.
Linkage refers to the tendency for genes to be inherited together; genes that are located further
from each other physically are less likely to be inherited together, and more likely to
undergo crossing over relative to each other. Thus, as opposed to asexual reproduction, which
produces identical offspring, sexual reproduction provides the advantage of great genetic
diversity, which is believed to increase the ability of a species to evolve and adapt to a changing
environment.
During metaphase I, homologous pairs (tetrads) align at the metaphase plate, and each pair
attaches to a separate spindle fiber by its kinetochore. Note the difference from mitosis: in
mitosis, each chromosome is lined up on the metaphase plate by two spindle fibers (one
from each pole); in meiosis, homologous chromosomes are lined up across from each other
at the metaphase plate and are held by one spindle fiber.
During anaphase I, homologous pairs separate and are pulled to opposite poles of the cell. This
process is called disjunction, and it accounts for Mendel’s first law (of segregation). During
disjunction, each chromosome of paternal origin separates (or disjoins) from its homologue
of maternal origin, and either chromosome can end up in either daughter cell. Thus, the
distribution of homologous chromosomes to the two intermediate daughter cells is random with
respect to parental origin. This separating of the two homologous chromosomes is referred to as
segregation.
During telophase I, a nuclear membrane forms around each new nucleus. At this point, each
chromosome still consists of two sister chromatids(that have gone through crossing over)
joined at the centromere. The cells are now haploid; once homologous chromosomes
separate(during Anaphase), only n chromosomes are found in each daughter cell (23 in
humans). The cell divides into two daughter cells by cytokinesis, not four daughter cells, making
answer choice (D) correct. Between cell divisions, there may be a short rest period, or
interkinesis, during which the chromosomes partially uncoil.
Meiosis II is very similar to mitosis in that sister chromatids—rather than homologues—are
separated from each other.
During prophase II, the nuclear envelope dissolves, nucleoli disappear, the centrioles migrate to
opposite poles, and the spindle apparatus begins to form.
During metaphase II, the chromosomes line up on the metaphase plate.
During anaphase II, the centromeres divide, separating the chromosomes into sister
chromatids. These chromatids are pulled to opposite poles by spindle fibers.
During telophase II, a nuclear membrane forms around each new nucleus. Cytokinesis follows,
and two daughter cells are formed. Thus, by completion of meiosis II, up to four haploid
daughter cells are produced per gametocyte. We use the phrase up to because oogenesis
may result in fewer than four cells if an egg remains unfertilized after ovulation.
Male Reproductive System
In males, the primitive gonads develop into the testes. The testes have two functional
components: the seminiferous tubules and the interstitial cells of Leydig. Sperm are produced
in the highly coiled seminiferous tubules, where they are nourished by Sertoli cells. (D) is
correct.
As sperm are formed they are passed to the epididymis, where their flagella gain motility, and
they are then stored until ejaculation. During ejaculation, sperm travel through the vas
deferens and enter the ejaculatory duct at the posterior edge of the prostate gland. The two
ejaculatory ducts then fuse to form the urethra, which carries sperm through the penis as they
exit the body.
As sperm pass through the reproductive tract they are mixed with seminal fluid, which is
produced through a combined effort by the seminal vesicles, prostate gland, and bulbourethral
gland. The seminal vesicles contribute fructose to nourish sperm, and both the seminal vesicles
and prostate gland give the fluid mildly alkaline properties so the sperm can survive in the
relative acidity of the female reproductive tract.
Cytokinesis during oogenesis is unique in that it is characterized by unequal division. Once a
woman reaches menarche (her first menstrual cycle), one primary oocyte per month will
complete meiosis I. This produces one primary oocyte which contains ample cytoplasm and one
polar body that is largely devoid of cytoplasm and organelles. A polar body is basically a small
package of DNA that is kicked off to allow for maximum cytoplasm to the secondary oocyte (or
ovum in the second division). The polar body generally does not divide any further and will
never produce functional gametes. The secondary oocyte, on the other hand, remains
arrested in metaphase II and does not complete the remainder of meiosis II unless
fertilization occurs.
Spermatogenesis is the formation of haploid sperm through meiosis, and occurs in the
seminiferous tubules. In males, the diploid stem cells are known as spermatogonia. After
replicating their genetic material (S stage), they develop into diploid primary spermatocytes .
The first meiotic division will result in haploid secondary spermatocytes, which then
undergo meiosis II to generate haploid spermatids. Finally, the spermatids undergo
maturation to become mature spermatozoa. Spermatogenesis results in four functional sperm
for each spermatogonium.
Mature sperm are very compact. They consists of a head (containing the genetic material), a
midpiece (which generates ATP from fructose), and a flagellum (for motility). The midpiece is
filled with mitochondria, which generate the energy to be used as the sperm swims through
the female reproductive tract to reach the ovum in the fallopian tubes. Each sperm head is
covered by a cap known as an acrosome. This structure is derived from the Golgi apparatus and
is necessary to penetrate the ovum. Once a male reaches sexual maturity during puberty,
approximately 3 million sperm are produced per day through the rest of life.
The production of female gametes is known as oogenesis. Although gametocytes undergo the
same meiotic process in both females and males, there are some significant differences between
the two sexes. First, there is no unending supply of stem cells analogous to spermatogonia in
females; all of the oogonia a woman will ever have are formed during fetal development. By
birth, all of the oogonia have already undergone DNA replication and are considered primary
oocytes. These cells are 2n , like primary spermatocytes, and are actually arrested in prophase
I. Once a woman reaches menarche (her first menstrual cycle), one primary oocyte per
month will complete meiosis I, producing a secondary oocyte and a polar body. The division
is characterized by unequal cytokinesis, which doles ample cytoplasm to one daughter cell (the
secondary oocyte) and nearly none to the other (the polar body). The polar body generally does
not divide any further and will never produce functional gametes. The secondary oocyte, on the
other hand, remains arrested in metaphase II and does not complete the remainder of meiosis II
unless fertilization occurs.
If, during anaphase of meiosis, homologous chromosomes (anaphase I) or sister chromatids
(anaphase II) fail to separate, one of the resulting gametes will have two copies of a particular
chromosome and the other gamete will have none. Subsequently, during fertilization, the
resulting zygote may have too many or too few copies of that chromosome. Nondisjunction can
affect both autosomal chromosomes (such as trisomy 21, resulting in Down syndrome) and
the sex chromosomes (such as Klinefelter’s and Turner syndromes).
Down’s Syndrome occurs when a person has a trisomy at chromosome 21. This is usually due to
an error in meiosis I or II where a nondisjunction event leads to unequal segregation of
chromosomes. Microtubules are involved in the segregation of chromosomes during
anaphase, and the malfunctioning of microtubules may lead to nondisjunction events.
Sickle cell disease is a single nucleotide mutation that causes sickled hemoglobin( which is a
point mutation that could cause a frameshift mutation which is a mutation caused by insertions or
deletion which changes the reading frame which can either change the protein that will be
produced or alter a stop CODON (UAG, UAA, UGA)
A woman takes a pregnancy test measuring human chorionic gonadotropin (hCG) levels. The test
result comes back negative. However, she is in her sixteenth week of pregnancy. Which of the
following explains the false-negative result?
In pregnancy, the zygote will develop into a blastocyst that will implant in the uterine lining and
secrete human chorionic gonadotropin (hCG). This hormone is an analog of LH, meaning that
it looks very similar chemically and can stimulate LH receptors. This maintains the corpus
luteum. hCG is critical during first trimester development because it is the estrogen and
progesterone secreted by the corpus luteum that keep the uterine lining in place. By the second
trimester, hCG levels decline because the placenta has grown to a sufficient size to secrete
progesterone and estrogen by itself (C). The high levels of estrogen and progesterone continue
to serve as negative feedback mechanisms, preventing further GnRH secretion.
Ventilation center
• Regulates ventilation
• A collection of neurons in the medulla oblongata
• Can be controlled consciously through the cerebrum, although the medulla can override the
cerebellum during extended periods of hypo or hyperventilation
Negative pressure breathing
• The diaphragm and external intercostal muscles expand the thoracic cavity, increasing. The
volume of the interplureal space, which
• Decreases the intraplueral pressure
• - pressure differential ultimately expands the lungs, dropping their pressure and drawing in air
form the environment
Visceral pleurae lies adjacent to the lung itself
Intrapleural space- lies between the visceral and parietal pleura and contains thin layer of fluid
which lubricates the two surfaces
Kidney medulla is the deeper region of the kidney containing the loop of hence regions often
nephrons
Kidney cortex is the filtering layer of the kidneys jam packed with nephrons
• Aldosterone is released and promoted during low blood pressure
• Aldosterone stimulates the kidney to retain sodium ions and water
• Aldosterone acts on Renal collecting ducts
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