Kaplan MCAT Biology: Chapter 2

Diploid

(2n) contain two copies of each chromosome, 46

Haploid

(n) contain one copy of each chromosome, 23

Cell Cycle

A series of specific phases in which the cell grows, synthesizes DNA, and divides.

The Cell Cycle Phases

G1, S, G2, M

Interphase

first three stages of the cell cycle, longest portion.

G0 Stage

Normal cell/no div offshoot of G1 phase.

Chromatin

less Condensed form of chromosomes during interphase.

G1 Stage

Create organelles - energy/protein production

increasing size (mitochondria, ribosomes, and endoplasmic reticulum)

governed by restriction points.

S Stage

genetic material replicates - each daughter identical. Each chromosome = two identical chromatids, bound at centromere.

Ploidy does not change (still 46 chromosomes, although 92 chromatids)

chromatids have doubled = twice as much DNA. Cells entering G2 have 2x amt of DNA as in G1.

G2 Stage

Cell passes through another quality control checkpoint to ensure DNA replicated correctly so error isnt passed down through M phase to progeny cells.

M Stage: Mitosis

mitosis and cytokinesis.

G1/S Checkpoint

DNA good enough for synthesis? (p53 protein) Restriction Point; If there is damage, cell arrests and p53 fixes problem.

G2/M Checkpoint

DNA of adequate size?

organelles replicate to support the daughter cells? (p53 protein)

Cyclins and Cyclin-Dependent Kinases

responsible for the cell cycle, via fluctuating levels of cyclin Cyclins bind CDK to activate them.

activated CDK-cyclin complex phosphorylates transcription factors for genes required for the next stage of the cell cycle.

p53

tumor suppressor gene binds to CDK to inhibit their activity if error occurs - prevent cell passage into next phase.

TP53

the gene that produces p53, the tumor suppressor gene. If damaged, can cause cancer.

Mitosis

in somatic cells, two identical daughter cells are created from a single cell.

Centrosomes

an organelle near the nucleus of a cell that contains the centrioles (in animal cells) and from which the spindle fibers develop in cell division.

Prophase

Condensation of the chromatin into chromosomes. Centriole pairs separate - move to poles of the cell. centrioles form spindle fibers (microtubules) from centrosomes.

Asters anchor the centrioles to cell membrane.

nuclear membrane dissolves -> spindle connection to the chromosomes.

Kinetochores appear at the centromeres to attach to specific fibers of the spindle apparatus (to rip them apart)

Metaphase

kinetochore fibers interact with the fibers of the spindle apparatus to align the chromosomes at the metaphase plate (equatorial plate, center of cell).

Anaphase

Centromeres split, each chromatid has its own distinct centromere

chromatids separate-via shortening of the kinetochore fibers (not really more like the shortening of microtubules).

Telophase

Spindle apparatus disappears

nuclear membrane reforms around chromosomes

nucleoli reappears.

chromosomes uncoil.

Cytokinesis

Separation of the cytoplasm/organelles -> daughter survive on own

general difference between meiosis and mitosis

mitosis separates 2 chromosomes of the same number (23 of them), one from each parent, so that each cell gets a new copy of each of the parent's chromosomes. Meiosis separates these chromosomes into different cells (each cell only has mother's or father's chromosome), and then each one of those cells divides to create gametes with only one parent's chromosome (but crossing over happens, so it ends up getting mixed up anyway)

Meiosis

in gametocytes cells

four nonidentical sex cells (gametes).

one round of replication + two rounds of division.

Meiosis I

Homologous chromosomes being separated, making haploid daughter cells (reductional division). Homologous pairs represent two different chromosome. 2n --> 1n

Prophase I

Chromatin condenses into chromosomes,

spindle app forms

nuclear membrane disappears.

Homologous chromosomes come together / intertwine (synapsis).

Each synaptic pair = tetrad.

Synaptonemal complex holds homologous chromosomes together (basically only hold together so crossing over can happen)

Chromatids break at the point of synapsis = chiasma in process called crossing over

dif-w/mitosis=occurs between homologous NOT SISTER CHROMATIDS (one homologous chromosome in ach cell after meiosis one

Crossing Over

homologous chromosomes break at synapsis, (called chiasma)

pieces of DNA are exchanged = recomb of genes.

Mendel's Second Law of Independent Assortment

Inheritance of one allele has no affect on the likelihood of inheriting certain alleles for other genes. Alleles separate independently in gametes. Crossing over explains this law. Linked genes are problematic because genes inherited together, instead of independently of one another.

Metaphase I

Tetrads align at metaphase plate,

each pair attaching to a separate SINGLE spindle fiber by kinetochore. (Double is for mitosis because it is actually pulling something apart, needs 2 points of attachment.

Anaphase I

Disjunction - each chromosome of paternal origin separates from its homologue of maternal origin (either chromosome can end up in either daughter cell). aka known as segregation.

explains mendels law of segregation

Mendel's First Law of Segregation

Separation of chromosome pairs in a random and unique way. Alleles separate from each other into each of daughter cells...Y to first cell and y to other cell (one allele from each parent). Anaphase I of meiosis demonstrates this law.

Telophase I

Nuclear membrane forms around each new nucleus.

Cells now haploid w/ two sister chromatids joined by a centromere (only has one of the homologous pairs, with modifications by crossing over).

Interkinesis- short period where chromosomes partially uncoil, may happen

Meiosis II

Sister chromatids separate.

Prophase II

Nuclear envelope dissolves,

nucleoli disappear

centrioles migrate to opposite poles

spindle app begins to form.

Metaphase II

Chromosomes line at metaphase plate.

Anaphase II

Centromeres divide,

sister chromatids pulled to opposite poles.

Telophase II

Nuclear membrane forms around each new nucleus. Cytokinesis follows.

Sex

Determined by the 23rd pair of chromosomes.

XX = female, XY = male.

Sperm can = X or Y

ova X chromosome only

X Chromosome

sizable amount of genetic info

can cause sex-linked disorders if mutated.

Hemizygous

Only one copy of the X chromosome RE males. Males offspring don't have a backup X, only one from mom, so if mom is carrying disorder on one of her X, so is screwed.

Why is being hemizygous as a male so dangerous when it comes to X-linked disorders?

Males offspring don't have a backup X, only one from mom, so if mom is carrying disorder on one of her X, son is screwed 50% of the time even if it is recessive. Daughters can at least inherit an X from the father who doesn't have the disorder and never express the disorder if it is recessive.

Carriers

females with a disease-causing X allele that do not exhibit the disease.

Y Chromosome

little genetic information.

has SRY (sex-determining region Y),

Testes

Primitive gonads in males.

functional components:

seminiferous tubules & interstitial cells (of Leydig).

DUCTUS DEFERENS muscle around vas deferens can raise/lower testes to maintain proper temp. for sperm development.

Leydig/Interstitial cells

Produce testosterone (in testes)

Seminiferous Tubules

in testes;

Produce sperm, and nourish them via Sertoli cells. Sperm develops between sertoli cells...

Epididymis

A coiled, tubular structure located on the posterior surface of each testis in the male reproductive system

Ejaculation

Sperm travels up the vas deferens to the ejaculatory duct at the posterior edge of the prostate gland.

2 ejaculatory ducts fuse to form urethra to carry sperm through penis and out of body.

Urethra

Fusion of the ejaculatory ducts, which carries sperm through the penis to exit the body.

Mature Sperm

Head, with genetic material, midpiece (which generates ATP from fructose), and flagellum (motility).

Midpiece

Filled with mitochondria, to generate ATP for flagella.

Head

Covered by acrosome cap derived from Golgi apparatus and is necessary to penetrate the ovum.

Ovaries

Female gonads that produce estrogen and progesterone. They are located on the pelvic cavity and consist of thousands of follicles that are sacs which nourish and protect the immature ova.

Ovulation

One egg ovulated a month through the peritoneal sac, lining the abdominal cavity. Egg is then drawn into fallopian tube (oviduct) via cilia in tube propelling egg forward. Egg goes onto uterus.

Fallopian Tube

Lined with cilia to move the egg forward.

Uterus

Connected to the fallopian tubes, sight of fetal development.

Cervix

Lower end of the uterus, connects to the vaginal canal, where sperm is deposited during intercourse.

Vulva

External female anatomy.

Primary oocyte

already done DNA replication, still diploid (2n)

Menarche

the first menstrual period

Secondary oocyte

one primary oocyte a month will complete meiosis I, producing a polar body and this

Polar body

a small cell containing little cytoplasm that is produced along with the oocyte and later discarded, all the cytoplasm goes to the secondary oocyte

Zona Pellucida

Surrounds the oocyte and is an acellular mixture of glycoproteins that protect the oocyte and contain the compounds necessary for sperm cell binding.

Corona Radiata

Lies outside of the zona pellucida, layer of cells that adhered to the oocyte during ovulation. Meiosis II is triggered when a sperm penetrates the corona radiata and zona pellucida via acrosomal enzymes.

Fertilization

Sperm cell penetrates the layers with acrosomal enzymes, triggering meiosis II. This indicates another meiotic division creating the mature ovum and another polar body.

Mature Ovum

Consists of all things needed to help the zygote.

Pronuclei

Haploid aspect of sperm. Joins ovum to make the zygote.

seminiferous tubules

site of sperm production

interstitial cells of the Leydig

secrete testosterone and other male sex hormones (androgens)

Sertoli cells

nourish sperm in the seminiferous tubules

Ejaculation pathway

epididymis, vas deferens, 2 ejaculatory ducts fuse, urethra, penis

Seminal fluid

substance in which sperm are suspended that is produced by three glands in the abdominal cavity. Contribute fructose to nourish sperm. Seminal vesicles used

Seminal vesicles and prostate gland give the fluid mild alkalinity to survive the relative acidity of the female reproductive tract. Seminal vesicles also contribute fructose to nourish sperm. Bulbourethral cleans out any remnants of urine and lubricates the urethra.

What does seminal vesicle, prostate gland, and bulbourethral gland do for sperm?

Diploid stem cells are spermatogonia, primary spermatocytes after replicating genetic material, meiosis 1 is secondary spermatocytes, meiosis 2 is spermatids, maturation is spermatozoa

stages of spermatogenesis

midpiece

Filled with mitochondria, to generate ATP for flagella.

acrosome

A vesicle at the tip of a sperm cell that helps the sperm penetrate the egg

Hypothalamus

This restricts the production of gonadotropin-releasing hormone (GnRH) prior to puberty.

It also releases pulses of GnRH when puberty begins, which triggers release FSH and LH

Anterior pituitary gland

What structure stimulates the production and release of FSH and LH?

FSH stimulates Sertoli cells and triggers sperm maturation while LH causes interstitial cells to produce more testosterone.

What do FSH and LH do in males?

Estrogens are secreted as a result of FSH and develop and maintain reproductive system and develops secondary sex characteristics> also thickens endometrium each month.

Progesterone is secreted as a result of LH by the corpus luteum (remains of ovarian follicle following ovulation. Involved in development and maintenance of endometrium.

What does FSH and LH do in females?

Follicular phase

begins when the menstrual flow begins (shedding of previous lining). APP increases concentrations of estrogen and progesterone. Starts to produce follicles, which has negative feedback and causes GnRH, LH, and FSH concentrations to level off. Estrogen stimulates regrowth of endometrial lining, stimulating vascularization and glandularization of the decidua (mucus lining uterus that is shed off).

Ovulation

Estrogen levels raise higher and higher which causes positive feedback and GnRH, LH, and FSH levels spike. Surge in LH induces ovulation (release of ovum from ovary into abdominal cavity)

Luteal phase

After ovulation, LH causes ruptured follicle to form corpus luteum, which secretes progesterone. Progesterone maintains uterine lining. Progesterone levels rise causing negative feedback on GnRH, FSH, and LH, preventing ovulation of multiple eggs.

Menstruation

If implantation does not occur, corpus luteum loses stimulation from LH, progesterone levels decline, and uterine lining is sloughed off. The loss of high levels of estrogen and progesterone removes block on GnRH so that next cycle can begin.

Pregnancy

Zygote develops into blastocyst which implants in uterine lining and secretes human chorionic gonadotropin (hCG), an analog of LH. Maintains the corpus luteum. hCG critical during first trimester because the estrogen and progesterone secreted by corpus luteum keep uterine lining in place. By the second trimester, hCG levels decline bc corpus luteum can produce enough estrogen and progesterone itself. High levels of estrogen and progesterone continue to serve as negative feedback on GnRH secretion.

Menopause

Ovaries become less sensitive to FSH and LH resulting in ovarian atrophy. Endometrium also atrophies, and menstruation stops. Because negative feedback is removed on FSH and LH, blood levels of the two hormones rise.