MCAT Biology - Reproduction

p 143

diploid (2n)

two copies of each chromosome; autosomal cells

haploid (n)

one copy of each chromosome; viruses, gametes

cell cycle

a series of phases during which a (eukaryotic) cell grows, synthesises DNA, and divides

four stages: G₁, S, G₂, M

cancer

can be caused by derangemnts of the cell cycle

interphase

first three stages of cell cycle collectively; longest part of cell cycle - 90% of time spent; DNA in chromatin - not visible by light microscopy

G₀ phase

phase for cells that don’t divide; offshoot of G₁; simply living and carrying out functions without preparation for division

chromatin

less condensed form of DNA available for gene transcription

G₁ phase

cells create organelles for energy and protein production; increases in size

restriction point

cell cycle checkpoint in the G₁ phase of the animal cell cycle at which the cell becomes "committed" to the cell cycle; certain criteria (contain proper complement of DNA, etc) must be met to pass

S phase

cell replicates it’s genetic material for daughter cells; each chromosome has two chromatids; but still keeps ploidy

chromatid

a DNA molecule that is one half of a replicated chromosome

96 chromatids but still 46 chromosomes

centromere

the region where chromatids are bound together

G2 phase

cell checks for DNA replication. organelles and cytoplasm

M phase

mitosis & cytokinesis

p53

main protein in charge of restriction point; a regulatory protein that prevents cancer formation by binding to DNA and regulating gene expression

cyclins

proteins that control the progression of a cell through the cell cycle by activating cyclin-dependent kinases (CDK); concentrations of the various cyclins increase and decrease during specific stages; cyclins bind to CDKs, creating an activated CDK–cyclin complex

cyclin-dependent kinases (CDK)

predominant group of protein kinases involved in the regulation of the cell cycle and its progression; require the presence of certain cyclins; cyclins bind to CDKs, creating an activated CDK–cyclin complex, which phosphorylate transcription factors

Transcription factors

a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence

cancer

a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body; when cell cycle control goes amiss and/or damaged cells are allowed to undergo mitosis

TP53

mutation of the gene that produces p53; fails to appropriately stop cell cycle to repair damaged DNA; allows mutations to accumulate and cancer cells to divide continuously

tumor

a mass of abnormal and excessive growth of tissue; originates from neoplasm; disease that causes tumors is called cancer

metastasis

local invasion/distant spread of cancerous cells through bloodstream/lymphatic systems; possible if cancer cells can produce proteases that digest basement membranes or factors that prompt blood vessel growth

oncogenes

genes that actively promote cell division when mutated

tumor suppressor genes

genes that lose their ability to regulate or arrest the cell cycle when mutated

Mitosis

process by which two identical daughter cells are created from a single cell; somatic cells

prophase

first phase in mitosis; condensation of chromatin into chromosomes; centriole pairs separate and move toward opposite poles

centrosome

location of centrioles, outside the nucleus

spindle fibers

formed by centrioles; made of microtubules

centrioles

paired cylindrical organelles; responsible for correct division of DNA

microtubule organizing centers

centrosome and basal body of flagellum/cilium

aster

a cellular structure shaped like a star, consisting of a centrosome and its associated microtubules during the early stages of mitosis

Kinetochores

protein structures on the centromeres that serve as attachment points for the (kinetochores) fibers of the spindle apparatus

spindle apparatus

the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells

metaphase

centriole pairs at opposite ends; kinetochore fibers align the chromosomes at the equator

metaphase/equatorial plate

plane which chromosomes line up, equidistant from the poles of the cell

anaphase

centromeres and sister chromatids split so each chromatid has its own centromere; pulled toward the opposite poles of the cell by shortening kinetochore fibers

telophase

reverse of prophase; spindle apparatus disappears; nuclear membrane and nucleoli reform around each set of identical chromosomes; chromosomes uncoil

cytokinesis

end of telophase; separation of cytoplasm and organelles

Number of divisions in life

human somatic cells: 20-50

somatic cells

cells uninvolved in sexual reproduction

gametocytes (germ cells)

precursors to gametes, undergoes meiosis

gametes

sex cells; products of meiosis

Meiosis I

results in homologous chromosomes being separated, generating haploid daughter cells

Meiosis II

similar to mitosis; without additional duplication

reductional division

divison reducing ploidy

equational division

division maintaining ploidy

homologues/homologous pairs

a pair of chromosomes that have the same genes in the same loci, one inherited from each parent

prophase I

the chromatin condenses into chromosomes, the spindle apparatus forms, and the nucleoli and nuclear membrane disappear, synapsis

synapsis

homologous chromosomes come together and intertwine

tetrad

two sister chromatids/four chromatids

synaptonemal complex

group of proteins that hold together homologous chromosomes

chiasma (pl. chiasmata)

point of contact of homologous chromosomes where they cross over

crossing over/crossovers

homologs interact at chiasmata and exchange equivalent pieces of DNA; characterized by the number of crossover events that occur in one strand of DNA, ex. single crossovers, double crossovers, chromatids involved are left with an altered but structurally complete set of genes

genetic recombination

exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent

Linkage

the tendency for genes to be inherited together; genes that are located farther from each other physically are less likely to be inherited together, and more likely to undergo crossing over relative to each other

Mendel’s second law

independent assortment; the inheritance of one allele has no effect on the likelihood of inheriting certain alleles for other genes

metaphase I

homologous pairs (tetrads) align at the metaphase plate, and each pair attaches to a separate spindle fiber by its kinetochore

anaphase I

homologous pairs separate and are pulled to opposite poles of the cell

disjunction

separation of chromatids/homologs

Mendel’s first law

segregation; the distribution of homologous chromosomes to the two intermediate daughter cells is random with respect to parental origin

telophase I

a nuclear membrane forms around each new nucleus; cells are now haploid; once homologous chromosomes separate, only n chromosomes are found in each daughter cell; cell divides into two daughter cells by cytokinesis

interkinesis

a short rest period between cell divisions during which the chromosomes partially uncoil

Nondisjunction

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

autosomal nondisjunction: trisomy 21, resulting in Down syndrome

sex chromosome nondisjunction: Klinefelter and Turner syndromes

Meiosis II

very similar to mitosis in that sister chromatids are separated from each other.

prophase II

the nuclear envelope dissolves, nucleoli disappear, the centrioles migrate to opposite poles, and the spindle apparatus begins to form

metaphase II

the chromosomes line up on the metaphase plate

anaphase II

the centromeres divide, separating the chromosomes into sister chromatids, pulled to opposite poles by spindle fibers

telophase II

a nuclear membrane forms around each new nucleus; Cytokinesis follows, and two daughter cells are formed

Chromosomal sex

determined by the 23rd chromosomes, with XX being female and XY being male

X chromosome

carried by ovum and sperm; carries a lot of genetic information

sex-linked (X-linked) disorders

caused by mutations in X gene; mostly recessive

hemizygous

only having one copy of a gene,

i.e, males and X chromosome

carrier

carrying a diseased allele without exhibiting symptoms

i.e. females carrying one copy of sex-linked disorder

Y chromosome

can only be carried by sperm; contains very little genetic information, SRY gene → production of androgens → male sexual differentiation

SRY (sex-determining region Y)

codes for a transcription factor that initiates testis differentiation and, thus, the formation of male gonads

presence → male

abscence → female

testes

male gonads

contain the seminiferous tubules and interstitial cells of Leydig

seminiferous tubules

produce sperm, highly coiled

sperm

male gametes

Sertoli cells

nourishes sperm

interstitial cells of Leydig

secrete testosterone/androgens

testosterone

a specific androgen; increases dramtically during puberty; remains high to adulthood, then decreases

negative feedback on hypothalamus and anterior pituitary

androgen

male sex hormone

scrotum

external pouch that holds testes and hangs below the penis; allows it to remain 2-4°C lower than body

ductus deferens

muscle around vas deferens that can raise and lower the testis to maintain the proper temperature for sperm development

epididymis

after sperm are formed, move here; flagella gain motility; stored until ejaculation

ejaculation

release of sperm from penis

vas deferens

sperm travel through this after the epididymis

ejaculatory ducts

meet at the posterior edge of the prostate gland; fuse to form the urethra

urethra

how urine/sperm leaves the body

In males, the reproductive and urinary systems share a common pathway; this is not the case in females.

prostate

organ that produces some fluid for semen; mildly alkaline to combat the relatively acidic environment of the female reproductive tract; surrounds the urethra, under the bladder

enlarges with age - benign prostatic hyperplasia: higher urinary frequency, urgency, nighttime awakenings

seminal fluid

mixes with sperm; produced by seminal vesicles, prostate gland, bulbourethral gland

seminal vesicles

contribute fluid and fructose to nourish sperm; mildly alkaline to combat the relatively acidic environment of the female reproductive tract

bulbourethral (Cowper’s) gland

produces clear viscous fluid that clears out urine and lubricates the urethra during sexual arousal

semen

sperm and seminal fluid

spermatogenesis

occurs in the seminiferous tubules

Spermatogonium (pl. -ia)

undifferentiated diploid male sex stem cells

primary spermatocytes

male stem sex cells after S stage; diploid

primary spermatocytes

male stem sex cells after meiosis I; haploid

spermatid

male stem sex cells after meiosis II; haploid

spermatozoon (pl. -zoa)

mature male sex cells, aka sperm