67 terms

Kaplan MCAT Biology 04

Kaplan MCAT Biology 04
- a process by which an organism perpetuates itself and its species
- divided into: cell division, asexual reproduction and sexual reproduction
Cell Division
- a process where a cell doubles its organelles and cytoplasm, replicates its DNA and then divides in two
- unicellular organism: reproduction
- multicellular organism: growth, development & replace old cells
Binary Fission
- prokaryotes' way of of cell division
- a type of asexual reproduction
- splits into two equal halves, each daughter cell receives a complete copy of the original chromosome
Somatic Cells
- or called autosomal cells
- everything but the gamete
- contain diploid number of chromosomes characteristic of its species (2N)
- N is the number of chromosomes found in haploid cell (gamete)
- in human, 2N = 46 and N = 23
The Cell Cycle
- four stages: G₁, S, G₂ and M
- interphase: first three stages
- mitosis includes the actual cell division
- the longest part of the cell cycle
- a cell normally spends at least 90% of the cycle in interphase
G₁ Stage (presynthetic gap)
- intense biochemical activity and growth
- cell doubles in size and new organelles are produced
- passing "restriction point", where a cell is committed to continue through the rest of the cell cycle and divide
- some cells like skeletal muscle cells and nerve cells never pass this point, and enter a nondividng phase sometimes referred to as G₀
- 2N number of chromosomes
S Stage (synthesis)
- each chromosome is replicated so that during division, a complete copy can be distributed to each daughter cell
- two identical sister chromatids held together at a region called the "centromere"
- telomeres: ends of the chromosomes
- 2 x 2N number of chromosomes
G₂ Stage (postsynthetic gap)
- cell continues to grow in size
- assembly of new organelles and other cell structures continues
M Stage (Mitosis)
- mitosis: division and distribution of the cell's DNA to its two daughter cells such that each cell receives a complete copy of the original genome
- cytokinesis: division of cytoplasm that follows
- happens on somatic cells only
- 2N --> 2N (ends up with two diploid cells)
- granular DNA during interphase
Chromosome Movement
- dependent on these cytoplasmic organelles
- centrioles: found in pairs, cylindrical organelles
- centrosome: an area outside the interphase nucleus
- spindle fibers: composed of microtubules, appears near each pair of centrioles, radiate outward.
- asters: spidle fibers in radiating structure
- spindle apparatus: asters extending toward the center of the nucleus, shortens to move chromosomes toward
opposite poles of the cell during the later stages of mitosis
Stages of Mitosis
a) Prophase
b) Metaphase
c) Anaphase
d) Telophase
- chromosomes condense
- centriole pairs separate and move toward opposite poles of cell
- spindle apparatus form
- nuclear membrane dissolves
- kinetochores, attached kinetochore fibers, appear at the chromosome centromere
- chromosomes align
- sister chromatids separate
- telomeres are the last part of the chromatids to separate
- spindle apparatus disappears
- new nuclear membrane forms around each set of chromosomes
- nucleoli reappears
- chromosomes uncoil
- cytokinesis occurs
- happens at the end of telophase
- cytoplasm divides into two daughter cells
- in animal cells, a cleavage furrow forms, cell membrane indents along the equator of the cell and cell split
Asexual Reproduction
- essentially genetic carbon copies of parent cells
- identical to parent cells except for random mutations
- different types: binary fission, budding, regeneration, parthenogenesis
Binary Fission
- a simple form of asexual reproduction in prokaryotes
- circular chromosome replicates
- a new plasma membrane and cell wall grow inward along the midline of the cell, dividing it into two
- each daughter cell contains a duplicate of the parent chromosome
- replication of the nucleus followed by unequal cytokinesis
- the cell membrane pinches inward to form a new cell that is smaller in size but genetically identical to the parent cell
- can grow to an adult size
- new cell may separate immediately from the parent or remain attached to it
- occurs in hydra and yeast
- the regrowth of a lost or injured body part
- replacement of cells occurs by mitosis
- ex: hydra and starfish
- in higher animals, regeneration is usually limited to the healing of tissues
- some internal organs (liver) can regenerate considerably as long as part of the organ remains viable
- development of an unfertilized egg into an adult organism
- occurs naturally in certain lower organisms (bees and ants, some salamander)
- eggs of some organisms can be induced to develop parthenogenetically (not done naturally)
- since the organism develops from a haploid cell, all of its cells will be haploid
Sexual Reproduction
- fusion of two gametes (specialized sex cells produced by each parent)
- meiosis is the process where the sex cells are produced
- mitosis preserves the diploid chromosome number while meiosis halves it
- somatic cells undergo mitosis, gametocytes undergo meiosis
- during fertilization, two haploid gametes fuse, restoring the diploid number
- only occurs in the sex cells
- meiosis I produces two intermediate daughter cells
- meiosis II is similar to mitosis, it separates sister chromatids and results in four genetically distinct haploid gametes
Prophase I
- homologous chromosomes come together and intertwine
- at this stage, chromosome consists of two sister chromatids
- where recombination and crossing over happens, responsible for increased genetic diversity
Homologous Chromosomes
chromosomes that code for the same trait, one inherited from each parent
homologous chromosomes come together and intertwine
chromosome consisting of two sister chromatids
where chromosomes are joined
Metaphase I
- homolgous pairs (tetrads) align at the equatorial plane
- each pair attaches to a separate spindle fiber by its kinetochore
Anaphase I
- disjunction: homologous pairs separate and are pulled to opposite poles of the cell
- distribution of homologous chromosomes to intermediate daughter cells are random
Telophase I
- a nuclear membrane forms around each new nucleus
- each chromosomes still consists of sister chromatids joined at the centromere
- cells divide into two daughter cells
- between cell divisions, there might be "interkinesis", a short rest period where chromosomes partically uncoil
Meiosis II
- similar to mitosis
- not preceded by chromosomal replication
Prophase II
- centrioles migrate to opposite poles
- spindle apparatus forms
Metaphase II
- chromosomes line up along the equatorial plane
- centromeres divide
- chromosomes separate into pairs of sister chromatids
Anaphase II
- sister chromatids are pulled to opposite poles by the spindle fibers
Telophase II
- nuclear membrane forms around each new haploid nucleus
- two daughter cells are formed after cytokinesis
- by completion of meiosis II, four haploid daughter cells are produced per gametocyte
- sperm and egg
- produced in gonads
- male: testes
- female: ovaries
Male Reproductive Anatomy
- testes contain two functional components: seminiferous tubules and interstitial cells (cells of Leydig)
- sperm are produced in seminiferous tubules, nourished by Sertoli cells
- interstitial cells secrete testosterone and anderogens
- testes have to be 2-4°C lower than body temperature
Pathway of Sperm
Seminiferous tubules
Vas deferens
Ejaculatory duct
where sperm acquire motility and stored here till ejaculation
Seminal Fluid
- mixed with sperm, aids in sperm transport by lubricating passageways
- semen: sperm + seminal fluid
- produced by:
a) seminal vesicles: fructose-rich fluid, serves as energy source
b) prostate gland: alkaline milky fluid to protect from acidic environment in female's reproductive tract
c) bulbourethral glands: small amount of viscous fluid, function unknown
- "sperm production"
- occurs in seminiferous tubules
- after a male reaches sexual maturity, about 3 million primary spermatocytes begin spermatogenesis per day
- maturation takes about 65-75 days
Spermatogenesis Process
- spermatogonia (diploid) differentiate into primary spermatocytes (diploid)
- primary spermatocytes undergo the first meiotic division and yield two secondary spermatocytes (haploid)
- secondary spermatocytes undergo second meiotic division and yield four spermatids (haploid)
- spermatogonia (2N) --> 1° spermatocytes (2N) --> meiosis I --> 2° spermatocytes (N) --> spermatids (N) --> spermatozoa (N)
- spermatids after a series of changes, becomes a mature sperm
- mature sperm is an elongated cell with a head, neck, body and tail
- head consists almost entirely of the nucleus
- tail (flagellum) propels the sperm
- mitochondria in the neck and body provide energy for locomotion
- cap-like structure, over the anterior half of mature sperm's head
- derived from Golgi apparatus
- contains enzymes to penetrate the tough outer covering of the ovum
- once in contact with ovum cell membrane, sperm forms a tubelike structure called the acrosomal process
- acrosomal process extends and fuses with ovum and enters the ovum's cytoplasm
Female Reproductive Anatomy
- once a month, an immature ovum is released from the ovary into the abdominal cavity and drawn into the nearby fallopian tube
- inner surface of the fallopian tube is lined with cilia that move the ovum into and along the tube and toward the uterus
- mammalian female's reproductive and excretory systems are distinct from one another (urethra and vagina aren't connected)
- female gonads
- found in the abdominal cavity
- produce eggs (ova)
- secrete esterogen and progesterone
- consist of thousands of follicles
- multilayered sac of cells that contains, nourishes and protects and immature ovum
- follicle cells produce estrogen
- site of fetal development
- lower, narrow end of the uterus
- connects with the vaginal canal
Vaginal Canal
- site of sperm deposition during intercourse
- passageway through which a baby is expelled during childbirth
- external female genitalia
- production of female gametes
- occurs in the ovarian follicles
- one primary oocyte completes meiosis I per month after menarche
- yields two secondary oocyte and a polar body
- secondary oocyte is expelled from the follicle during ovulation
- meiosis II doesn't occur till fertilization
Primary Oocyte
- immature ova
- diploid cells that form by mitosis in the ovary
- at birth, all of the primary oocyte that a female will produce during her lifetime are already in her ovaries
- the first time a female gets her period
Meiosis II
- doesn't occur until fertilization
- triggered when zona pellucida and corona radiata are penetrated by a sperm cell
- fertilization yields two haploid cells, a mature ovum and another polar body
- the mature ovum is a large cell containing a lot of cytoplasm, RNA, organelles and nutrients needed by a developing embryo
Zona Pellucida
- inner layer of the oocyte cell membrane
Corona Radiata
- outer layer of the oocyte cell membrane
Ovulation and Menopause
- women ovulate about once every four weeks
- menapause occurs between age 45 and 50
- during menopause, ovaries become less sensitive to the hormones that stimulate follicle development (FSH and LH), and eventually they atrophy
- remaining follicles disappear, estrogen and progesterone levels greatly decline and ovulation stops
- happened 12-24 hours after ovulation
- occurs in the lateral, widest portion of the fallopian tube
- sperm travels through vaginal canal, cervix, uterus, and into the fallopian tubes to reach the ovum
- sperm penetrates corona radiata, then zona pellucida, contact with ovum cell membrane, become acrosomal process, fuse sperm with ovum
- sperm nucleus enters ovum's cytoplasm and ovum completes meiosis II
Cortical Reaction
- triggered by acrosomal reaction
- calcium ions released into the cytoplasm, initates a series of reaction to form the fertilization membrane
- calcium ion also stimulates increase in ovum's metabolic rate
- this is followed by fusion of the sperm nucleus with ovum nucleus to form a diploid zygote
Fertilization Membrane
- hard layer that surrounds the ovum cell membrane
- prevents multiple fertilizations
Monozygotic (identical) Twins
- result when a single zygote splits into two embryos
- if splitting occurs at the two-cell stage of development, embryos will have separate chorions and separate placentas
- if it occurs at blastula stage, embryos only have one chorionic sac and share a placenta (and probably amnion)
Dizygotic (fraternal) Twin
- results when two ova are released in one ovarian cycle
- fertilized by two different sperm
- two embryos implant in the uterine wall individually
- each develops its own placenta, amnion and chorion
- share characteristics in the level of siblings