BIOL 216 ch1 reproduction

reproduction

• biological process by which new individual organisms (offspring) are produced from parents

• fundamental to all known life

• each individual organisms exists from reproduction

sexual reproduction

• reproduction involving two individuals or gametes from each opposite sex

• introduces variability to offspring

gametogenesis

differentiation of primordial germ cells into gametes

primordial germ cells

early embryonic cells which acquire developmental potential to develop into gametes

oogenesis

process of egg development

spermatogenesis

process of sperm development

egg cell characteristics

• large, non-motile

• abundant stored raw materials and machinery

• yolk

• totipotent

• haploid genome

• egg coat

• cortical granules

eggs are ready for cell division

eggs contain abundant stored raw materials and machinery (proteins, RNAs, nucleic acids, ribosomes, mitochondria)

yolk

provides energy supply

totipotent

developmental potential to develop into any cell type of the organism

components of egg coat

• shell
• jelly coat

cortical granules

specialized golgi structures that protect against polyspermy

egg cell functions

• contributes one half of diploid genome to organism

• provides mechanisms for sperm recognition, binding, fusion, prevention of polyspermy

• provides raw materials and machinery to carry out early development without zygotic genomic expression

sperm cell characteristics

• haploid genome
• small, motile
• few organelles
• acrosomal vesicle

acrosomal vesicle

specialized golgi structure used by sperm to bind and penetrate the egg coat

sperm cell functions

• reach and fuse with egg
• contribute one half of diploid genome to organism
• activate egg

egg activation by sperm

launches processes to begin the first cleavage after fertilization

genomic event

any process that affects an organism's development and determines early cell fate

genomic event in C elegans

• P elegans: complexes of RNA and proteins initially evenly distributed throughout egg
• move to posterior end of zygote before the first cleavage
• final localization in the gonad of adult germ cells

genomic event in mice

• specification of primordial germ cells in the early embryo through cell signaling
• initiated by high levels of BMP to activate expression of transcription factors

precursor germ cells

primordial germ cells that have to migrate to reach gonads

PGC migration to gonads

• repeated division through the gut
• spermatogenesis in testes or oogenesis in ovary results in mature germ cells

PGC migration in mice

• PGC induction in proximal epiblast by signals from extra-embryonic ectoderm
• move to posterior during gastrulation to become specified
• migrate to gonads once specified

genes involved in sex determination in mice

Sry
Sox9
Fgf9
Rspo1
Wnt4

male pathway of mice

• Sry gene present
• activate Sox9 (high)
• activate Fgf9 (high)
• no Rspo1 activation (off)
• no Wnt4 activation (off)

female pathway of mice

• no Sry gene present
• Rspo1 activated (high)
• Wnt4 activated (high)
• Sox9 inhibited (off)
• Fgf9 inhibited (off)

Sry gene

• sex determining region Y
• gene located on Y chromosome initiating development of testes and male characteristics

Wolffian duct

develops into male internal genetalia

Mullerian duct

develops into female internal genetalia

male germ cells

germ cells that commit first from bipotential cells

mitosis purpose in gametogenesis

expansion of primordial germ cell population

meiosis purpose in gametogenesis

development of diploid germ cells into haploid eggs or sperm to generate genetic variability

asexual reproduction

reproduction by which offspring arise from a single parent (common in archaea, bacteria, protists)

mitosis chromosome number

46 total (23 each from mother/father gamete)

interphase

phase of cell cycle where cell grows and DNA replicates

mitotic phase

phase of cell cycle where replicated DNA and cytoplasmic contents are separated when cell divides

G1 phase

• cell accumulates building blocks of chromosomal DNA and associated proteins
• builds up energy reserves to complete chromosomal replication in nucleus

S phase

• DNA replication forms two sister chromatids attached at centromere (one chromosome)
• centrosome duplicates to give rise to mitotic spindle

G2 phase

• cell replenishes energy stores
• synthesizes proteins for chromosome manipulation
• duplicates some organelles and breaks down cytoskeleton to rebuild mitotic spindle

prophase

• chromosomes condense and become visible
• spindle fibers emerge from centrosomes
• nuclear envelope breaks down into small vesicles
• centrosomes move toward opposite poles

prometaphase

• chromosomes continue to condense
• each sister chromatid attaches to spindle microtubules at the centromere via kinetochore protein
• mitotic spindle microtubules attach to kinetochores

metaphase

• all chromosomes line up at center plate
• each sister chromatid is attached to spindle fibers from opposite poles
• maximum condensation of chromosomes

anaphase

• centromeres split in half
• sister chromatids (individual chromosomes) pulled toward opposite poles
• cell becomes visibly elongated as microtubules not attached at kinetochores slide against each other

telophase

• chromosomes arrive at opposite poles and begin to decondense
• nuclear envelope forms around each set of chromosomes
• mitotic spindle breaks down to reassemble cytoskeleton
• spindle fibers continue to push poles apart

cytokinesis

physical separation of cytoplasmic components into two daughter cells

cytokinesis in animal cells

cleavage furrow forms from contraction of actin filaments just below plasma membrane

cytokinesis in plant cells

buildup and expansion of a cell plate forms a new plasma membrane and cell wall of cellulose

G0 phase

cells not actively preparing to divide

meiosis I

• reduction phase of meiosis
• recombination of genetic information to create unique chromosomes

meiosis II

• separation of sister chromatids in a similar way as mitosis

prophase I

• nuclear envelope breaks down
• proteins holding homologous chromosomes together form synapsis
• chromosomes continue to condense but homologous chromosomes remain attached at chiasmata

synapsis

• tight pairing of homologous chromosomes in prophase I
• genes on chromatids of homologous chromosomes are perfectly aligned with each other
• crossing over

chiasmata

X-shaped shape where homologous chromosomes segments are physically connected after crossing over

tetrad

visible structure of 4 sister chromosomes of each pair of homologous chromosomes

prometaphase I

• attachment of spindle fiber microtubules to fused kinetochore between sister chromatids
• microtubules grow toward middle of cell
• each homologous chromosome is attached to one pole and its pair is attached to the other pole
• complete breakdown of nuclear envelope

metaphase I

• homologous chromosomes arranged in the center of the cell with kinetochores facing opposite poles
• random orientation of each pair (each pair has an equal chance of facing either pole)

independent assortment

• random alignment of homologous chromosomes at metaphase plate
• results from the equal chance that a microtubule fiber will encounter a maternal or paternally inherited chromosome

total possible tetrad arrangements

2^n, where n is the number of chromosomes per set (23 in humans)

anaphase I

• spindle fibers pull chromosomes apart
• sister chromatids remain attached at centromere (1 chromosome)
• chiasma connections between homologous chromosomes are broken

telophase I

• separated chromosomes arrive at opposite poles
• chromosome decondensation and formation of nuclear envelopes may or may not occur depending on organism

cytokinesis (meiosis I)

• separation of cell contents by cleavage furrow or cell plate
• each daughter cell contains only one full set of chromosomes with duplicates of sister chromatids
• results in haploid cells

interkinesis

brief interphase between meiosis I and II in some species that lacks an S phase

prophase II

• decondensation of chromosomes (if condensed in telophase I)
• fragmentation of nuclear envelope (if formed in telophase I)
• duplicated centrosomes from interkinesis separate to opposite poles and form new spindles

prometaphase II

• nuclear envelopes fully broken down
• new spindles are fully formed
• each sister chromatid forms an individual kinetochore
• microtubules attach to individual kinetochores of sister chromatids
• no chiasmata between sister chromatids

metaphase II

individual sister chromatids are maximally condensed and aligned at the center of cell

anaphase II

individual sister chromatids are pulled apart by spindle fibers and move toward opposite poles

telophase II

• chromosomes arrive to opposite poles and begin decondensing
• nuclear envelopes form around individual sister chromatids (chromosomes)

cytokinesis (meiosis II)

• separation of 2 daughter cellsinto 4 genetically unique haploid cells

• all nuclei are haploid and have only one copy of a single set of chromosomes

DNA replication in mitosis vs. meiosis

mitosis: occurs during interphase before nuclear division begins

meiosis: occurs once during interphase before meiosis I begins

number of division in mitosis vs. meiosis

mitosis: 1
meiosis: 2

synapsis of homologous chromosomes in mitosis vs. meiosis

mitosis: does not occur

meiosis: joining of homologous chromosomes during prophase I forms tetrads which cross over and recombine

number of daughter cells and genetic composition in mitosis vs. meiosis

mitosis: 2, 2n, genetically identical to parent cell and each other

meiosis: 4, n, genetically nonidentical to parent cell and each other

role in animal body of mitosis vs. meiosis

mitosis: enables multicellular adult to arise from zygote, produces cells for growth and tissue repair

meiosis: produces gametes, reduces chromosome number in half and introduces genetic variability among gametes

2 ways of generating variation in meiosis

1. independent assortment (segregation of chromosomes)
   metaphase I

2. crossing over (homologous recombination)
   prophase I

chromosome nomenclature

chromosome - refers to both a pair of sister chromatids joined at centromere and one sister chromatid with a centromere
tetrad - refers to the group of 4 sister chromatids joined at the centromere after replication and before meiosis I
chromatid - refers to a single copy of a chromosome

oogonia

produced from primary germ cells in a delayed progression

primary oocytes

• more mature cells produced from mitosis of oogonia

• present in females at birth and reside in follicles

production of primary oocytes

begins with meiosis at birth and arrests at prophase I

follicle

fluid-filled cavity in ovary containing primary oocytes that is released during ovulation

follicle stimulating hormone in oogenesis

• reproductive hormone secreted during puberty that stimulates follicles to grow and mature

• aligned with menstrual cycle

secondary oocyte

larger cell resulting from the uneven division of meiosis I

metaphase II of oogenesis

stage of oogenesis that secondary oocyte development pauses at

ovulation

• the process of breaking open a mature follicle to release the secondary oocyte

• induced by the spike in LH levels

sperm penetrates the secondary oocyte

how meiosis II of oogenesis resumes

cytokinesis of meiosis II products

produces:

• second polar body

• mature egg fused with sperm cell

fertilization

fusion of haploid sperm nucleus with haploid egg nucleus

corpus luteum

• temporary collection of cells that forms from the ruptured follicle to thicken uterus lining
• produces estrogen and progesterone to thicken uterine lining
• degenerates if fertilization does not occur

progesterone

• hormone secreted from corpus luteum that thickens uterine lining to faciliate implantation of fertilized embryo
• regulates menstrual cycle

polar body

• small haploid cell formed from uneven division during meiosis in oogenesis
• generally cannot be fertilized and degenerates without enough cytoplasm

GnRH, LH, FSH

hormones produced by the hypothalamus and pituitary gland that trigger puberty in humans

stimulates hypothalamus and GnRH release

effects of high levels of estradiol on endocrine system

inhibits anterior pituitary gland and FSH/LH release

effects of low levels of estradiol on endocrine system

estrogen

• female reproductive hormone released from developing follicles
• assists in thickening of uterine lining, ovulation, calcium absorption, and development of female reproductive organs

FSH in females

hormone that stimulates development of egg cells in follicles

inhibin in females

hormone produced by follicle cells which inhibit FSH production

LH in females

hormone that influences development of egg cell, induction of ovulation, stimulation of estradiol

estradiol

• most potent and prevalent form of estrogen
• produces secondary sex characteristics in females
• regulates menstrual cycle

ovarian cycle

regulates endocrine tissues and release of eggs

menstrual cycle

regulates preparation and maintenance of uterine lining

slowly rising levels of FSH and LH

hormones that cause growth of follicles on ovary surface to prepare egg for ovulation