Lecture 5 - Reproduction Flashcards
Define Asexual Reproduction
A single individual is the sole parent and passes copies of all its genes to its offspring
- 1 parent
- clone offspring
Define Sexual Reproduction
Two parents give rise to offspring that have unique combinations of genes inherited from each parent
Advantage of asexual reproduction
Energetically cheaper
- no time/energy wasted on finding a mate (courtship rituals, etc)
- every member of the population can yield offspring
Animals that asexually reproduce
- not mobile or live in sparse populations
- cannot search for or find a mate - in constant environments
- genetic diversity is less important for reproductive success
- few pressures on them, their “plan works”
* rare
Type of Asexual Reproduction
- Budding
- Regeneration
- Parthenogenesis
Budding
Asexual
- small offspring buds from larger parents
- bud grows by mitotic cell division and differentiates before bud breaks away
Regeneration
Asexual
- replacement of damaged or lost limbs, or entire individual
- example starfish cut into pieces will regenerate as many starfish as pieces
Parthenogenesis
Asexual
- development of an unfertilized egg into a new individual
- only practiced by females - species also often practice sexual reproduction
ex: bees: fertilized eggs develop into females, unfertilized into males
Advantages and disadvantage of sexual reproduction
Adv: provides genetic diversity
Disadv: Is energetically more expensive *but 99.9% of eukaryotes do it… so seems to work
General Process of Sexual Reproduction
- Gametogenesis
- making haploid cells or gametes
- meiotic cell division - Spawning/mating
- bringing gametes together - Fertilization
- union of sperm and egg to produce a diploid zygote with two sets of chromosomes, one from each parent
Gametogenesis
Produces: gametes
- eggs - non-motile
- sperm - move by flagella
Occurs: gonads
- female ovaries
- male testes
Process:
- diploid (2 sets of each chromosome) to haploid (only one set of each)
Germ Cells
- precursors to gametes
- diploid
- small collection of cells in early mammalian embryo are induced to become primary germ cells through signals from their neighbors
- develop differently from somatic cells of rest of the body
- proliferate and migrate to developing gonads
- signaled to develop into sperm or eggs depending on whether gonads have started developing into testes or ovaries
* not based on the chromosomes they have themselves
*Single gene on Y chromosome (SRY) important for dictating gonad development into testes
How do germ cells develop into sperm or eggs?
- signaled by the developing gonads, whether male or female
*not based on their sex chromosomes
Spermatogenesis (steps)
- after germ cells have taken up residence in testes
- TIME: begins at puberty
1. Germ Cells (until puberty) - mitosis - differentiate, proliferate
2. Spermatogonia - mitosis - differentiate
- some on track to become sperm, others just divide over and over
3. Primary Spermatocytes (1 diploid)
*meiosis I*
- Spermatids (2 haploid - connected by cytomplasmic bridges)
*meiosis II*
- More spermatids (4 haploids - cytoplasmic bridgeS)
- mature - Sperm (4 haploid)
Spermatogenesis: Where, when, how long?
- seminiferous tubules tightly coiled within each testes (move from the outside in)
- after puberty, continuous
- ~ 65 days
Oogenesis (Part I)
(where, when, how)
During embryonic development
in the ovaries
- Germ cells
(mitosis)
- oogonium
(mitosis)
- oocytes
(meiosis)
- Stop at prophase I
* at this point all primary oocytes are present
Ovarian Cycle
- all primary oocytes are present in embryo at birth
- When Oogenesis resumes
- involveds ovulation and corpus luteum development
Primary Oocyte
- each primary oocyte is surrounded by layer of cells that form a follicle
- begin puberty and menopause 6-12 mature each month
- others remain paused
- oocyte and rest of follicle will grow
- after about 1 week 1 will be larger than the others (out of the 6-12)
- the big one will continue to mature, others degenerate
- just before ovulation primary oocyte will finally undergo meiosis
How manny primary oocytes develop each month?
- 6-12 from puberty to menopause
- only 1 actually matures all the way, others degenerate
Oogenesis (Part II)
- Primary oocyte undergoes mitosis very close to the edge of the cell
- 1 daughter cell gets almost all cytoplasm (secondary oocyte), other gets very little (first polar body) - secondary oocyte divides in meiosis, forms one large OOTID), other is a second pilary body
- ootid develops into ovum
*primary oocyte
* secondary oocyte
* first polar body
*secondary polar body
*ootid
* ovum
Result of Oogensis
- 1 mature haploid egg per primary oocyte
- 1 mature egg per month
* mature means it is through metaphase II
When does the egg complete its cell division?
- does not complete until fertilization occurs
- when expelled from ovary, it is arrested in metaphase of meiosis II
Oogenesis vs spermatogenesis
Oogensis
- initiated once in a finite population of cells
- one gamete produced per meiosis
- completion of meiosis delayed for months or years
- meiosis arrested at first meiotic prophase and reinitiated in a smaller population fo cells
- diff of gamete occurs with diploid, in first meiotic prophase
Spermatogenesis
- meiposis initiated continuously in a mitotically dividing stem cell pop
- four gametes produced per meiosis
- meiosis completed in days or weeks
- meiosis and differentiation proceed continuously without cycle arrest
- diff of gametes occurs while haplois, after meiosis ends
Ovulation
- after 2 weeks of follicular growth, follicle ruptures and releases egg
- follicle cells that remain in the ovary continue to proliferate and form a mass of tissue about the size of a marble
- corpus luteum aka yellow body develops
Corpus Luteum
- important in ovulation
- functions as an endocrine gland producing estrogen and progesterone for about 2 weeks
- degenerates unless pregnancy occurs
Implantatoin
- egg release into body cavity cloe to openings of oviducts (fallopian tubes)
- egg drawn into oviduct
- where fertilization takes place
- first few divisions in oviduct
- moves down oviduct to uterus, attaches to epithelial lining
- implantation in endometrium of uterus
Hormones Leading up to Ovulation
- puberty: hypothalamus increases releas eof gonatrophin releasing hormone (GnRH)
- anterior piruitary secretes 2 tropic hormones
- FSH - follicle stimulating hormone
- LH - leutenizing hormone - FSH and LH trigger small sets of primary oocytes to mature in ovaries
- follicles of oocytes steadily increase estrogen - Estrogen
- initially negative feedback on FSH and LH - keeps in check
- ~ day 12, switches to positive feedback
- surge of LH (and some FSH) occurs
- triggers ovulation: follicole ruptures and releases egg and develops in to CL
Hormones after ovulation
- spike of LH has just cuase ovulation
- LH and FSH levels are high, estrogen low
1. corpus luteum - secretes progesterone (and some estrogen)
- controls growth and maintenance of endometrium
- inhibits GnRH, LH, FSH release (prevents new follicles from maturing)
2. If egg not fertilized - CL disintigrates
- GnRH, FSH, LH all increase - hypothalamus and pituitary relased from negative feedback
- cycle begins again
Increased GnRH leads to…
Increased FSH and LH
Follicle development and increased estrogen leads to..
- decreased FSH and LH
- THEN increased FSH and LH
LH surge leads to…
ovulation
Increased progesterone leads to…
- endometrium dev
- decreased GnRH, FSH, LH
Corpus luteum degenerates..
- decreased progesterone
- increased GnRH, FSH, LH
Female Hormone Cycle
Fertilization results in a…
zygote
Events in fertilization:
- Sperm and egg recognize eachother
- sperm is activated, enabling it to gain access to egg’s plasma membrane
- Plasma membrane of egg fuses with plasma membrane of single sperm
- egg blocks entry of all additional sperm
- egg is metabolically activated and stimulated to start development
- egg and sperm nuclei fuse to create the diploid nucleus of the zygote
Sperm - Egg recognition
- specific molecules mediate interactions
- direct sperm towards egg
- prevent fertilization of eggs by sperm of other species
- eggs release chemical attractant that increase motility of sperm and cause them to swim toward egg
Layers of the Egg
- sperm must successfully penetrate protective layers of egg
Two layers:
- Thick Jelly coat
- Protein Envelope
* callad “Zona Pellucida” in humans
THEN sperm can access plasma membrane of egg
Acrosome
- Membrane enclosed cap at front of sperm head
- contain enzymes for digesting protective layers as well as actin and other proteins to help sperm contact egg plasma
Sperm Activation
- sperm penetrates jelly coat
- acrosome binds to specific receptors in Zona Pellucida
- Binding to Zona Pellucida triggers *acrosomal reactoin*
- acrosomal enzymes are released
- digest a hole through zona pellucida
- create a pathway for sperm to get to the plasma membrane of the ovum
Fusion
Allowed by acrosomal pathway
- sperm reaches ovum, egg and sperm membranes are fused
- sperm nucleus brough into egg cytoplasm
Blocks to Polyspermy
- mechanisms that prevent more than one sperm from entering egg
FAST:
- influx of Na+ cations hanges charge difference across membrane (rapid depolarization)
- transiently prevents other sperm from binding to plasma membrane
SECONDARY:
- sperm entry tino egg stimulates local release of Ca++ from stroes in the ER into the cytoplasm
- this spreads in a wave
- causes *cortical granules* that are stored underneath the plasma membrane to fuse with the plasma membrane and release contents
- protein envelope is converte into a physical barrier that sperm cannot penetrate
Cortical Reaction
*protein envelope is converted to a physical barrier that sperm cannot penetrate
- influx of Ca++ causes relase of cortical granules
1. causes fertilization envelope (zona pellucisa) to harden
2. degrade sperm binding molecules on surface of envelope
(sometimes. ..)
3. protein envelope rises to form a fertilization envelope (NOT in mammals)
Fusion of Sperm and egg nuclei
- two halpoid nuclei must combine chromosomes into one dilpoid nuleus
- in mammals, nuclear membranes do not fuse directly together
- approach each other but remain distinct until membrane of each nucleus has broken down in preparation for sygote’s first mitotic division
