Reproduction

Asexual Reproduction

  • One parent

  • No reproductive organs

  • No genetic variation

  • Mitosis and Cytokinesis

  • Binary Fission, Budding, Spores, Regeneration, Vegetative Propagation

Sexual Reproduction

  • Two parents

  • Sex cells

  • Meiosis

  • Genetic variation 

    • Ensures survival of species

Sexual Reproduction in Lower Organisms

  • Conjugation

    • Conjugation bridge that is sent out to meet a partner

    • Exchange or transfer of genetic material

    • No male/female

    • Bacteria, paramecia, spirogyra

    • They only do this when they are about to die

Sexual Reproduction in Animals

  • Have male and female

  • Have sex organs (Gonads)

    • Ovaries - ova or egges (Haploid)

    • Testes - Sperm (Haploid)

  • Hermaphrodites

    • Ovaries and testes in the same animal

    • In slow or sessile organisms (worms, hydra, snail)

    • Self fertilization rare, instead they reproduce with other hermaphrodites

How are gametes made?

  • Gametogensis

    • Oogenesis

      • From one primary oocyte, only one ovum forms and three polar bodies form to support the ovum

      • There is only a small number of ovums created because the female can only support a limited number of fertilizations during her reproductive lifespan, thereby ensuring that each potential offspring has the necessary resources to develop successfully

    • Spermatogenesis

      • From one primary spermatocytes, four spermatozoa, or sperm are formed

Sperm vs Egg

  • Sperm

    • Small

    • Many

    • Mobile

    • Complex Shape

    • Has digestive enzymes to break down the jelly layers of the egg

    • They will die in 24 hours, so it has to work fast

    • It has its own mitochondria, to create energy to swim

  • Egg

    • Larger

    • Few

    • Nonmobile

    • Round

    • May have yolk

    • Has many jelly layers

Fertilization

  • Sperm swims to egg

  • Sperm releases enzymes to make hole in egg

  • Sperm injects nucleus into egg

  • Fertilization membrane form around egg

  • No future fertilization

  • Joining of sperm nucleus and egg nucleus

  • Form diploid zygote

  • Fertilization can happen outside of the female (External fertilization)

  • Fertilization can happen inside of the female (Internal fertilization)

External Fertilization

  • Female releases eggs in water

  • Male releases sperm in water

  • The sperm has to find the eggs

  • There are many gametes made

  • Spawning behavior or Amplexus

  • There are many dangerous risks to this because the sperm and the egg could get lost, or even get eaten by other animals

  • Seasonal spawning of Salmon is when the male and the female release their eggs into the water in many amounts in order to get a higher chance of reproduction

  • Amplexus is when amphibians like frogs get on top of the female and squeezes her until she releases her eggs, then the male will release his sperm

Internal Fertilization

  • Safer

  • Terrestrial animals and some aquatic

  • Need moist female reproductive tract

  • Fewer eggs needed

  • Short life span of gametes (24 hours)

  • Need specialized organs

  • Time release of gametes (Hormonal)

Adults with no fertilization

  • Parthenogenesis

    • Egg develops without sperm

    • Insects

    • Unfertilized eggs of queen bee = male drone

    • Fertilized eggs of queen bee = female workers or queens

    • The male bees don’t do anything other than reproduce with the queen

    • The female bees do everything

What happens after fertilization?

  • Development

  • Embryology = study of embryo development

Stages of development

  • Cleavage

    • Mitotic cell division

    • 2 cells, 4, 8, …… into a morula = a solid ball of multiple cells

    • No growth in cell size (cells get smaller)

    • Blastula stage = Ball hollows out and has fluid in core (Blastocoel)

  • Gastrulation

    • Growth in cell size

    • Embryo changes shape (cells migrate)

    • Development of 3 primary Germ layers; Ectoderm, Endoderm, and Mesoderm

    • Ectoderm

      • Nervous system, lining of mouth, nostrils and anus

      • Epidermis, hair, nails

    • Mesoderm

      • Bone muscles, blood vessels, repro and excretory system, dermis

    • Endoderm

      • Lining of digestive system, respiratory system, liver, pancreas, glands, bladder

  • Differentiation

    • Embryonic cells become specialized

    • Embryonic Induction = “organizers” influence cells to take on a certain role in life

    • Where a cell is located will determine what it will become

    • Scientists used to believe that differentiated cells could not revert back to stem cells but this is not true

External vs Internal Development

  • External

    • Water

      • Embryo feeds on yolk

      • Wastes and Oxygen exchange via diffusion

      • Little to no care, dangerous

      • Many fish do this

      • Less than 2% of fish are “Mouth-brooders”

        • Fertilized eggs protected in mouth

    • Land

      • Embryo feeds on yolk in a SHELLED egg

      • Pores in shells for gas exchange 

      • Reptiles lay eggs that are LEATHERY SHELLED, and they leave the eggs

      • Birds lay eggs that are HARD SHELLED, and they protect the eggs

  • Internal

    • Some organisms: nutrients mostly from yolk and babies born self-sufficient

    • Others: development in the womb (Uterus)

    • High protection

    • Little yolk, nutrients from mom

    • Born underdeveloped

    • Mammals continue to feed via mammary glands

Extraembryonic membranes of Bird Eggs

  • Yolk sac = Nutrients

  • Amnion = shock absorption

  • Allantois = metabolic waste storage and gas exchange

  • Chorion = under shell, gas exchange

Placental vs Nonplacental Mammals

  • Placental

    • Have placenta = site of nutrient and waste exchange between mom and baby

    • Baby side and mom side with space between

    • No blood-blood contact

  • Nonplacental

    • *Egg-laying mammals (External Development) 

    • Monotremes

      • Spiny anteater and duckbill platypus

      • Eggs with yolk develop outside

      • Babies feed on mammary gland milks

    • Pouched mammals (Marsupials)

    • Some internal development

    • Poorly developed babies born and remain in a pouch feeding on milk

Human Male Reproductive System

  • Testes

    • Semineferous tubules

      • This is where meiosis happens to create sperm

    • Sperm production

    • Testosterone production

    • In scrotum sac

  • Epididymis

    • Sperm storage and maturation site

  • Vans deferens

    • Sperm ducts

  • Urethra

    • Exit route for sperm through penis

  • Glands

    • Seminal vesicles = thick, nutrient-rich fluid

    • Cowper’s Gland (Bulbourethral glands) = pre-ejaculation lubricant

    • Prostate Gland = milky, alkaline fluid (Semen os sperm and fluids)

      • Semen has a bleach smell, which means it is a base with a high pH

      • This is supposed to form a neutralization reaction with the female’s acidic reproductive tract

Human Female Reproductive System

  • Ovaries

    • Matures eggs located in follicle sacs

    • Make estrogen

    • Usually on egg matures/month

    • Mature egg ovulated

    • Egg enters oviducts 

  • Oviduct

    • Site of fertilization

    • Could burst during ectopic pregnancy

  • Uterus

    • Site of embryonic development

    • Nurtures baby for 9 months

  • Cervix

    • Doorway into Uterus

    • Dilates during labor

  • Vagina

    • Birth canal and sperm receiving area

Human Sexuality

  • Fertilization

    • Hundreds of millions of sperm ejaculated into vagina

    • Fertilization in oviduct

      • In vitro fertilization is done in a lab

      • In vivo fertilization is done naturally

    • Fraternal Twins

      • 2 eggs and 2 sperm

    • Identical Twins

      • Same egg and sperm which divide into 2

      • “Identical” Twins may have some genetic differences; they can have different qualities of a gene

Human Extraembryonic Membranes

  • Chorion = surrounds other membranes

    • Chorionic villi

    • Villi+uterine wall = placenta

      • Placenta = nutrient exchange, no blood-blood contact

  • Amnion = surrounds fetus

    • Amniotic Fluid-shock

  • Yolk Sac and Allantois develop into umbilical cord

Birth

  • Gestation Time = Time on Womb

    • 9 months for humans

    • 20 days for mouse

    • 21 months for elephant

  • Labor 

    • Uterine contractions

    • Cervix enlargement

    • Amnion breaks 

    • Baby’s head out first

  • Afterbirth

    • Placenta comes out

Terms to Know

  • Embryo = zygote from fertilization till 8 weeks

  • Fetus = 8 weeks - birth

Menstrual Cycle and Stages

  • Starts at puberty (10 to 14 years old)

  • Stops at menopause (45-50 years old)

  1. Follicle Stage

    1. FSH from pituitary gland matures one follicle

    2. Follicle secretes Estrogen

    3. Estrogen enhances Uterine Lining

    4. High Estrogen levels inhibit the pituitary gland from secreting FSH

    5. Pituitary gland starts to secrete LH

    6. Days 1-13

  2. Ovulation

    1. High levels of LH cause egg to pop out of follicle

    2. Day 14

  3. Corpus Luteum Stage

    1. Ruptured follicle releases Progesterone

    2. Progesterone maintains uterine lining

    3. Progesterone inhibits FSH release

  4. Menstruation - if no fertilization

    1. LH levels drop

    2. Corpus luteum breaks down

    3. Progesterone levels drop

    4. Estrogen levels drop

    5. Uterine lining sheds

    6. FSH no longer inhibited

    7. Starts maturing a new follicle

  • If fertilization does occur, then the body continues releases chemicals to maintain the uterine lining to support a baby.

    • The day-after pill forces menstruation to happen if there is fertilization

Estrous Cycle

  • Only humans and other primates have a menstrual cycle

  • Others have estrous cycle

    • Periodic changes in female sex organs

    • Periodic desire to mate

    • Seasonal fertility (in heat)


Asexual Reproduction

  • Involves a single parent organism that reproduces without the involvement of specialized reproductive organs.

  • This method results in offspring that are genetically identical to the parent, leading to no genetic variation. This can limit adaptability in changing environments.

  • Asexual reproduction primarily uses processes like Mitosis (the division of a cell nucleus) and Cytokinesis (the division of the cytoplasm).

  • Common forms of asexual reproduction include:

    • Binary Fission: Seen in prokaryotes like bacteria, where the organism splits into two equal parts.

    • Budding: A new organism develops from an outgrowth or bud from the parent, common in yeast and hydra.

    • Spores: Both unicellular and multicellular organisms, like fungi and certain plants, release spores that can develop into new individuals.

    • Regeneration: Certain organisms like starfish can regrow lost parts, which can also lead to the formation of new individuals.

    • Vegetative Propagation: Plants can reproduce asexually from vegetative parts; for instance, runners in strawberries or tubers in potatoes.

Sexual Reproduction

  • Involves two parent organisms contributing genetic material, leading to genetic variation among offspring. This process is essential for the evolution and adaptation of species over time.

  • In sexual reproduction, specialized sex cells (gametes) are produced through Meiosis, which reduces the chromosome number by half and allows for the mixing of genetic material.

  • Sexual reproduction enhances the chances of survival of a species due to variability, which can result in better adaptation to environmental changes.

Sexual Reproduction in Lower Organisms

  • Conjugation: A form of sexual reproduction primarily seen in microorganisms like bacteria and protozoa, where genetic material is exchanged via a conjugation bridge. This is crucial for genetic diversity.

  • It is interesting to note that some organisms may only engage in conjugation when under stress or facing unfavorable conditions to increase survival odds.

Sexual Reproduction in Animals

  • Involves male and female sexes, each with specific reproductive organs called gonads:

    • Ovaries: Produce ova (eggs), which are haploid reproductive cells.

    • Testes: Produce sperm, also haploid.

  • Some species are hermaphrodites, possessing both male and female reproductive capabilities. This is common in slow-moving or sessile organisms like worms and snails, which enhance mating opportunities through cross-fertilization.

    • Self-fertilization is rare; they tend to reproduce with other hermaphrodites when possible.

Gamete Formation

  • Gametogenesis: The process of producing gametes is crucial for sexual reproduction and includes:

    • Oogenesis: Involves the development of oocytes into ova. Typically, only one mature ovum is produced from each primary oocyte, with three polar bodies being formed to assist in nourishment.

    • Spermatogenesis: Involves the production of sperm from primary spermatocytes. This process results in four functional sperm cells from each primary spermatocyte.

Sperm vs. Egg

  • Sperm Characteristics:

    • Typically smaller, numerous, and highly mobile due to their flagella.

    • Designed to penetrate the egg's outer layers using enzymes.

    • They contain specialized mitochondria for energy to swim towards the egg within a short lifespan of approximately 24 hours.

  • Egg Characteristics:

    • Generally larger, fewer in number, and non-mobile. They often contain yolk to provide nutrients for the developing embryo after fertilization.

    • Surrounded by multiple protective layers to aid in nutrient absorption and prevent desiccation.

Fertilization

  • The process where sperm successfully penetrates the egg leads to the formation of a diploid zygote.

  • Fertilization can occur:

    • Externally: In many aquatic animals, where eggs and sperm are released into the water. This carries risks, like predation or environmental loss of gametes.

    • Internally: In terrestrial animals, where fertilization occurs within the female reproductive system, offering a safer environment for developing embryos.

Development After Fertilization

  • Embryonic development progresses through stages:

    • Cleavage: The zygote undergoes mitotic divisions leading to a morula (a solid ball of cells).

    • Blastula Formation: The development of a hollow ball of cells that marks the transition to the next stage.

    • Gastrulation: Formation of three primary germ layers, leading to differentiation into various tissues.

  • Differentiation: Cells become specialized, influenced by their position and signaling from surrounding cells, an essential process in forming complex structures and organs.

External vs Internal Development

  • External Development: Taking place outside the parent's body, typical in many aquatic species, includes less parental care and is usually risky for embryos due to predation and environmental hazards.

  • Internal Development: Seen in most land mammals, this method provides high protection and resource access for the developing embryo, reflecting a more efficient nurturing process with less nutrient dependence on the yolk.

Conclusion

  • Understanding both asexual and sexual reproduction is fundamental, not just in biological education but also in recognizing the diverse strategies life has developed to perpetuate its existence across different environments. This knowledge also informs studies in ecology, genetics, and evolutionary biology.