Meiosis is the Basis of Sexual Reproduction Chapter 6 Meiosis is the Basis of Sexual Reproduction

Sexual Reproduction Requires two parents and results in genetic variation Produces offspring that are genetically different from each other, from parents and from any other member of their species. Genetic variation occurs because the offspring inherits half of its genetic material from the female parent and the other half of its genetic material from the male parent.

Terminology: Sexual Reproduction Haploid: half the genetic content Half of the genetic material or (Haploid) are carried in gametes which are specialized cells needed for reproduction. Gametes often referred to as sex cells. Fertilization: male sex cell combines with female sex cell to produce a zygote (diploid) Diploid: Genetic content equal to the parent Embryo: the zygote undergoes mitosis and cell division to produce an embryo.

Meiosis Meiosis occurs in gametes (eggs and sperm) and is a process that produces eggs and sperm with half the number of chromosomes as body cells. Meiosis occurs in two parts, Meiosis I and Meiosis II. Interphase occurs before Meiosis I begins (this is when the cell increases in size, makes proteins and organelles, and DNA duplicates: Basically the cell gets ready to divide)

Meiosis I: Similar to mitosis, EXCEPT in meiosis I, a pair of matching chromosomes (one from each parent) called a homologous pair, lines up at the equator The homologous pair separates and moves to opposite poles, and two daughter cells are formed at the end of Meiosis I The phases are called Prophase I, Metaphase I, Anaphase I, Telophase I

Prophase I: Metaphase I: Homologous chromosomes shorten and thicken and the membrane around the nucleus disappears. Metaphase I: Homologous pairs are lined up at the equator of the cell

Anaphase I: Telophase I: Homologous pairs move to opposite poles of the cell. Telophase I: Membrane for the nucleus forms around each set of chromosomes. Now there are two nuclei in one cell and the cell is ready to divide again.

Meiosis I

Meiosis II: DNA is NOT replicated again before Meiosis II. Meiosis II is like mitosis because in both processes, the chromatids of each chromosome are pulled to opposite poles. The stages are Prophase II, Metaphase II, Anaphase II and Telophase II Each daughter cell inherits one chromatid from each chromosome. End result is 4 haploid cells, each with half the number of chromosomes as the original parent cell.

Meiosis II

Comparing Mitosis and Meiosis Core Lab Comparing Mitosis and Meiosis

Similarities and Differences between Mitosis and Meiosis: In body cells In sex organs to produce sex cells Two daughter cells Four daughter cells Same number of chromosomes as parent cell One half the number of chromosomes as parent cell Asexual Reproduction Sexual Reproduction

Methods of Fertilization External Fertilization: a sperm cell and an egg cell unite outside the bodies of the parents. Example: fish, mosses and ferns How do mosses reproduce? The male and female organs develop on the end of the stems or branches of the plant Fertilization cannot occur without water. Sperm cells produced by the moss either swim across the damp ground or are splashed by raindrops into female parts of the plants Fertilization results in a new plant

Life cycle of Moss

How do flowering plants reproduce? Internal Fertilization: sperm cells are deposited inside the female’s body where they meet an egg How do flowering plants reproduce? Internal fertilization occurs through a process called pollination Pollination is the transfer of male gametes in structures called pollen from the male reproductive part (stamen) to the female reproductive part of a plant (pistol) After pollen lands on the pistol, structures form to deliver the sperm cells to the egg cells. Zygote forms, grows into an embryo and is nourished by the food stored in the seed. The seed’s outer covering protects the embryo.

Flowering Plants

Sexual Reproduction in Insects Internal fertilization Metamorphosis: a change in the form of an insect as it matures Incomplete Metamorphosis: involves subtle changes through three life stages: egg, nymph, and adult These changes are mostly due to growing The nymph (immature) phases look much like smaller versions of the adult The nymph is usually wingless and unable to reproduce The nymph has to moult (shed exoskeleton) in order to grow and develop wings and reproductive organs

Incomplete metamorphosis

Another type of internal sexual reproduction Complete Metamorphosis: a change in the form of an insect as it matures. The adult form of the insect is completely different from the larval stage. Occurs in 4 stages: egg, larvae, pupa, adult. The larva (grub or caterpillar) is wingless and wormlike The larva’s job is to eat and grow After several moultings, the larva enters the pupa stage and energy reserves are used in reorganizing organs and developing new adult structures such a wings. Adult’s main purpose is to reproduce and in many species the adult does not eat.

Complete metamorphosis

Comparing Complete and Incomplete Metamorphosis Number of Parents 2 Type of Reproduction Sexual (internal) Number of Stages in Life Cycle 4 3 Habitat ??

Advantages and Disadvantages of Sexual Reproduction Little energy required to find a mate for external fertilization More energy required to find a mate for internal fertilization External fertilization, greater numbers of offspring Internal fertilization fewer offspring are produced Internal fertilization, there is more protection to the embryo and more prenatal care External fertilization, offspring are unprotected and are often preyed upon Greater genetic variation. More likely to survive disease or other threats that appear in a population Some good traits may not be passed from parent to offspring

Compare/Contrast Sexual and Asexual Reproduction Number of Parent Cells 1 2 Gametes (eggs or sperm) None (cell divides) 2 (cells unite to from a zygote) Variation in offspring Lesser Greater Amount of Energy required Parental care

Male Reproductive System Structure Function Testes Produce sperm (male gametes) by meiosis and release hormones Scrotum Protects the testes, keeping them at a cooler temperature Vas deferens Muscular tubes in which sperm mixes with fluids to produce semen as the sperm move from the testes to the urethra Urethra Opening through which sperm leave the body Penis Contains the urethra for the delivery of sperm

Male Reproductive System

Female Reproductive System Structure Function Ovaries Produce eggs (female gametes) by meiosis and release hormones Oviducts/Fallopian tubes Location of fertilization. Connect the ovaries to the uterus Uterus Protects and nourishes the zygote during development. Connects the oviducts to the cervix Cervix Sperm travel through this opening on the way to the uterus. It opens (dilates) to allow the baby to leave the body during childbirth Vagina Sperm are deposited here. Opening though which the baby leaves the body, or where unfertilized eggs leave the body.

Female Reproductive System

Stages of Human Development Fertilization First Trimester (weeks 1 – 12) Second Trimester (weeks 13 -2 4) Third Trimester (weeks 25-38) -occurs in fallopian tube -egg and sperm unite to produce a zygote -movement from fallopian tube to uterus occurs -zygote now called an embryo, attaches to uterine wall -placenta and amniotic sac form -embryo becomes a fetus -by the end of this trimester, all major organs have begun to develop -skeleton forms -nervous system begins to function -mother begins to feel movement -organs continue to develop -rapid increase in size -immune system develops -increased movement Note: Once fertilization occurs, mitosis controls the growth and development from the zygote into adulthood.

Possibly ask a public health nurse to address the next two slides and last few slides on reproductive systems

Visible Signs of Pregnancy These signs vary from one person to another. Signs include: Menstrual flow stops Widening of the hips Enlargement of the breasts Morning sickness Weight gain Cravings

Major Shifts in Scientific Understanding of Genetics Gregor Mendel (mid 1800’s): Experimented with inherited traits in pea plants. Working with several generations, he showed that traits were inherited from parent plants He isolated the parts in the seeds that passed on the traits of color and shape

Major Shifts… Francis Crick and James Watson (1953) Described the structure of DNA Showed that DNA is an organization of genes into a double helix shape, like a twisted ladder The specific makeup of this ladder helped explain how a cell could make exact copies. The ladder came apart and each side acted as a template on which a new side could form

Major Shifts… Human Genome Project: Genome: consists of the full set of genetic material that makes up an organism Scientists around the world collaborated for about 20 years to identify every gene in the human DNA A big surprise was how few genes make up human cells. Only one sixth the estimated number Another surprise, how alike the genes of very different animals are

Major Shifts… Genetic Engineering Scientists are now selectively taking fertilized eggs with particular genes (such as gender, or absence of particular disease like Huntington’s disease) and implanting them into the female parent. This is embryo screening to select a certain offspring. Definitely some ethical issues arise from this process

Genetic Conditions that Cannot be Cured Down Syndrome: the results of a chromosome mutation Caused by an extra chromosome People with this syndrome have characteristic facial features and shorter stature Embryo screening currently exists for this condition but if an individual is born with this condition it is incurable.

Genetic Conditions … Cystic Fibrosis Embryo screening exists for this condition. Caused by a mutation Protein that is responsible for moving a substance (chloride ion) into and out of the cell is not made correctly Mucus thickening in the lungs occurs and breathing is difficult Lung infections can occur more easily

Genetic Conditions… Allderdice Syndrome A local example, restricted to an isolated community of Sandy point on the west coast (later resettled to St. George’s) Caused by an inverted insertion of a chromosome Leads to low birth weight, and and facial abnormalities and psychomotor dysfunction

Evaluate information and evidence gathered on the topic of genetics and genetic engineering ???????