Asexual Reproduction

Some organisms can create “offspring” without having egg and sperm

Bacteria – Binary Fission (no nucleus)

Animals – Asexual Reproduction (natural cloning)

Examples of asexual reproduction(animals)

• Hydra budding (this is an animal)

• Some sponges

• Planaria

• Starfish

Asexual Reproduction and Longevity

• Swedish scientists claim:

• Animals that reproduce asexually have exceptionally good health and can delay aging.

• Sea squirts can activate the enzyme telomerase which protects the DNA

– this enzyme is also more active in people who reach a great (very old) age.

Sexual Reproduction(most animals, many plants)

• Accomplished when an organism (of a species) creates a cell with half the number of chromosomes (haploid)

• Haploid in humans is 23 chromosomes. This cell is either an EGG or a SPERM

• This cell can only create new life if joined to another gamete

Sexual vs. Asexual Reproduction

DNA from how many parents?

Offspring

Identical?

Example

species

Asexual

Sexual

Chromosome Terms

• Diploid: two of each type of chromosome (one from each parent)

• Haploid: one of each chromosome

• Most human cells are Diploid

–We have 46 chromosomes

•22 pairs of body chromosomes

•2 sex chromosomes (XX or XY)

• What would happen if we made Egg and Sperm cells through mitosis?– Fertilized egg would have 92 chromosomes!

• What do we have to do when forming these cells?

–Divide the DNA in half!

Major differences between Mitosis and Meiosis

• one cell goes through two divisions to make a total of 4 cells

• cells created at the end are different from the cell they came from because of crossing over

Meiosis

• After the first division, cells are haploid and no longer diploid

• Happens only in reproductive cells

WHY IS SEXUAL REPRODUCTION BENEFICIAL? Genetic Variation:

- diversity between individuals – survival

DIVERSITY IS ACHIEVED BY

1. combining genes from two individuals

2. swapping alleles with a homologous chromosome prior to creating the haploid cell (crossing over)

Meiosis I: 1st division

1.Homologous chromosomes pair up to form tetrads

2.Crossing over occurs: homologous chromosomes swap info (blonde allele for brown allele)

Meiosis I: 1st division

Similar steps to mitosis but:

3. Cells end up haploid but chromosomes are still duplicated

Meiosis II: 2nd division

Just like mitosis

- each cell from Meiosis 1 divides

- Creates 4 cells total

- all are different because of crossing over - all have ½ the original chromosomes

Compare and Contrast Mitosis and Meiosis

(see handout)

Humans and Meiosis

• Germ Cell – any biological cell that gives rise to the

gametes of an organism that reproduces sexually.

– Found in the gonads (organs that make gametes) Testis (male) Ovary (female)

Spermatogenesis

• Germ cell is called Spermatogonium – 2N– Makes sperm (1N) through meiosis– Makes additional spermatogonia (2N) through

mitosis

– Meiosis I• Reduces chromosome number from 2N 1N• BUT.. Still have duplicated chromosomes

– Meiosis II• Splits sister chromatids each chromatid becomes a

chromosome

Oogenesis

• Asymmetrical cell division leads to formation of the ovum and 3 polar bodies

• Polar bodies eventually disintegrate.

Karyotype

• Photomicrograph (microscopic picture) of chromosomes in a normal dividing cell

– Organized based on size of the chromosome and position of the centromere.

– Grouped by homologous pairs. Sex chromosomes are the 23rd pair

• Usually made from chromosomes in prophase (duplicated)

Types of Chromosomes

• Sex chromosomes– Determine the gender of an organism and

may also carry other genes

• X or Y XX XY

• Autosomes– All other chromosomes (pairs 1-22)

Chromosomal Mutations

Mutation

a change in the nucleotide-base sequence of a gene or DNA molecule.

Possible Effects:

Germ cell mutations - occur in gametes

Do not affect the organism, but affect the offspring.

Somatic cell mutations – affect the body cells (but not gametes) and do affect the organism.

Classification of Mutations

Lethal mutations– Cause death, often before birth

Beneficial mutations– Mutations which provide higher survivability or

reproducibility

Silent mutations– neither harmful nor beneficial

Chromosomal vs. Gene Mutations

• Chromosomal Mutations – Involve changes in the structure of a chromosome or the

loss or gain of a chromosome.– Involve more than one gene– Generally seen in a karyotype

• Gene mutations – WE WILL STUDY LATER– The addition or removal of a single nucleotide within a

single gene– AKA point mutation.

Karyotyping and Chromosomal Mutations

• Karyotypes ONLY IDENTIFY chromosomal mutations

– Can involve losing or gaining a “whole” or part of a chromosome.

Chromosome Mutations• Deletion

– Loss of a piece of chromosome due to breakage (genetic information is missing)

• Non-disjunction – non-separation of homologues or sister chromatids

Chromosomal Diseases

• Learn about these at the following site

• http://learn.genetics.utah.edu/content/begin/traits/

• Know the karyotype of:– Down syndrome (one extra chromosome 21) – Turner syndrome XO, – Klinefelter syndrome (XXY)

Questions to Answer

• What is the difference between monosomy and trisomy? How can both occur “at the same time”?– Monosomy (missing one chromosome)

• Example: in Turner syndrome you have only ONE X chromosome and NO other sex chromosome

• In humans, that means you have 45 chromosomes and not 46

– Trisomy (one extra chromosome• Down’s syndrome – you have three of chrom. 21• Klinefelter syndrome : you have 2 X and 1 Y

Deletions

• What are two possible ways deletions can occur? – You can lose a whole chromosome

(monosomy) because of non-disjunction

– You can lose a part of a chromosome because the chromosome breaks (deletion)

Stem Cells

• Read article on Stem Cells. Be sure that you can answer all questions

Stem Cells

• have the ability to either divide again and again to create more of itself (exact copies), OR to differentiate into another type of cell

• . Because of their ability to develop into any cell type, they could potentially provide an unlimited source of adult cells, such as bone, muscle, liver, or blood cells.

Where can we find stem cells?

• An embryo– These cells are totipotent (morula) or

pleuripotent (gastrula)

• In areas of the body where “new cells” develop and differentiate (multipotent)– Bone marrow– Umbilical cord of newborn babies

Three types

• Totipotent– Occur during cleavage (first few hours after

fertilization)

• Pluripotent– One “step” differentiated– Can form many but not all cell types (most

researched currently for medical use)

• Multipotent– Bone marrow cells for all blood cells. Best

understood.

Stem Cell Technology

• Can we “grow an organ”?– NOT yet.

• We can grow tissue• We can put a stem cell in a tissue and the stem

cell will differentiate into that tissue• But…we cannot make an organ (or make people

walk) …YET

Stem Cell Controversy

• Controversy:– When the source of stem cells is embryonic,

there is controversy about “taking a life”• Is it a “ball of cells” or is it a “potential child”?

– Why not just use adult stem cells (multipotent)• Limitations of adult stem cells

– Bone marrow cells can only form cells found in bone and blood

– Umbilical cord blood – has cells that can be used only for blood diseases like leukemia.

Embryology- links to meiosis and stem cells

• A zygote is formed when egg and sperm unite

• When a zygote cell begins dividing, it is called an embryo (this term is used until the embryo is 8 weeks old

• Totipotent and pleuripotent stem cells come from embryos.

Fertilization

Egg and sperm join

Sperm cell breaks down but leaves the chromosomes

Zygote:

A Fertilized Egg

Zygote begins to divide (mitosis!)

2-cell stage 4-cell stage (Day 2)

8-cell stage

(Day 3)

Morula: solid ball of 10 to 30 cells (Day 4)

Blastula: Hollow, fluid filled ball of

cells;

(Day 5)

Cells continue to divide and start to

rearrange themselves

Still dividing, outer cells start to fold inward

GASTRULATION

More dividing, three distinct layers of cells are formed

Ectoderm (outer)

Nervous

System and

skin

Mesoderm (middle)

Muscle, skeleton

Dermis, Heart, Kidneys

Endoderm (inner)

Lining of digestive system

Gastrula (W3)

No G1 or G2 phases just M and S during cleavage

1. Are the egg and sperm haploid or diploid? What about the zygote?

HAPLOID. DIPLOID

2. What type of division is occurring to create a morula? A blastula? What term is used to describe the first divisions of the zygote?

MITOSIS, Again MITOSIS, CLEAVAGE

3. In which stage do you see definite differentiation? What causes this?

GASTRULA. Caused by specific “starter genes” being turned on (and then off at a later point)

4. Where would you find totipotent cells? MORULA

Pleuripotent cells? GASTRULA

Different view of previous slide

Other Important Terms:

• Gamete – a haploid reproductive cell that unites with another haploid

reproductive cell to form a zygote.

• Zygote - The cell that results from the union of gametes; also called a

fertilized egg

Fertilization – the union of a male and female gamete to form a zygote

• Embryo – an organism in the early stages of development . A developing

human is called an embryo from two weeks after conception until the end of

the eighth week.

• Differentiation – the structural and functional specialization of cells during an

organisms’s development

SEE NEXT PAGE FOR TWINS

Other Important Terms:

• Identical twins (monozygotic)- --

• - single egg fertilized by one sperm.

• Fertilized egg splits into two embryos

early in development.

• Most IT share the same placenta. (get

oxygen and nutrients from the mother

and get rid of wastes through the

placenta.)

• Usually in separate amniotic sacs. Fraternal twins (dizygotic) - Fraternal

twins develop when two eggs are fertilized by two

separate sperms. The fetuses have separate

placentas and amniotic sacs.