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?
Asexual
Sexual
Offspring
Identical?
Example
species
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/begi
n/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
Zygote
begins to
divide
(mitosis!)
Sperm cell
breaks down
but leaves the
chromosomes
Zygote:
A Fertilized
Egg
Cells continue to
divide and start to
rearrange
themselves
Morula: solid
ball of 10 to 30
cells (Day 4)
8-cell stage
4-cell stage
(Day 2)
(Day 3)
2-cell stage
No G1 or G2 phases just M and S during cleavage
GASTRULATION
More dividing, three
distinct layers of
cells are formed
Still dividing,
outer cells start
to fold inward
Blastula: Hollow,
fluid filled ball of
cells;
(Day 5)
Gastrula (W3)
Ectoderm
(outer)
Nervous
System and
skin
Mesoderm
(middle)
Muscle, skeleton
Dermis, Heart,
Kidneys
Endoderm
(inner)
Lining of
digestive
system
Different view of previous slide
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
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
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.)
•
Fraternal twins (dizygotic)
- Fraternal
Usually in separate amniotic sacs.
twins develop when two eggs are fertilized by two
separate sperms. The fetuses have separate
placentas and amniotic sacs.