Transcript asexual reproduction

Reproduction:
The passing on of genetic
information from one
generation to the next
Why do living things reproduce?
•Living things need to reproduce so that they can pass on
the genetic information in the nucleus of their cells
from one generation to another in order to continue
their species. This way their genes live on into the
future.
•If a species could not reproduce any more it would not
take long for it to become become extinct.
•There are actually two types of reproduction, sexual
and asexual...
Asexual Reproduction
•Asexual reproduction means (1) that an individual, or part of an individual
can produce an offspring that has identical genetic makeup to itself, or
(2) grow or regenerate new tissue that is also genetically identical.
•The genetically identical offspring or the cells that make the new tissue
resulting from asexual reproduction are known as a clones.
•Asexual reproduction is the opposite of sexual reproduction in that it
does not involve the fusion of both male and female gametes (sex cells).
•Which of the photos on the title page show organisms undergoing asexual
reproduction?
•Asexual reproduction uses a process of cell division called mitosis to
produce the new cloned cells.
Here’s one about my toes for my sister…
“My toes, my toes sis...
They are all clones, the cells in my toes sis…”
Asexual Reproduction
Advantages
•Organism doesn’t need to invest
time & energy in finding & attracting
(or travelling to) a partner as they
can reproduce on their own.
•Organism doesn’t have to waste
energy travelling to find a mate.
•Less chance of offspring having a
harmful genetic mutation as they are
clones of a successful parent.
•Maintains desirable traits.
•Less chance of anything going wrong
as it is a simpler process.
Disadvantages
•No opportunity for genetic
variation by mixing of genes from
two different individuals.
•Little genetic change within a
species gene pool means little
chance of species improvement
and evolution.
•Lack of genetic change is
particularly harmful to species
when environmental conditions
change and the organism can’t
adapt to suit its environment and
may become extinct.
Sexual Reproduction
•Sexual reproduction is the fusion of two different sex cells, or
gametes, that combine to create a genetically new individual.
•The male gamete is the sperm and the female gamete is the ovum and
they fuse to produce a zygote.
•Most of the time the male gamete and female gamete comes from
different parents, but if a single organism has both male and female
gametes (they can therefore fertilize themselves), they are known as a
hermaphrodite. Examples include worms and plants.
•The offspring of sexual reproduction are not clones, they resemble
their parents but are not identical to them or to each other.
•Gametes for sexual reproduction are produced by a type of cell
division known as meiosis.
Here’s another for my sister about my O…
“My O, my O sis…
You’re different to me sis because of my O sis…”
Sexual Reproduction
Advantages
•Plenty of opportunity for
genetic variation by mixing of
genes from the two gametes.
•Genetic change within a species
gene pool means more chance of
species improvement and
evolution.
•Genetic change is particularly
useful to species when
environmental conditions change
as the organism can adapt to
change via genetic mutation.
Disadvantages
•Organism needs to invest time and
energy to select a partner.
•Organism may have to travel or
compete to propagate its species.
•More chance of harmful genetic
mutation.
•More chance of anything going
wrong as it is a more complex
process.
•Can lose desirable traits.
•Can take a long time to produce
fewer offspring.
How does Reproduction take place?
•Because reproduction is the transfer of genetic
information from one generation to the next, the nucleus
of a cell plays a very important role in reproduction
because that is where the genetic information is located.
•The genetic material in the nucleus is organised in a
particular way that makes it easy to be copied and then
divided into 2 to made 2 new cells.
•Remember that in asexual reproduction cells divide by
mitosis and in sexual reproduction cells divide by meiosis.
•Mitosis and meiosis take place in the nucleus of the cell.
How does Reproduction take place?
This cell cut in half so we
can see inside.
Each chromosome is
one molecule of DNA.
A gene is a section
of DNA that
contains the code
for a particular
characteristic.
The nucleus holds the genetic
information.
The tiny cross here
represents a
chromosome in the
nucleus.
If we ‘unwind’ a
chromosome we see a
helical molecule of
DNA that contains our
genes. We have
thousands of genes on
each chromosome.
Let’s take a look inside a nucleus
All these 46 chromosomes make up the 6th nucleus. We can
see the chromosomes because this nucleus is undergoing
reproduction by a type of cell division known as mitosis.
This is a photo of 6
different nuclei taken
under the microscope.
The other parts of the
cell have been
chemically treated and
destroyed so that only
the nucleus remains.
1
2
3
5
4
Can you count the 6
nuclei?
Let’s take a look inside a nucleus
A pair of chromosome #2 one from mum & one from dad
•Scientists have taken a photo of the
chromosomes from a single nucleus
undergoing mitosis, cut them out, and
lined them up in pairs.
•This is known as a karyotype.
•A human karyotype (in our body
cells/somatic cells) has 46 individual
chromosomes or 23 pairs.
•One chromosome of each pair comes
from one parent. This makes us a
mixture of both our parents.
•Remember that each chromosome is
The last pair is the sex pair and are one DNA molecule & there are
XX for females and XY for males thousands of genes on each
chromosome.
Mitosis
•The process of mitosis is the single division of the nucleus when a
cell is undergoing asexual reproduction.
•The cells produced by mitosis are exactly identical to each other
and are known as clones. They start with 46 chromosomes (diploid)
and end with 46 chromosomes (diploid).
•In humans, mitosis takes place in the body (somatic) cells.
•Mitosis allows us to grow by making lots of identical new cells.
•Mitosis helps us repair our body tissue if we get hurt and damage
any part of our selves. An example is after a cut when new blood
cells and new skin cells are made.
Hi there! Can you think of some other places in our
bodies where asexual reproduction is taking place?
Mitosis
46
46
46
46
46
46
46
…and so on…and so on…and so on…generation after generation...
Meiosis
•There is another type of nuclear division which is known as meiosis
and this occurs during sexual reproduction.
•Meiosis does not take place in the somatic (body) cells, but in the
male and female reproductive organs (gonads) - the testes and the
ovaries.
•The cells produced by meiosis are not clones, they contain half the
number of chromosomes. They start with 46 chromosomes (diploid)
and end up with 23 chromosomes (haploid).
•Meiosis allows the genetic information to change from one generation
to another providing genetic variation. That is why we look different
from a brother or a sister even though we both contain half the
genetic make up of our parents.
•The cells produced by meiosis are called sex cells or gametes. The
female gamete is the ovum and the male gamete is the sperm.
Meiosis in humans
•There are two divisions
in meiosis which leaves
the cells with half the
number of chromosomes
than when they started.
•This means that when
the sperm and egg fuse
together they can make
a cell - or zygote with 46
chromosomes.
What percentage of the
sperm are female (X) and
what percentage are
male (Y)?
•The zygote then divides
asexually using mitosis so
that it can grow into
another human being.
http://anthro.palomar.edu/synthetic/synth_3.htm
Recap of mitosis and meiosis
Mitosis
•Occurs in somatic cells.
•Has only one division.
•Produces identical daughter
cells known as clones.
•Does not provide genetic
variation.
•Occurs in asexual
reproduction and allows
organisms to reproduce easily
and quickly.
Meiosis
•Occurs in testes - where sperm
is produced and ovaries - where
ova are produced.
•Has two divisions.
•Produces daughter cells that
have half the genetic makeup of
their parents.
•Occurs in sexual reproduction
and allows organism to undergo
genetic change which is important
for the species to evolve and
adapt to new environments.
The reproductive systems
•We need specialized reproductive organs to carry out the production
of gametes by meiosis for sexual reproduction.
•We also need specialized reproductive organs to transfer the male and
female gametes to a place where they can meet and attempt to fuse and
undergo fertilization.
•We also need specialized reproductive organs to produce sex
hormones.
•Females also need specialized reproductive organs to nurture the
developing offspring.
•All of the organs that work together for sexual reproduction to take
place are collectively called the reproductive systems.
•Let’s take a look at the reproductive systems in human females and
males...
The female reproductive system
Structure
Oviduct/fallopian
tube
Oviduct/Fallopian
tube
Ovary
Ovary
Uterus
Uterus
Cervix
Ligaments
Vagina
Cervix
Rectum
Urethra
Vagina
Bladder
Pubic bone
The female reproductive system
Function
•Ovary – produces gametes (ooocyte) and releases one once a month. Produces
the hormones oestrogen and progesterone that are responsible for sexual
characteristics. Also produce small amounts of testosterone that is responsible
for sexual desire.
•Fallopian tube (oviducts) – tubes leading from the ovaries to the uterus. They
are connected to the uterus but not the ovary. The fimbriae hover over the
ovary and collect the oocyte once it is released at ovulation. They move the ova
towards the uterus. The sperm fertilize the ova here.
•Uterus – thick walled structure where the zygote implants and where the
fetus develops. It swells to prepare for implantation and if fertilization does
not take place it releases the tissue on its inside wall as menstrual blood.
•Cervix – small entrance into the uterus from the vagina. Allows sperm into
uterus and menstrual blood out.
•Vagina – muscular tubular cavity leading to the cervix. Directs the penis
towards the uterus.
The female reproductive system
Hormones
•FSH – made in the pituitary (base of the brain) and is responsible for
oocyte production.
•LH – made in the pituitary and is responsible for oocyte production.
•Oestrogen – produced by the ovary and control female sexual development
such as breast, uterus and genital development.
•Progesterone – produced by the ovary and responsible for preparing for
pregnancy. During pregnancy it is produced by the placenta for maintaining
nourishment.
•Testosterone – small amounts are produced by the ovaries and is responsible
for sexual stimulation.
The male reproductive system
Structure
Bladder
Vas deferens
Seminal vesicle
Bladder
Prostate gland
Vas deferens
Rectum
Urethra
Gland
Penis
Epididymis
Epididymis
Sperm
producing
tubes
Urethra
Testis
Testis
Scrotum
Penis
The male reproductive system
Function
•Scrotum – sac made up of lose but muscular skin that holds the testes outside
the body and helps control their temperature.
•Testis – ball of thin tubules that produce sperm (by meiosis). Also produce
testosterone.
•Epididymis – coiled tube that stores and matures sperm so that it is capable of
fertilization. Releases the sperm in the vas deferens during sexual arousal.
•Vas Deferens – long tube that transports the sperm from the epididymis to the
ureathra just prior to ejaculation
•Seminal Vesical – sac like pouches attached to the vas deferens that produce a
sugary fluid to help provide the energy for the sperm and also helps it to move.
Most of the ejaculate is make up of this fluid.
•Prostrate gland – rounded structure that also provides nourishment to the
sperm.
•Penis – made up of a sponge like tissue that fills with blood in order to become
erect so that it can transfer sperm into the vagina during sexual intercourse.
The male reproductive system
Hormones
•FSH – made in the pituitary (base of the brain) and is responsible
for sperm production.
•LH – also made in the pituitary and stimulates the production of
testosterone.
•Oestrogen – small amounts are produced in the testis. Its role is
not clear.
•Testosterone – made in the testes and is responsible for the
production of sperm. Also responsible for male characteristics,
including muscle mass and strength, fat distribution, bone mass and
sex drive.
Fertilization
When the zygote is
formed, the oocyte
always donates the X
chromosome and the
sperm can donate
either an X or a Y
chromosome.
So, at fertilization...
If the sperm is X then:
X(ovum) + X(sperm) = XX (female)
If the sperm is Y then:
X(ovum) + Y(sperm) = XY (male)
The end
http://www2.hu-berlin.de/sexology/ECE2/html/fertilization.html
•Cleveland Clinic Health System (2004) Structure and Function of the Male Reproductive System. Retrieved from the website
http://www.cchs.net/health/health-info/docs/2300/2376.asp?index=9117 April 2005.
•Haeberle, Erwin J. (2003) Fertilization. Retrieved from the website http://www2.hu-berlin.de/sexology/ECE2/html/fertilization.html April 2005.
•O’Neill, Denis (2005) Recombination. Retrieved from the website http://anthro.palomar.edu/synthetic/synth_3.htm April 2005.
References