Download Sexual reproduction

Reproduction
Define ‘reproduction’:
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.
Advantaged
Disadvantages
•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.
•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.
Advantages
Disadvantages
•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.
•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.
Let’s look inside a nucleus
•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 one DNA molecule & there are 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.
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.
•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.
•The zygote then divides asexually using mitosis so that it can grow into another human being.
Recap of mitosis and meiosis
Mitosis
Meisosis
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...
Female
Structure
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.
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.
Male
Structure
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.
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
Definition:
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