Download REPRODUCTION

REPRODUCTION
REPRODUCTION: making offspring
•
A characteristic of living organisms is the
ability to reproduce and pass on genetic
material to maintain the survival of species.
• Reproduction can be identified as either:
4. Asexual
- involves a single parent
- offspring or daughter cells are clones (ie.
identical copies of each other and of the single
parent)
- offspring produced by mitosis
2. Sexual
- Involves genetic contributions from two separate
parents
- Gametes ( sex cells) are the two sources in the
form of eggs (female) and sperm (male)
- Offspring are different from parents and each
other
- Variation due to meiosis
TABLE: Differences between asexual and sexual reproduction.
FEATURE
ASEXUAL
REPRODUCTION
SEXUAL
REPRODUCTION
No. parents or
parental contributions
• one
• two
Processes involved
• binary fission (prok)
• cell replication
involving mitosis (euk)
• absent
• gamete production
involving meiosis (euk
only)
• no genetic variability;
offspring are clones of
single parent
• faster
• offspring vary from
parents and each
other
• slower
Fertilisation
Offspring
Time for completion
• fusion of gametes
required
ASEXUAL REPRODUCTION
• Is reproduction that does not involve the fusion (joining
together) of two gametes (sex cells) is called asexual
reproduction.
In Prokaryotes:
• binary fission (fission= ‘splitting’; binary= ‘two’)
- replication of the circular molecule of DNA of the bacterial
cell.
- attachment of the two DNA molecules to the plasma
membrane
- lengthening of the cell
- physical division of the cell into two via a constriction
across the middle of the cell
→ each new cell contains one circular molecule of
DNA
• Simpler and faster than asexual reproduction in eukaryotic
organisms
• If resources are available one bacteria could produce 16
million descendants over an eight-hour period
→ process is called exponential growth
• Organisms that are formed from the cells of a single
parent are genetically identical to the parent.
In Eukaryotes:
• Some animals are capable of reproducing both
asexually and sexually depending on the environmental
conditions
Forms of asexual reproduction in animals are:
– Regeneration – parent breaks into 2 or more pieces
and a new individual will arise from the piece e.g.
planaria
– Splitting in Half – e.g. amoeba
– Budding – small groups of cells may bud or break
away from the main organism and then settle on a
substrate and reproduce mitotically to produce a
new organism e.g. sponges, hydro
– Parthenogenesis – ‘virgin birth’. Young are produced
from unfertilised eggs. Offspring are identical to
female parent e.g. honey bee
•
The ability to asexually reproduce is economically important.
Examples of vegetative reproduction or vegetative propagation in
plants:
• Cuttings – piece of parent plant can be planted and new
plants will grow from it e.g. roses
•
Runners (horizontal stems) – e.g. strawberries e.g. ferns and
blackberry
•
Rhizomes – plants propagate through underground stems
which sprout new daughter plants e.g. grasses and bracken
•
Suckers – plants that sprout from the roots of the parent plant
•
Plantlets without sex – e.g. ‘piggy-back’ plant. These are new
small plants that drop off the parent and can strike
•
Gemme – small multicellular bodies produced in cup-like
structures e.g. liverworts
Artificial cloning of mammals
Several techniques developed include:
• Cloning using embryo cells
- artificially separating the cells of an early
embryo
- transfer separated cells into surrogate
mothers
- results in identical twins, triplets or larger
multiple identical siblings
• Cloning using somatic nucleus
- still an experimental and dramatic
technique
- first came to world attention through the
birth of ‘Dolly’ in July 1996
- a somatic nucleus is cloned and
transferred to an egg cell
MEIOSIS
• A cell divides twice to produce four cells with
genetically different sets of chromosomes and
half the original chromosome number.
• This occurs in sexually reproducing organisms to
produce GAMETES.
• One diploid (2n) cell divides twice to form four
genetically different haploid (n) cells.
Meiosis Continued…
Meiosis Continued…
Meiosis Continued…
Meiosis Continued…
Meiosis Continued…
Meiosis Continued…
Meiosis Continued…
Meiosis Continued…
Meiosis Continued…
•
•
Products of meiosis are not identical.
The table below shows the possible outputs from meiosis – gametes
in animals and spores in plants.
SEXUAL REPRODUCTION IN PLANTS
• Flower plays a key role in sexual reproduction
Some Important Terms:
• CALYX – made up of a ring of SEPALS, which are green
and leaf-like
• SEPALS – protect the flower when in bud
• PETALS – form a ring inside the sepals (usually brightly
coloured) an together form COROLLA
• STAMEN – male reproductive parts form a ring inside
corolla. Has a stalk called a FILAMENT with an ANTHER
at its tip
Sexual Reproduction in Plants Continued…
•
PISTIL – female reproductive part found at the centre of flower. At
the base is an OVARY that extends into the STYLE which has an
enlarged sticky part on top, the STIGMA
•
CARPEL – comprises an ovary, style and stigma. Some flowers
have 1 carpel, some may have many
Sexual Reproduction in Plants Continued…
• Most flowering plants have both pollenproducing organs (stamens) and eggproducing organs (carpels).
→ these are called monoecious (= ‘one
household’) plants, eg. Corn, oak
• In other flowering plants the different
flower types are on different plants
→ these are called dioecious (= ‘two
household’) plants, eg. Holly, conifers
Sexual Reproduction in Plants Continued…
EGG PRODUCTION
• An ovary contains at least one OVULE – a group
of diploid (2n) cells attached to the wall of the
ovary.
• An inner cell divides by meiosis with three of the
haploid cells degenerating
• Mitosis eventually produces a 7-cell structure
called the FEMALE GAMETOPHYTE or embryo
sac
• The cell closest to the micropyle (hole in layer
around ovule) forms the egg
Sexual Reproduction in Plants Continued…
POLLEN PRODUCTION
• Each anther contains four pollen sacs (diploid
cells)
• Each inner cell divides by meiosis and results in
a 4 haploid spore
• Each spore divides by mitosis once and
develops into a POLLEN grain (microspore)
containing 2 haploid cells, the tube cell and
generative cell
• The pollen grain is the MALE GAMETOPHYTE
Sexual Reproduction in Plants Continued…
POLLINATION
• Occurs when pollen is transferred from an anther to a
stigma
• Transfer may be by:
– Wind
– Insects
– Birds
– Bats
Two kinds of pollination exist:
• 1. Self – Pollination – pollen from a flower falls onto the
stigma of the same flower. Rarely happens because the
anthers and ovules of the same flower mature at different
times
• 2. Cross – Pollination – more common. Occurs when pollen
from one plant falls onto the stigma of another plant
Sexual Reproduction in Plants Continued…
FERTILIZATION
• Once pollen is attached to stigma, the outer coat of the
pollen grain begins to germinate
• The POLLEN TUBE grows down through the style to the
ovary
• 2 nuclei enter the tube. One fertilises the egg and
produces a zygote then an embryo. Second sperm fuses
with the two polar nuclei and forms the ENDOSPERM, a
food store for the developing embryo
• Because 2 sperm cells involved – DOUBLE
FERTILIZATION
• Flowering plants DO NOT require presence of free water
for fertilization
Sexual Reproduction in Plants Continued…
SEED FORMATION
• After fertilization a SEED forms
• Seeds consist of an outer seed coat
formed from the outer layers of the ovule
& an inner endosperm
• Seeds are usually surrounded by a fruit
that develops from the ovary of the plant
Sexual Reproduction in Plants Continued…
LIFE CYCLE OF A MOSS
• Some plants require moisture for reproduction
• Moss plants are of 2 distinct parts – the
GAMETOPHYTE (haploid) and the
SPOROPHYTE (diploid)
• The green plant we see is the gametophyte.
Male & female reproductive organs are on the
end of separate stalks of the same plant or
different ones
• Eggs and sperm are produced by MITOSIS
Sexual Reproduction in Plants Continued…
• Sperm must swim to the egg therefore water
must be present
• Zygote formed after fertilization is DIPLOID and
divide by MITOSIS to form the sporophyte
• In the capsule on the tip of the sporophyte
diploid cells within undergo MEIOSIS to give rise
to SPORES (haploid)
• Both flowering plants and moss show this
ALTERATION OF GENERATION but it is more
obvious in a moss plant
Sexual Reproduction in Plants Continued…
SEXUAL REPRODUCTION IN ANIMALS
EXTERNAL FERTILIZATION
• Eggs are laid and then male sheds sperm
onto eggs and then fertilization occurs.
• Many eggs released, increase chance of
fertilisation but wastage is high.
• Examples are animals that live in aquatic
environments like most fish (not sharks),
many amphibians, coral
Sexual Reproduction in Animals Continued…
INTERNAL FERTILIZATION
• Copulation of male and female sees the transfer
of sperm to the female.
• The zygote is retained and develops in female.
• Few eggs are produced, wastage is low.
• Occurs in most land animals e.g. mammals,
birds, reptiles, insects.
Internal Fertilisation Continued…
• Insects- complex genital structures at the end of the
abdomen of a male insect enable him to couple with a
female and transfer sperm packages into the
reproductive tract of a female.
• Reptiles- the first vertebrates (apart from sharks) to
develop a penis.
- the presence of a penis enables sperm to be directly
transferred into the reproductive tract of a female.
- high chance of fertilisation and low gamete wastage.
- an evolutionary change was the egg with a protective
outer shell, series of internal membranes and a food
supply for the developing embryo. This self-contained
aquatic environment allowed the egg to be laid on land
Internal Fertilisation Continued…
Lay Eggs
No. of Eggs
Parental care
REPTILES
BIRDS
Moist
environments
Up to 150
(turtles)
Some
In nest
2-3
Lots
• Yolk of egg provides nutrients for a developing
zygote
• Albumin (egg white) provides water
Internal Fertilisation Continued…
• Birds- sperm transferred through close contact
of the urogenital openings (cloacas) of male and
female birds.
- also produce shelled eggs where the embryo
develops
• Mammals- in monotremes (platypus & echidna),
embryonic occurs within a a shelled egg.
- in marsupials, fertilised egg retained and
embryonic development occurs in uterus. Young
born at a very undeveloped stage.
- in placental mammals, fertilised egg retained
and embryonic development occurs in uterus.
Young born at a much more developed stage.
Sexual Reproduction in Animals Continued…
• In some species, a single organism has both eggproducing and sperm-producing organs. These
organisms are called hermaphrodites.
eg. Garden snail and earthworm
• When both sperm-producing and egg-producing organs
are active at the same time it is called simultaneous
(synchronous) hermaphrodism.
• Animal species that can change sex are called
sequential hermaphrodites.
eg. Coral fish like anemonefish, wrasses
• These fish start life as one sex and can transform to the
other under certain conditions.
Sexual Reproduction in Animals Continued…
• Involves the fusion of two cells called gametes
• Results in offspring that are quite different from each
other
• Essential process that ensures that when a ZYGOTE
is formed after FERTILIZATION it has the same
number of chromosomes as their parents – MEIOSIS
• Meiosis occurs in reproductive tissues called
GONADS
• Gonads (‘gamete factories’) of female mammals –
OVARIES; male mammals - TESTES
Sexual Reproduction in Animals Continued…
• Number of chromosomes in a somatic
(body) cell is called the diploid number
eg. Humans is 46 (2n = 46)
• The number of chromosomes in a gamete
(sex cell) is called the haploid number
eg. Humans is 23 (n = 23)
Sexual Reproduction in Animals Continued…
REPRODUCTION IN HUMANS
• Eggs have small amount of yolk
• Sperm are motile
• Internal fertilisation occurs so gametes are
always in moist environment.
MALE REPRODUCTIVE SYSTEM
• Two testes are located in a loose pouch of skin – SCROTUM
• Testes found outside body because lower temperature
(35.5˚C) better for sperm development
• Each testicle produces sperm and hormones
• Onset of PUBERTY – Luteinising Hormone released from
pituitary gland acts on testes and the testes produce
TESTOSTERONE
• Testosterone brings about:
–
–
–
–
–
Sex organ change
Voice changes
Muscle development
Body hair growth
Production of sperm
Male Reproductive System Continued…
Male Reproductive System Continued…
MEIOSIS IN HUMAN MALES (SPERMATOGENESIS)
• Sperm produced in TUBULES
• Precursor sex cells develop during foetal
development and remain dominant until puberty
• Precursor sex cells reproduce by mitosis –
results in primary spermatocytes
• primary spermatocytes meiotically divide to
produce 4 mature sperm
Male Reproductive System Continued…
Male Reproductive System Continued…
Male Reproductive System Continued…
Mature sperm move from:
A testis
↓
EPIDIDYMIS – temporary storage
↓
VAS DEFERENS – runs from each epididymis to
urethra
↓
URETHRA – sperm + a nutrient-rich fluid (SEMEN)
secreted from SEMINAL VESICLE, COWPER’S
GLAND and PROSTATE GLAND
Male Reproductive System Continued…
• During sexual arousal, large amount of blood
enters PENIS, which becomes firm and erect
• Semen leaves body through the urethra by
EJACULATION
• Each ejaculation – between 2.4 – 6mL of semen
• Each mL of semen contains 50 – 100 x 106
sperm
FEMALE REPRODUCTIVE SYSTEM
• Female gametes – EGGS (or OVA) are
produced in OVARIES
• Precursor sex cells ↑ in no. by mitosis and
enlarge to form potential egg cells
• At birth each ovary has approx. 200,000 eggs
• Ovaries also produce hormones –
OESTROGEN & PROGESTERONE
• MEIOSIS starts before birth but stops before
completion and remains dormant until puberty
Female Reproductive System Continued…
Female Reproductive System Continued…
MEIOSIS IN HUMAN FEMALES (OOGENESIS)
• At puberty an ovarian cycle begins
• One egg cell increases in size in response to hormonal
signals
• One egg cell divides – 2 cells of unequal size are
produced
- Smaller one is POLAR BODY and degenerates
- Larger cell continues to develop within a FOLLICLE &
is released at OVULATION into the FALLOPIAN TUBE
• Meiosis continues after egg is released
• The empty follicle becomes CORPUS LUTEUM
Female Reproductive System Continued…
Female Reproductive System Continued…
Female Reproductive System Continued…
• The egg moves into the funnel-like opening of a fallopian
tube (10cm long) → UTERUS – muscular and shaped
like an inverted pear
• If egg is fertilised in fallopian tube then it becomes
embedded in the uterus’ lining – ENDOMETRIUM
• Endometrium is rich in blood vessels supplying
developing EMBRYO with oxygen and nutrients
• Lower end of uterus is CERVIX, which opens into
VAGINA
• Vagina has 2 functions:
– Sperm enter female reproductive organs
– Passageway for a baby during birth
MENSTRUATION & MENSTRUAL CYCLE
• From puberty → approx. 50 years old
• Lasts 28 days
• Governed by hormones – chemical messages
transported in the bloodstream
• Hormones released are:
1. Pituitary Gland - F.S.H – follicle stimulating hormone
- L.H – luteinising hormone
2. Ovaries
- Oestrogen
- Progesterone
Menstruation & Menstrual Cycle Continued…
Menstrual cycle begins with:
 Pituitary gland releases FSH into the bloodstream
stimulating eggs to develop. All but 1 degenerates.
 As ovum grows ovary releases OESTROGEN.
Increasing levels causes 2 things:
– Inhibits production of FSH so no more ova develop
– Prepares lining of uterus (endometrium – capillaries)
for fertilised ovum
f. As ovum continues to ripen & oestrogen levels increase,
the pituitary gland is stimulated to release LH.
Menstruation & Menstrual Cycle Continued…
 LH causes release of ripe hormone ovum from
ovary to nearby oviduct – OVULATION &
causes follicle which held ovum to develop into
CORPUS LUTEUM
 Corpus luteum releases PROGESTERONE
into the bloodstream preventing pituitary
releasing LH or FSH
 Increase in progesterone and decrease in LH
causes basal body temp to rise by approx.
1˚C. This temp rise occurs just after
OVULATION and is maintained till
MENSTRUATION
Menstruation & Menstrual Cycle Continued…
 If ripe ovum is not fertilised within a few days,
corpus luteum stops producing progesterone
and breaks up
 Decreasing the amount of progesterone
causes the thickened lining of the uterus to
break up and MENSTRUATION occurs 14 -16
days after ovulation
 Decrease in progesterone will stimulate
release of FSH and a new ovum will start to
grow and the cycle starts again
Menstruation & Menstrual Cycle Continued…
If Fertilization Occurs:
c.
If fertilization takes place, the placenta develops and
produces 2 hormones responsible for preventing the
corpus luteum breaking up so it continues to produce
progesterone, therefore it:
i. Prevents release of FSH so no more eggs
mature
ii. Prevents shredding of uterine lining
NOTE:
At time of ovulation egg is still undergoing meiosis.
Two cells are produced and uneven. Again the smaller
degenerates and becomes a polar body. The larger
contains most of the cytoplasm and forms the mature
egg.
HUMAN LIFE CYCLE
COMPARISON OF ANIMAL AND PLANT LIFE CYCLES