reproduction Flashcards
Asexual Reproduction
Asexual reproduction does not involve sex cells or fertilisation
Only one parent is required so there is no fusion of gametes and no mixing of genetic information
As a result, the offspring are genetically identical to the parent and to each other (clones)
Asexual reproduction is defined as a process resulting in genetically identical offspring from one parent
examples of asexual reproduction
Advantages & Disadvantages of Asexual Reproduction
Sexual Reproduction
Sexual reproduction is a process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other
Fertilisation is defined as the fusion of gamete nuclei
Gametes & Zygotes
A gamete is a sex cell (in animals: sperm and ovum; in plants pollen nucleus and ovum)
Gametes differ from normal cells as they contain half the number of chromosomes found in other body cells – we say they have a haploid nucleus
This is because they only contain one copy of each chromosome, rather than the two copies found in other body cells
In human beings, a normal body cell contains 46 chromosomes but each gamete contains 23 chromosomes
When the male and female gametes fuse, they become a zygote (fertilised egg cell)
This contains the full 46 chromosomes, half of which came from the father and half from the mother – we say the zygote has a diploid nucleus
Advantages & Disadvantages of Sexual Reproduction
Most crop plants reproduce sexually and this is an advantage as it means variation is increased and a genetic variant may be produced which is better able to cope with weather changes, or produces significantly higher yield
The disadvantage is that the variation may lead to offspring that are less successful than the parent plant at growing well or producing a good harvest
Flowers & Pollination
Flowers are the reproductive organ of the plant
They usually contain both male and female reproductive parts
Plants produce pollen which contains a nucleus inside that is the male gamete
Unlike the male gamete in humans (sperm), pollen is not capable of locomotion (moving from one place to another)
This means plants have to have mechanisms in place to transfer pollen from the anther to the stigma
This process is known as pollination and there are two main mechanisms by which it occurs: transferred by insects (or other animals like birds) or transferred by wind
The structure of insect and wind-pollinated flowers are slightly different as each is adapted for their specific function
Parts of a Flower
The pollen produced by insect and wind-pollinated flowers is also different:
Insect-pollinated flowers produce smaller amounts of larger, heavier pollen grains that often contain spikes or hooks on the outside so they are better able to stick to insects
Wind-pollinated flowers produce large amounts of small, lightweight pollen grains that are usually smooth
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Self and Cross-Pollination
Cross-pollination occurs when the pollen from one plant is transferred to the stigma of another plant of the same species
This is the way most plants carry out pollination as it improves genetic variation
Occasionally, the pollen from a flower can land on its own stigma or on the stigma of another flower on the same plant – this is known as self-pollination
Self-pollination reduces genetic variety of the offspring as all the gametes come from the same parent
Lack of variation in the offspring is a disadvantage if environmental conditions change, as it is less likely that any offspring will have adaptations that suit the new conditions well
On the other hand, cross-pollination relies completely on the presence of pollinators and this can be a problem if those pollinators are missing (eg the reduction in bee numbers is of great importance to humans as bees pollinate a large number of food crops)
Fertilisation occurs when
a pollen nucleus fuses with an ovum nucleus in the ovule
Fertilisation
As the pollen has no ‘tail’ to swim to the ovary, instead it grows a pollen tube
This only happens if it has landed on the right kind of stigma (ie of the same species as the flower the pollen came from)
The nucleus inside the pollen grain slips down the tube as it grows down the style towards the ovary
The ovary contains one or more ovules which each contain an ovum with a female nucleus that a male pollen nucleus can fuse with
Once the nuclei (pl) have joined together, that ovule has been fertilised and a zygote has been formed
The zygote will start to divide and eventually form a seed with the ovule
As different plants have different numbers of ovules, this explains why different fruits (which develop from the ovary) have different numbers of seeds (which develop from the ovules)
Gametes and Fertilisation
Fertilisation is the fusion of the nuclei from a male gamete (sperm cell) and a female gamete (egg cell)
It occurs in the oviducts
Gametes have adaptations to increase the chances of fertilisation and successful development of an embryo
sperm cell structure
An egg cell structure
Adaptations of Gametes Explained
The Placenta
During the gestation period the foetus develops and grows by gaining the glucose, amino acids, fats, water and oxygen it needs from the mother’s blood
The bloods run opposite each other, never mixing, in the placenta
The foetus’s blood connects to and from the placenta by the umbilical cord
The mother’s blood also absorbs the waste from the foetus’s blood in the placenta; substances like carbon dioxide and urea are removed from the foetus’s blood so that they do not build up to dangerous levels
Movement of all molecules across the placenta occurs by diffusion due to difference in concentration gradients
The placenta is adapted for this diffusion by having a large surface area and a thin wall for efficient diffusion
The placenta acts as a barrier to prevent toxins and pathogens getting into the foetus’s blood
Not all toxin molecules or pathogenic organisms (such as viruses, eg rubella) are stopped from passing through the placenta (this usually depends on the size of the molecule)
This is why pregnant women are advised not to smoke during pregnancy as molecules like nicotine can pass across the placenta
After the baby has been born, the umbilical cord is cut – this does not hurt as there are no nerves in it, just two blood vessels
It is tied off to prevent bleeding and shrivels up and falls off after a few days leaving the belly button behind
The placenta detaches from the uterus wall shortly after birth and is pushed out due to contractions in the muscular wall of the uterus – known as the afterbirth
Growth & Development of the Fetus
After fertilisation in the oviduct, the zygote travels towards the uterus
This takes about 3 days, during which time the zygote will divide several times to form a ball of cells known as an embryo
In the uterus, the embryo embeds in the thick lining (implantation) and continues to grow and develop
The gestation period for humans is 9 months
Major development of organs takes place within the first 12 weeks, during which time the embryo gets nutrients from the mother by diffusion through the uterus lining
After this point the organs are all in place, the placenta has formed and the embryo is now called a fetus
The remaining time is used by the fetus to grow bigger in size
The fetus is surrounded by an amniotic sac which contains amniotic fluid (made from the mother’s plasma)
This protects the fetus during development by cushioning it from bumps to the mother’s abdomen
The umbilical cord joins the fetus’s blood supply to the placenta for exchange of nutrients and removal of waste products
Breastfeeding
During pregnancy the mammary glands enlarge and become prepared to secrete milk
Shortly after birth, the mother will be stimulated to release milk due to the sucking action of the baby at the breast
Some mothers struggle to breastfeed successfully and so may feed the baby using formula milk in a bottle
Some changes occur to both boys and girls including
growth of sexual organs and growth of body hair
Emotional changes also occur due to the increased levels of hormones in the body
These include more interest in sex and increased mood swings
oestrogen and progesterone in the Menstrual Cycle
Oestrogen levels rise from day 1 to peak just before day 14
This causes the uterine wall to start thickening and the egg to mature
The peak in oestrogen occurs just before the egg is released
Progesterone stays low from day 1 – 14 and starts to rise once ovulation has occurred
The increasing levels cause the uterine lining to thicken further; a fall in progesterone levels causes the uterine lining to break down (menstruation / ‘period’)
FSH and LH
FSH (Follicle Stimulating Hormone) is released by the pituitary and causes an egg to start maturing in the ovary
It also stimulates the ovary to start releasing oestrogen
The pituitary gland is stimulated to release Luteinising Hormone (LH) when oestrogen levels have reached their peak
LH causes ovulation to occur and also stimulates the ovary to produce progesterone
Interaction between all four of the menstrual cycle hormones:
The pituitary gland produces FSH which stimulates the development of a follicle in the ovary
An egg develops inside the follicle and the follicle produces the hormone oestrogen
Oestrogen causes growth and repair of the lining of the uterus wall and inhibits production of FSH
When oestrogen rises to a high enough level it stimulates the release of LH from the pituitary gland which causes ovulation (usually around day 14 of the cycle)
The follicle becomes the corpus luteum and starts producing progesterone
Progesterone maintains the uterus lining (the thickness of the uterus wall)
If the ovum is not fertilised, the corpus luteum breaks down and progesterone levels drop
This causes menstruation, where the uterus lining breaks down and is removed through the vagina – commonly known as having a period
If pregnancy does occur the corpus luteum continues to produce progesterone, preventing the uterus lining from breaking down and aborting the pregnancy
It does this until the placenta has developed, at which point it starts secreting progesterone and continues to do so throughout the pregnancy
