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
Stages of Birth
Amniotic sac breaks
Muscles in the uterus wall contract
Cervix dilates (gets wider)
Baby passes out through the vagina
Umbilical cord is tied and cut
Afterbirth is delivered
Antenatal Care
Antenatal (before birth), care is the name given to the care and advice given to expectant mothers along with checks on fetal growth and development
Whilst pregnant, expectant mothers are given advice on:
diet including the need to take folic acid to prevent developmental issues with the fetus and the importance of a balanced diet
exercise to stay fit
health precautions such as avoiding infections, tobacco, alcohol and other drugs
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
Secondary Sexual Characteristics
Primary sexual characteristics are present during development in the uterus and are the differences in reproductive organs etc between males and females
Secondary sexual characteristics are the changes that occur during puberty as children become adolescents
They are controlled by the release of hormones – oestrogen in girls and testosterone in boys
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
The menstrual cycle is controlled by hormones released from
the ovary and the pituitary gland in the brain
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
Birth control methods are important in
keeping family sizes small and in limiting the increase in human population
Humans can use …methods to prevent a pregnancy
mechanical, chemical, surgical and natural contraceptive
Some birth control methods also give protection from …
sexually transmitted infections
Natural
Abstinence
avoiding sexual intercourse completely
Rhythm method
avoiding sexual intercourse during the fertile period of the menstrual cycle when ovulation occurs
the exact time ovulation happens can be worked out by monitoring body temperature and quality of cervical mucus
this is the least reliable method of birth control
Chemical
IUD / IUS
an intrauterine device or intrauterine system is a small device fitted inside the uterus by a doctor or nurse
it releases sex hormones which thicken the mucus produced in the cervix, making it difficult for sperm to swim into the uterus
it also thins the lining of the uterus, making it more difficult for a fertilised egg to implant
an IUD also interferes with passage of sperm through the uterus, in which way it is acting as a barrier method of birth control
Contraceptive pill, implant, injection
may contain just progesterone or a mixture of progesterone and oestrogen
very effective when taken regularly
the hormones can also be delivered from a small skin implant or an injection, both of which last several months and increase the effectiveness as they remove the risk of forgetting to take a pill regularly
Barrier
These all work by preventing sperm from reaching the egg
Condom
latex sheath worn over the penis
prevents sperm entering the vagina as ejaculate remains in condom
also protects against STIs
Femidom
latex sheath inserted into the vagina
prevents entry of sperm into the vagina
Diaphragm
a rubber cap that fits over the entrance to the cervix
prevents entry of sperm into uterus
often used with a spermicide (cream which kills sperm)
Surgical
Vasectomy
the sperm ducts are cut, meaning that no sperm is present in the semen when ejaculation occurs
very effective but difficult to reverse
Female sterilisation (tubal ligation)
the oviducts are cut or tied off, preventing eggs from reaching the uterus or sperm from reaching the eggs
very effective but difficult to reverse
How do Contraceptive Hormones in The Pill Work?
They work by mimicking some of the hormone levels during pregnancy
By raising the levels of progesterone and oestrogen, the uterus lining is maintained and development of another egg cell is prevented
This means that sex at any time of the month cannot cause pregnancy as no egg is released to be fertilised
In situations where couples find it difficult to conceive,… can improve their chances
fertility treatments
Artificial Insemination (AI)
If the male is not producing healthy sperm, donor sperm can be used
The sperm are placed into the female’s vagina at the fertile point in her menstrual cycle
Social issues to consider include:
The male must be able to cope with the fact that the child is not biologically his
Sperm donors may father multiple children who are not able to know their parentage
Fertility Drugs
This method is used when the female is not producing enough eggs
Hormones, including FSH, are given to her to stimulate egg production
Social issues to consider include:
Several eggs can be released at once so this increases the chance of multiple births (twins or triplets etc)
In Vitro Fertilisation (IVF)
If the female cannot conceive naturally even after taking fertility drugs, or if there are issues with both male and female fertility in a couple, IVF can be used
This involves fertility drugs being given to the female to stimulate egg production before they are harvested from the ovary
The eggs are then inseminated in a petri dish using sperm from the male (‘in vitro’ means ‘in glass’) and, once embryos have formed, they are placed back into the uterus of the female
Several embryos are implanted to increase the chance of one developing further
Social issues to consider include:
IVF is relatively expensive and not all couples can afford it
As several embryos are implanted, the risk of multiple births is quite high
Some women use IVF to get pregnant at a later age than they would be able to conceive naturally
Unprotected sexual intercourse can lead to
the transfer of pathogens via exchange of body fluids
Unprotected sexual intercourse can lead to the transfer of pathogens via exchange of body fluids
Infections passed on in this way are known as
sexually transmitted infections (STIs
An example of an STI is
s HIV, the virus that usually leads to the development of acquired immunodeficiency disease (AIDS)
HIV can also be spread via
sharing needles with an infected person, blood transfusions with infected blood and from mother to fetus through the placenta and mother to baby via breastfeeding
How HIV Affects the Immune System
Immediately after infection, people often suffer mild flu-like symptoms
These symptoms pass and for a period of time infected people might not know they are infected
The virus infects the lymphocytes of the body’s immune system
Normally lymphocytes seek out and destroy pathogens that enter the body
However HIV avoids being recognised and destroyed by lymphocytes by repeatedly changing its protein coat
It then infects a certain type of lymphocyte and uses the cells to multiply
This reduces the number of lymphocytes, and also the number of antibodies that can be made
This decreases the body’s ability to fight off infection, eventually leading to AIDS
Controlling the Spread of STIs
The spread of STIs such as HIV are best controlled by:
Limiting the number of sexual partners an individual has
Not having unprotected sex, but making sure to always use a condom
Getting tested if unprotected sex or sex with multiple partners has occurred
Raising awareness by education programmes
