Category: Section 3: Reproduction and Inheritance

Menstrual Cycle video

This is a great summary video on the hormones of the menstrual cycle.  IGCSE students should not worry about the role of GnRH from the hypothalamus, but should instead focus on the hormones FSH, LH, oestrogen and progesterone.

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Differences between Sexual and Asexual Reproduction: 3.1 3.2

All organisms have the potential to reproduce.  Reproduction is one of the 7 characteristics of life (8 if you study EdExcel IGCSE Biology…..) and of course it means the ability to produce new individuals.  But over the 4 billion or so years life has been around on the planet, evolution has developed a myriad ways of producing new individuals.  So as biologists are simple folk (well all the ones I work with are….), it makes sense to group different reproductive strategies together to make it easier to understand.

The major distinction between different ways of reproducing is to divide them into asexual and sexual reproduction.  This works fairly well, although it seems to be a subject many GCSE students don’t understand too well.  So here goes….

The first big idea to dispel is that the number of parents involved determines whether reproduction is sexual or asexual.  Too often I am told that asexual involves one parent, sexual two.  But although this works in most cases, sexual reproduction can happen with only one parent, for example in flowering plants that self-pollinate.  So we need a different way of deciding whether reproduction is sexual or asexual.

And in fact a clear distinction does exist and it is to do with genetics.  If the offspring produced are genetically identical to the parent (i.e. a clone) then it is an asexual form of reproduction.  If the offspring produced are genetically different to the parents, then it is sexual.

Often sexual reproduction involves the process of fertilisation.  This allows two parents to each contribute half their genetic material to their offspring thus generating individuals with new and unique genetic make ups.   These specialised cells that contain half the genetic material are called gametes and as you all know, they are made by a special type of cell division called meiosis.  Meiosis is vital for sexual reproduction as it produces cells that are haploid (one member of each pair of chromosomes) and all genetically unique.

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So this diagram shows two humans each producing gametes by meiosis.  The parents on the left will have 23 pairs of chromosomes and so each haploid gamete will have 23 individual chromosomes.  Fertilisation restores the diploid number.  Mitosis is then used to turn this single cell, the zygote into a multicellular embryo and then indeed into a new individual.  (You will remember mitosis is a type of cell division that always produces genetically identical daughter cells)

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The examples of asexual reproduction on the left all involve only this second type of cell division, mitosis.  There are no gametes, no fertilisation and hence no genetic variation.  The simplest type of asexual reproduction is shown as (A) and this is called binary fission.  A single-celled organism can divide in two to produce two genetically identical daughter cells.  Hydra (B) are a simple type of animal and they reproduce by budding.  A new individual just grows off the side and when it is big enough, it drops off….. And many plants can reproduce asexually using a technique called vegetative propagation.  The sweet potato plant in (C) can produce several offspring plants from each potato but as they are all clones of each other, this is definitely asexual reproduction.

Students do get confused with this topic so please ask me a question using the comment feature below the post.  Keep revising hard!

3.28 Variation: A* understanding for iGCSE Biology

Variation within a population is a critical and required component for natural selection.  If you have understood all the work on DNA, chromosomes and Mendelian genetics, you should now have a good understanding of where the genetic causes of this variation comes from.  But remember that variation can also be caused by the environment.  Indeed all variations in reality come from an interplay between genetic and environmental factors.  Genes by themselves cannot cause variation as without the environment of a cell to produce the protein, genes alone cannot affect the phenotype.

Genetic causes of variation

Sexual reproduction is the key to genetic variation in a population.  Meiosis produces gametes that are haploid and genetically different from each other.  The gametes may have a different combination of randomly “shuffled” chromosomes.  Crossing over in meiosis also allows alleles that would not otherwise be combined in a gamete.  So there are massive genetic differences between one gamete and the next.  And then there is random fertilisation so that any one male gamete is equally likely to fuse with any female gamete.  Random fertilisation (of gametes that meiosis has made genetically different to each other) is the key to genetic variation in a population.

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A final cause of genetic variation has nothing to do with sexual reproduction and is mutation.  DNA replication is not 100% accurate – the enzymes in eukaryotes make one error every billion base-pairs.  These mutations are random and can lead to new alleles appearing in a population.  Chromosomal mutations can also occur where chromosomes do not separate properly in meiosis or parts of a chromosome break off and re-join somewhere else….

Environmental causes of variation

Some of the differences seen in populations are due not to differences in genes but due to the differing environments in which an organism lives.  Peas which have inherited two copies of the T allele (for tallness) will never grow tall unless they are planted in well-watered soil and given access to sunlight.  You would never be a school teacher as a career without understanding that the environment a brain develops in can affect a person’s outcomes.  Environment is as important as genes in many variations in the human population, in particular to do with health and disease.  This is why so much emphasis for health for example is placed on promoting balanced diets, altering smoking habits, and moderating alcohol consumption.

Genes and Environment always interact to determine Phenotype

Don’t allow yourself to fall into the lazy thinking of the “nature-nurture” debate.  It is lazy thinking because the debate is a nonsense.  Nothing is either determined by your genes or your environment – it is always both.  So when you read of a ‘gene for obesity’ or a ‘gene for domestic violence’ treat with extreme caution (and switch newspapers……)  If you read that playing violent computer games causes violent behaviour in humans, treat with caution.  None of these variations in a population will be just due to genes, none will be just do to the environment.  It will always be some complex interaction between the two.

Cell Division part 3: A* understanding of Meiosis for iGCSE 3.25 3.26

In sexually reproducing organisms two types of cell division are needed.  One is for the processes of growth, repair and asexual reproduction and it is called mitosis. Mitosis produces daughter cells that are diploid and genetically identical to the parent cell.

But when the organism wants to make gametes a different mechanism is needed. Gametes are not diploid like all the other body cells, but instead they only have one member of each homologous pair of chromosomes.  In order to make a haploid daughter cell, a second type of cell division, meiosis, is needed.

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You can see in the diagram above some of the key differences between mitosis and meiosis.  Both start with diploid cells (2n) but whereas mitosis involves one round of division and produces two identical diploid daughter cells, meiosis is different. Meiosis has two rounds of division, called Meiosis I and Meiosis II.  This results in four daughter cells and you can see that they are all haploid (n) cells.  These cells develop into gametes (sperm and egg cells in humans) and so when they fuse together in fertilisation, the diploid number is restored.

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Gametes are all genetically different to each other

Meiosis does not just produce haploid daughter cells.  It also introduces genetic variation into the daughter cells so each is genetically unique.  This means that random fertilisation will produce offspring that are all genetically different to either parent. How does this genetic variation in gametes come about?

Well to answer that, you need to understand a little bit more about how the chromosomes behave during meiosis.  I am not going to talk through all the various stages of meiosis (life is too short and you can read the diagram below) but there is a key event that happens in meiosis that never happens in mitosis…..

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It happens in prophase of the first meiotic division and is called synapsis.  As the nuclear membrane is degenerating, the two members of a homologous pair of chromosomes line up alongside each other to form a structure called a bivalent (or tetrad)

Synapsis-and-Crossing-over

In the first meiotic division, the two members of the homologous pair are pulled apart and separated.  Because these bivalents attach and assort independently of each other, this means that this random assortment can produce many different gametes.  A human cell with 23 pairs of chromosomes can produce 2^23 possible gametes just by random assortment.

But there is a second process called crossing over that happens during prophase 1 when the bivalents are formed.  As you can see in the diagrams, small sections of chromatid can be swapped between the chromatids of one chromosome and with its homologous partner.  This ensures that when the individual chromatids are separated in meiosis 2, each is different to each other.  This multiplies up the genetic variation by several orders of magnitude. (see diagram below)

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Now that is more detailed than you will need in an iGCSE exam, but it is good to understand where the genetic variation in gametes comes from.  Let’s finish with something more simple – the differences between mitosis and meiosis.

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One final point: please learn the spellings of these two types of cell division.  Spelling is only penalised in exams when the meaning is lost and any intermediate spelling (e.g. meitosis or miosis) has no meaning!  So if you are one of those people who finds spelling difficult, find a way of learning mitosis (produces identical diploid daughter cells and is used in growth) compared to meiosis (produces genetically different haploid cells and is used to make gametes)

Cell division video: a revision video for DNA structure and chromosomes

This is a summary video that might help those of you still struggling to get to grips with chromosomes and genes.  I apologise for the terribly amateur production values on the video but hope the biological content at least might be useful….

Cell Division part I: A* understanding for iGCSE 3.23 3.24

At the beginning of March each year, I get my Y11 classes to draw up a list of topics they want to go through again in revision. Cell Division is always there and it is not difficult to see why.  Mitosis doesn’t make any sense unless you understand the concept of homologous pairs of chromosomes and I think you already know that very few iGCSE students do…… (Please see the various posts and videos on the blog on this topic before attempting to understand mitosis)

But there is actually very little to fear in the topic of cell division. If your teacher has told you about the various stages of mitosis that’s fine but you will not be asked to recall them in the exam, at least not if you are studying EdExcel iGCSE.  So in this post I am going to try to focus on the key bits of understanding you need rather than bombarding you with unnecessary details.

1 Chromosomes come in pairs

This is the main idea you need before you start.  In almost all sexually-reproducing organisms the cells are DIPLOID.  This means that however many different sized chromosomes they have, in each cell there will be pairs of chromosomes (called homologous pairs)

So human cells contain 23 pairs of chromosomes (46 in total)

chromosomes

Remember that the number 46 only applies to humans.  Other species have very different numbers of chromosomes in each cell (see table below)

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So doves have 8 pairs of chromosomes, dogs have 39 pairs of chromosomes, rats 21 pairs of chromosomes.  The important point is not how many pairs each organism has but that they all have chromosomes that come in pairs!

The chromosomes any individual possesses is determined at the moment of fertilisation.  Sperm and Egg cells (gametes) do not have pairs of chromosomes.  They are the only cells in the body that are not diploid.  Gametes only have one member of each pair of chromosomes.  Cells which only have one member of each pair of chromosomes are called HAPLOID cells.

So every cell in the body is diploid and genetically identical apart from the gametes which are haploid.

2 Organisms that reproduce sexually need two different types of cell division

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The fertilised egg (zygote) is a diploid cell.  It has pairs of chromosomes that originate one from each parent via the gametes.  Every cell division in growth and development of the embryo and foetus until birth, every cell division in growth and repair after birth always produces two genetically identical and diploid cells from the one original cell.  This cell division that produces genetically identical diploid cells is called Mitosis.

Gametes (sperm and egg cells) need to be made by a different process.  If gametes were diploid then there would be a doubling of the chromosome number every generation and that clearly wouldn’t do.  So a different way of dividing the nucleus has evolved.  It doesn’t produce genetically identical diploid cells but produces gametes that are haploid and genetically unique.  This process is called Meiosis and is only used in the production of gametes.

3 Mitosis is involved in growth, repair, asexual reproduction and cloning

Any process in the body in which the outcome required is the production of genetically identical diploid cells will use mitosis.  (It is not too complicated an idea to see that if you don’t need to make gametes and fuse them together in fertilisation, you can just copy cells by mitosis over and over again.  All the daughter cells will be exact copies of each other and diploid.

Now I know this post is not going to satisfy everyone. I know some of you will want to read about the cell cycle, prophase, metaphase, centrioles, spindle fibres and the condensation of chromosomes, chromatids being pulled apart etc. etc.) And just for you, I will write a post later today on the details of Mitosis….. But please remember that if you are using the blog to revise for exams, none of this second post is necessary and none of it will be tested in the Edexcel iGCSE paper.  If you are doing revision, focus on the key understanding ideas discussed above.  And as always, please leave a reply below to ask questions, comment or leave feedback – all comments welcome!

3.8 Female Reproductive System: A* understanding for iGCSE Biology

The human male and female reproductive systems are made from the same embryonic cells and are perhaps more similar in structure and function than is first apparent.  There are two ovaries protected within the pelvic cavity.  The ovary is the site of egg cell production.  The egg cell is the female gamete and is haploid – it has only one chromosome from each homologous pair.  The ovaries are also endocrine organs that produce the female sex hormones oestrogen and progesterone.

[Indeed differences between the gametes is the essential difference between male and female organisms.  Females are always individuals who produce a small number of large, often immobile gametes.  You can easily remember this: female – few, fixed, fat.  Males are organisms that produce large numbers of small, motile games. Male – many, mini, motile.]

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This diagram shows the human egg cell after it has been released from the ovary into the Fallopian tubes (or oviduct). The egg cell is coloured pink in the diagram above (if you are being picky it is not really an egg but a cell called a secondary oocyte but I won’t stress over this now…)  The egg cell is surrounded by a thick jelly-like layer called the zona pellucida and then by a whole cluster of mother’s cells from her ovary – the corona radiata.  The big idea to remember is that the egg cell is very large compared to sperm cells:  it is one of the largest cells in humans with a diameter of about 500 micrometers.

The Fallopian tubes carry the egg down towards the uterus.  The lining of the Fallopian tubes is covered in a ciliated epithelium.  The cilia waft to generate a current that helps move the egg down towards the uterus.  Sperm cells have to swim against this current to reach the egg in the tubes.  The Fallopian tube is the usual site for fertilisation to occur.

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Once fertilisation has occurred, the newly formed zygote divides over and over again by mitosis to form a ball of cells called an embryo.  The embryo continues its journey down the Fallopian tube until it reaches the uterus.  The uterus (womb) is a muscular organ with a thickened and blood-rich lining called the endometrium.  Implantation occurs when the embryo attaches to the endometrium and over time, a placenta forms.  The embryo develops into a foetus and remains in the uterus for 9 months.

The cervix is a narrow opening between the uterus and the vagina.  It holds the developing foetus in the uterus during pregnancy but dilates (widens) at birth to form part of the birth canal.  The vagina is the organ into which sperm are deposited from the man’s penis during sexual intercourse.  The lining of the vagina is acidic to protect against bacterial pathogens and the sperm cells released into the vagina quickly start to swim away from the acidity in grooves in the lining.  These grooves lead to the cervix and hence into the uterus.

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3.8 Male Reproductive Systems: A* understanding for iGCSE Biology

I am slightly wary about writing about the male and female reproductive systems.  Not because I get embarrassed with this topic (5 terms of human dissection at medical school removed any squeamishness about body parts….)  But rather that I worry that the school’s internet filters might start blocking my website if the wrong words appear.  But you don’t know until you try, so here goes…..

Male Reproductive System

I will start with the male reproductive system as males are simpler than females in many, many ways… The male reproductive system has three functions:

  • to produce the male gametes, sperm cells, at a prodigious rate
  • to make the male sex hormone testosterone
  • to act as a delivery system to ensure sperm cells are carried into the female reproductive tract in conditions that will allow them to fertilise an egg

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The first two functions listed above happen in the testis.  There are cells in the testis that secrete the hormone testosterone into the blood from puberty onwards.  Testosterone switches on secondary sexual characteristics in the male (body hair growth, muscle development, change in pitch of voice etc.) after puberty.  The main part of the testis is made up of very long coiled tubules called seminiferous tubules in which the sperm cells are made.

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Humans have over a hundred meters of seminiferous tubules in total in both testes and this allows sperm cells to made at a very fast rate.  Even though it takes around 75 days to make an individual sperm cell, the testes make them at a rate of around 85 million sperm cells per day.  The epididymis is found next to the testis in the scrotum and is a coiled tube in which sperm cells continue to develop and mature.  Sperm are stored here too in readiness for ejaculation.

Everyone knows that in humans the testes are found outside the body cavity in order to keep them cool.  Sperm production happens at a maximal rate 3 degrees below core body temperature and having testes outside the body keeps them at this temperature.

The vas deferens is a tube lined with smooth muscle that carries sperm cells away from the testis for ejaculation.  As you can see it loops around the back of the bladder, before joining up with the urethra just below the bladder.  The urethra is the tube that carries urine away from the bladder but can also carry semen once the vas deferens has joined with it.

There are three accessory glands in the male system (prostate gland, seminal vesicle and the Cowpersglands)  These glands produce the fluid that when mixed with the sperm cells is called semen.  Semen contains a sugar fructose to provide energy for the sperm cells to swim.  It is slightly alkaline to neutralise the acidity in the vagina and also contains mucus to make the fluid easy to move along the tubes.

The sperm cells only acquire the ability to swim when in the epididymis and only become totally mature and able to fertilise the egg right next to the egg cell in the female tract.

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The penis is an organ that contains erectile tissue that can fill with blood to allow the penis to fit into the female vagina for ejaculation.