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Reproduction: The passing on of genetic information from one generation to the next 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. Here’s one about my toes for my sister… “My toes, my toes sis... They are all clones, the cells in my toes sis…” Asexual Reproduction Advantages •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. Disadvantages •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. Here’s another for my sister about my O… “My O, my O sis… You’re different to me sis because of my O sis…” Sexual Reproduction Advantages •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. Disadvantages •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. How does Reproduction take place? This cell cut in half so we can see inside. Each chromosome is one molecule of DNA. A gene is a section of DNA that contains the code for a particular characteristic. The nucleus holds the genetic information. The tiny cross here represents a chromosome in the nucleus. If we ‘unwind’ a chromosome we see a helical molecule of DNA that contains our genes. We have thousands of genes on each chromosome. Let’s take a look inside a nucleus All these 46 chromosomes make up the 6th nucleus. We can see the chromosomes because this nucleus is undergoing reproduction by a type of cell division known as mitosis. This is a photo of 6 different nuclei taken under the microscope. The other parts of the cell have been chemically treated and destroyed so that only the nucleus remains. 1 2 3 5 4 Can you count the 6 nuclei? Let’s take a look inside a nucleus A pair of chromosome #2 one from mum & one from dad •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 The last pair is the sex pair and are one DNA molecule & there are XX for females and XY for males 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. Hi there! Can you think of some other places in our bodies where asexual reproduction is taking place? Mitosis 46 46 46 46 46 46 46 …and so on…and so on…and so on…generation after generation... 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. Meiosis in humans •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. What percentage of the sperm are female (X) and what percentage are male (Y)? •The zygote then divides asexually using mitosis so that it can grow into another human being. http://anthro.palomar.edu/synthetic/synth_3.htm Recap of mitosis and meiosis Mitosis •Occurs in somatic cells. •Has only one division. •Produces identical daughter cells known as clones. •Does not provide genetic variation. •Occurs in asexual reproduction and allows organisms to reproduce easily and quickly. Meiosis •Occurs in testes - where sperm is produced and ovaries - where ova are produced. •Has two divisions. •Produces daughter cells that have half the genetic makeup of their parents. •Occurs in sexual reproduction and allows organism to undergo genetic change which is important for the species to evolve and adapt to new environments. 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... The female reproductive system Structure Oviduct/fallopian tube Oviduct/Fallopian tube Ovary Ovary Uterus Uterus Cervix Ligaments Vagina Cervix Rectum Urethra Vagina Bladder Pubic bone The female reproductive system 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. The female reproductive system 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. The male reproductive system Structure Bladder Vas deferens Seminal vesicle Bladder Prostate gland Vas deferens Rectum Urethra Gland Penis Epididymis Epididymis Sperm producing tubes Urethra Testis Testis Scrotum Penis The male reproductive system 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. The male reproductive system 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 When the zygote is formed, the oocyte always donates the X chromosome and the sperm can donate either an X or a Y chromosome. So, at fertilization... If the sperm is X then: X(ovum) + X(sperm) = XX (female) If the sperm is Y then: X(ovum) + Y(sperm) = XY (male) The end http://www2.hu-berlin.de/sexology/ECE2/html/fertilization.html •Cleveland Clinic Health System (2004) Structure and Function of the Male Reproductive System. Retrieved from the website http://www.cchs.net/health/health-info/docs/2300/2376.asp?index=9117 April 2005. •Haeberle, Erwin J. (2003) Fertilization. Retrieved from the website http://www2.hu-berlin.de/sexology/ECE2/html/fertilization.html April 2005. •O’Neill, Denis (2005) Recombination. Retrieved from the website http://anthro.palomar.edu/synthetic/synth_3.htm April 2005. References