reproduction

• sexual reproduction


  A reproductive process that involves two parents that combine their genetic material to produce a new organism, which is genetically different to either parent
• gametes


  sex cells produced by meiosis
• egg cell


  also called ovum; female gamete
• sperm


  male gamete
• how many chromosomes do gametes have?

  23 individual chromosomes - half the number of chromosomes in a normal cell (instead of have two of each chromosome, a gamete has just one of each)
• fertilisation


  fusing of a male gamete with a female gamete to form a zygote
• zygote


  a fertilised egg with 46 chromosomes (23 from the mother, 23 from the father)
• why does the offspring of sexual reproduction inherit features from both parents?

  because it receives a mixture of chromosomes from its mother and father (and it's chromosomes that decides traits)
• what produces variation in offspring?

  the mixture of genetic information due to the mixture of chromosomes received from each parent in sexual reproduction
• asexual reproduction


  a reproductive process that involves only one parent and produces offspring that are identical to the parent - happens through mitosis
• mitosis


  cell division in which the nucleus divides into nuclei containing the same number of chromosomes, producing two genetically identical daughter cells
• what produces asexually?


  bacteria, some plants and some animals
• why is there no genetic variation in asexual reproduction?

  offsprings are the genetically identical or a clone of the parent because there is no recombination of genetic material taking place. hence, there is no variation or genetic variability seen in the individual produced by asexual reproduction
• meiosis


  a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores
• where does meiosis take place?

  in reproductive organs (testes and ovaries)
• meiosis steps


  1. before the cell starts to divide, it duplicates its genetic information, forming two armed chromosomes - one arm of each chromosome is an exact copy of the other arm. after replication, the chromosomes arrange themselves into pairs

  2. in the first division of meiosis, the chromosomes arrange pairs lineup in the centre of the cell

  3. the pairs are then pulled apart so each new cell only has one copy of each chromosome - some of the father's and mother's chromosomes go into each new cell

  4. in the second division, the chromosomes line up again in the centre of the cell and the arms of the chromosomes are pulled apart

  4. the result is four gametes, each with only a single set of chromosomes in it. each of them are genetically different from each other because the chromosomes are shuffled up during meiosis and each gamete only gets half of them, at random
• what happens to the zygote produced by fertilisation?

  it replicates itself by mitosis, which repeats many times to produce lots of new cells in the embryo and as the embryo develops, these cells then start to differentiate into different types of specialised cells that make up a whole organism
• sexual reproduction advantages


  - genetic variation from the organism inheriting genes (and therefore features) from both parents

  - variation increases the chances of a species surviving a change in the environment, as it could lead to some offspring being able to survive

  - organisms with characters that make them better adapted to the environment have better chances of survival, meaning they are more likely to breed successfully and pass the genes for the characteristics on
• sexual reproduction disadvantages


  - energy costly

  - required two parents

  - courtship is time and resource communing

  - risk of birth defects
• survival advantage


  the outcome of a characteristic that helps an organism to live long enough to reproduce and pass on its genes.
• natural selection


  a process in which individuals that have certain inherited traits tend to survive and reproduce at higher rates than other individuals because of those traits
• asexual reproduction advantages


  - there only needs to be one parents

  - uses less energy and time as organisms do not need to find a mate

  - many identical offspring can be produces in favourable conditions
• asexual reproduction disadvantages


  - low genetic diversity

  - more prone to dying out from environmental change

  - inhibits adaption
• how do malarial parasites produce sexually and asexually?

  when a mosquito carrying the parasite bites a human, the parasite can be transferred to the human. the parasite reproduces sexually when its in the mosquito and asexually when its in the human host
• how do fungi produce sexually and asexually?

  many species of fungus can reproduce sexually and asexually. these species release spores, which can become new fungi when they land in a suitable place. spores can be produced sexually or asexually. asexually-produced spores form fungi that are genetically identical to the parent fungus. sexually-produced spores introduce variation and are often produced in response to an unfavourable change in the environment, increasing the chances that the population will survive the change
• how do plants produce sexually and asexually?

  in sexual reproduction, the pollen grain lands on the stigma and sticks to it, the pollen goes down the style into the ovary to fertilise the egg and a seed is produced. in asexual reproduction, plants can produce runners, which are horizontal stems growing away from the plant on the surface of the soil and at various points, a new and identical strawberry plant forms. bulbs
• sexual reproduction in plants

  the pollen grain lands on the stigma and sticks to it, the pollen goes down the style into the ovary to fertilise the egg and a seed is produced.
• asexual reproduction in plants

  - runners: plants can produce runners, which are horizontal stems growing away from the plant on the surface of the soil and at various points along the runner, a new and identical plant can form

  - bulbs: underground food storage organs with fleshy leaves that store food and can divide off, grow and develop into new plants (e.g. onions and garlic)

  - tubers: underground food stores. groups of cells in the tuber use this food to produce the energy needed for cell division. these cells then grow and develop into new plants
• what chromosomes decide sex?

  the 23rd pair, which are labelled XX or XY
• XY chromosomes


  male sex chromosome - the Y chromosome causes male characteristics
• XX chromosomes


  female sex chromosomes - the XX combination allows female characteristic to developc
• sex chromosomes in sperm

  when making sperm, the X and Y chromosomes are drawn apart in the first division in meiosis and there's a 50% chance each sperm cell gets either an X or Y chromosomes
• sex chromosomes in eggs

  all eggs have one X chromosome, as the original cell has two X chromosomes
• how is sex determined?

  by the presence or absence of a Y chromosome, where a man will have a Y and an X chromosome and a woman has two X chromosomes
• genetic diagrams


  - biological models used to show the possible genes of offspring

  - only one of the possibly outcomes would actually happen for any one offspring

  - Greger Mendel was the first to suggest separately inherited factors which we now call genes