Science Test 28/05

• DNA


  The blueprint of life
• Features such as hair colour, freckles and susceptibility to certain diseases tend to run in families. We inherit these features from our parents.
• The DNA in our cells carries the information that makes us who we are. This DNA originally came from an egg and a sperm produced by your parents.
• Characteristics that are passed from parent to child are said to be inherited.
• Characteristics are passed from both parents.
• It is impossible for two brown-eyed parents to have a child with blue eyes.
• Your weight is determined by your genetic make-up to some extent.
• A woman who eats a high protein diet during the first 2 months of pregnancy is more likely to give birth to a son.
• It is possible for a person to carry a gene for a genetic disorder and not know it.
• Some types of cancers are hereditary.
• Identical twins are natural clones.
• In the majority of cases, genetic disorders are due to a problem with the mother's egg cells rather than the father's sperm cells.
• A brother and sister who are twins have exactly the same DNA.
• Gene


  A piece of DNA (deoxyribonucleic acid)
• DNA


  A chemical found in the nucleus of cells that contains the instructions for constructing a particular polypeptide
• Protein


  Containing one or more polypeptides, can determine characteristics
• Chromosome


  Made up of DNA and other substances, including proteins, that can be seen using a light microscope when a cell is about to divide
• Most of the cells of healthy humans contain 23 matching pairs of chromosomes.
• Karyotype


  A picture showing the chromosomes organised in matching pairs in order of size from biggest to smallest
• People who have Down syndrome have 47 rather than 46 chromosomes in their cells.
• In Turner syndrome the karyotype reveals that there are only 45 chromosomes. Instead of two sex chromosomes there is only one X chromosome.
• The platypus has 10 sex chromosomes.
• Mitosis


  1. Produces daughter cells with the same number and type of chromosomes as the parent cell

  2. Occurs in certain parts of plants including the tips of the roots and stems

  3. Turns a single fertilised egg into the trillions of cells that make up an adult human

  4. Produces new skin cells to replace those lost
• Gametes contain only half the number of chromosomes of other body cells.
• Cytokinesis is the process where the cell membrane pinches in so that the cell divides in two after mitosis has occurred.
• Dr Elizabeth Blackburn discovered an important enzyme called telomerase that maintains and repairs the telomeres at the end of chromosomes.
• Mitosis is a type of cell division that produces daughter cells with the same number of chromosomes as the parent cell
• Meiosis is a type of cell division that produces daughter cells with half the number of chromosomes as the parent cell
• Meiosis produces gametes
• In gametes the chromosomes are not in pairs
• Meiosis


  1. Chromosomes line up in matching pairs (homologous pairs)

  2. Homologous pairs separate

  3. Chance determines which cell gets the maternal or paternal chromosome from each pair
• Humans have 23 pairs of chromosomes, so there are 2^23 or 8,388,608 different ways the chromosomes can be divided during meiosis
• Chromosomes can swap sections when in homologous pairs, further increasing variation
• At fertilisation, a male and female gamete combine to form a zygote with a diploid number of chromosomes
• The zygote's chromosomes are copied during mitosis, so all body cells have the same chromosomes as the zygote
• Sex chromosomes


  Females have two X chromosomes, males have one X and one Y chromosome
• The sex of a baby is determined by whether the sperm cell that fertilises the egg contains an X or Y chromosome
• A sperm cell with an X chromosome results in a baby girl, a sperm cell with a Y chromosome results in a baby boy
• A mother's diet during pregnancy has no effect on the sex of the baby
• Sex selection technology can be used to increase the chances of having a child of a particular sex