* Genetics

• Sexual Reproduction is where genetic information from two organisms is combined to produce offspring which are genetically different to either parent. In sexual reproduction, the father and mother produce gametes. Gametes only contain half the number of chromosomes of normal cells - haploid.
• At fertilisation, a male gamete fuses with a female gamete to produce a fertilised egg, also known as a zygote. The zygote then undergoes cell division and develops into an embryo. The embryo inherits characteristics from both parents, as it has received a mixture of chromosomes.

  
• Meiosis is a type of cell division. It is different to mitosis because it doesn’t proDuce identical cells. In humans, meiosis only happens in the reproductive organs (ovaries and testes).
• Meiosis -> Division 1

  • before the cell divides, it duplicates its DNA.
  • the chromosomes line up in pairs in the centre of the cell.
  • the pairs are then pulled apart, so each new cell only has one copy of each chromosome.
  • each new cell has a mixture of the mother and the father‘s chromosomes.

  Meiosis -> Division 2

  • chromosomes line up again in the centre of the cell.
  • the arms of the chromosomes are pulled apart again.
  • you get 4 haploid daughter cells - these are gametes.
  • the gametes are all genetically different.

  
• When cells reproduce asexually - they divide by mitosis, this results in 2 diploid daughter cells which are genetically identical to each other and to the parent cell.
• Sexual reproduction involves meiosis and the production of genetically different haploid gametes, which fuse to form a diploid cell at fertilisation.
• Advantages of Asexual Reproduction:

  • can produce lots of offspring very quickly
  • allows organisms to colonise a new area very rapidly.
• Disadvantages of Asexual Reproduction:

  • no genetic variation between offspring in the population
  • if the environment changes, the conditions become unfavourable and the whole population could be affected.
• Advantages of Sexual Reproduction:

  • creates genetic variation within the population so different individuals have different characteristics
  • if the environment changes, it is more likely that at least some individuals in the population will have the characteristics to survive the change
  • overtime, this can lead to natural selection and evolution as species become better adapted to their new environment.
• One of four different bases joins to each sugar. The bases are: A (adenine), T (thymine), C (cytosine) and G (guanine).
• A always pairs up with T and C always pairs up with G. This is called complementary base pairing. The complementary base pairs are joined together by weak hydrogen bonds.
• A DNA molecule has two strands coiled together in the shape of a double helix. Each base links to a base on the opposite strand in the helix.

  
• DNA strands are polymers made up of lots of repeating units called nucleotides. Each nucleotide consists of a sugar, a phosphate group and one ‘base’.

  
• Chromosomes are long, coiled up molecules of DNA . They are found in the nucleus of eukaryotic cells.

  A gene is a section of DNA on a chromosome that codes for a particular protein.

  All of an organisms DNA makes up its genome.
• How to extract DNA from fruit cells:

  1. mash some strawberries and put them in a beaker containing a solution of detergent and salt, mix it well.
  2. the detergent will break down the cell membranes to release the DNA.
  3. the salt will make the DNA stick together.
  4. filter the mixture to get the froth and big, insoluble bits of cell out.
  5. gently add some ice cold alcohol to the filtered mixture.
  6. the DNA will start to come out of solution as it is not soluble in cold alcohol - it will appear as a stringy white precipitate that’s can be carefully removed with a glass rod.
• DNA controls the production of proteins (protein synthesis) in a cell. Proteins are made up of chains of molecules called amino acids.
• The amino acid chains fold up to give each protein a different, specific shape - which means each protein can have a different function.
• Each amino acid is coded for by a sequence of 3 bases in the gene - this is called a base triplet.
• Many region of DNA are non-coding - that they don’t code for any amino acids.
• A mutation is a rare, random change to an organism’s DNA base sequence that can be inherited. If a mutation happens in a gene, it produces a genetic variant.
• Proteins are made in two stages: transcription and translation.
• Transcription:

  1. proteins are made in the cell cytoplasm by ribosomes.
  2. the information is transported from the DNA to the ribosome. In the cytoplasm.
• Transcription:

  1. The cell transports the information from the DNA to the ribosome in the cytoplasm.
  2. They use the molecule called the messenger RNA (mRNA).
  3. RNA polymerase binds to a region of non-coding DNA in front of a gene.
  4. The two strands unzip and the RNA polymerase moves along one of the strands of the DNA.
  5. It uses the coding DNA in the gene as a template to make the mRNA.
  6. Once made, the mRNA molecule moves out of the nucleus and joins with a ribosome.
• Translation:

  1. Once the mRNA is bound to a ribosome, the protein can be assembled.
  2. Amino acids are brought to the ribosome by another RNA molecule called transfer RNA (tRNA).
  3. The order in which the amino acids are brought to the ribosome matches the order of the base triplets in mRNA.
  4. Part of the tRNA’s structure is called an anticodon - it is complementary to the codon for amino acid.
  5. The amino acids are joined together by the ribosome. This makes a polypeptide (protein).
• Gregor Mendel was an Austrian monk who trained in mathematics and natural history. He noted how the characteristics in plants were passed on from one generation to the next.
• The results of his research were published in 1866 and eventually become the foundation of modern genetics.

  • in one experiment, Mendel crossed two pea plants of different heights - a tall pea plant and a dwarf pea plant. The offspring produced were all tall pea plants.
  • In his second experiment, he bred two of these tall offspring together. He found that when the offspring from the first cross were crossed with each other, three tall offspring were produced for every one dwarf offspring overall. In other words, he produced a 3 : 1 ratio of tall : dwarf plants.
• Mendel has shown that the height characteristics in pea plants was determined by separately inherited ’hereditary units’ passed on from each parent.

  The ratios of tall and dwarf plants in the offspring showed that the unit for tall plants, T, was dominant over the unit for dwarf plants, t.
• Mendel reached 3 important conclusions about hereditary in plants:

  1. characteristics in plants are determined by “hereditary units”.
  2. hereditary units are passed on to offspring unchanged from both parents, one unit from each parent.
  3. hereditary units can be dominant or recessive - if an individual has both the dominant and the recessive unit for characteristic, the dominant characteristic will be expressed.
• We know now that his ‘hereditary units’ are of course genes.

  But at the time, scientists didn’t have the background knowledge to properly understand Mendel’s findings - they had no idea about genes, DNA and chromosomes.

  It wasn’t until after his death that people realised how significant his work was and that the mechanism of inheritance could be fully explained.
• If an organism has two alleles for a particular gene that are the same, then it is homozygous for that trait.

  If its two alleles for a particular gene are different, then it is heterozygous.
• Dominant alleles overrule recessive alleles.
• Your genotype is the combination of alleles you have. Your alleles determine what characteristics you have - your phenotype.

  So different combinations of alleles give rise to different phenotypes.
• The inheritance of a single characteristic is called monohybrid inheritance. You can use a monohybrid cross to show how recessive and dominant traits for a single characteristic are inherited.
• There are 23 matched pairs of chromosomes in every human body cell. The 23 rd pair is labelled XX or XY. These are the 2 chromosomes which decide whether you turn out male or female.
• Males have an X chromosome (XY). The Y chromosome causes male characteristics.
• Females have two X chromosomes (XX). The XX combination causes female characteristic.
• You can use a family pedigree chart to show the inheritance of a trait in a family.
• The Y chromosome is smaller than the X chromosome and carries fewer genes.
• Disorders caused by faulty alleles located on sex chromosomes are called sex-linked genetic disorders.
• Colour blindness is caused by a faulty allele carried on the X chromosome.