inheritance

• sexual reproduction involves the joining (fusion) of male and female gametes (sex cells)
• sexual reproduction involves sperm and egg cells in animals and pollen and egg cells in flowering plants
• fertilisation is the fusion of gamete nuclei
• in sexual reproduction there's a mixing of genetic information which leads to variety (or variation) in the offspring
• asexual reproduction involves only one parent and no fusion of gametes, meaning there is no mixing of genetic information
• in asexual reproduction, offspring are genetically identical (or clones)
• in asexual reproduction only mitosis is involved
• sexual reproduction advantages --

  • increases genetic variation
  • so if the environment changes, variation gives the species a survival advantage through natural selection
  • and disease is less likely to affect the entire population
  • natural selection can be sped up by humans in selective breeding to increase food production and quality
• sexual reproduction disadvantages --

  • takes more time and energy to find mates
  • difficult for isolated members of the species to reproduce
• asexual reproduction advantages --

  • only one parent needed
  • so more time and energy efficient as do not need to find a mate
  • faster than sexual reproduction
  • many identical offspring can be produced when conditions are favourable
• asexual reproduction disadvantages --

  • very limited genetic variation in the population
  • so the population is vulnerable to changes in conditions and may only be suited to one environment
  • and disease is likely to affect the whole population
• some organisms reproduce both sexually and asexually depending on circumstance
• malarial parasites reproduce sexually in the mosquito but asexually in the human host
• fungi reproduce asexually by spores but also reproduce sexually to give variation
• plants produce seeds sexually but also reproduce asexually through runners (strawberry plants) or bulb division (daffodils)

  • 'runners' are stems that grow horizontally away from the parent plant, at the end of which a new identical offspring plant forms
  • 'bulb division' is when new bulbs form from the main bulb underground and then grow into new identical offspring plants
• cells in reproductive organs divide by meiosis to form gametes
• the number of chromosomes must be halved when gametes are formed, otherwise there would be double the normal number of chromosomes in a zygote after fertilisation
• in animals, meiosis occurs in the ovaries and testes.
• in plants, meiosis occurs in the ovaries and anther
• meiosis:

  1. copies of the genetic information are made in the parent cell, so the DNA is replicated
  2. in first division the chromosome pairs line up along the centre of the cell and are then pulled apart so that each new cell has only one copy of each chromosome
  3. second division occurs where the chromosomes line up along the centre of the cell and the arms of the chromosomes are pulled apart
  4. this leaves 4 haploid gametes with a single set of chromosomes
  5. all the gametes are genetically different from each other
• • meiosis leads to non-identical cells being formed while mitosis leads to identical cells being formed
  • mitosis occurs in somatic cells whereas meiosis occurs in reproductive organs only
• • gametes are formed through meiosis in reproductive organs
  • gametes are haploids
  • in fertilisation, gametes join to restore the normal number of chromosomes
  • when male and female gametes fuse a zygote is formed
  • the zygote contains the full number of chromosomes and divides by mitosis to form an embryo
  • cells differentiate as the embryo develops
• a haploid is a cell that has half the normal number of chromosomes (23 instead of 46 in humans)
• a diploid is a cell that has the full set of chromosomes (23 pairs, or 46 chromosomes)
• • DNA is a polymer made up of two strands forming a double helix
  • the two strands are held together by chemicals called 'bases'
  • the genetic material in the nucleus of a cell is composed of DNA
  • DNA is contained within structures called chromosomes
• a gene is a small section of DNA on a chromosome that codes for a specific protein
• the genome of an organism is the entire genetic material of that organism
• The Human Genome Project was an international scientific research project with the goal of determining the sequences of chemical bases which make up all of human DNA
• The Human Genome Project was completed in 2003, so now the whole human genome has been studied.
• understanding the human genome is important because it allows us to:

  • search for genes linked to different types of diseases
  • understand inherited genetic disorders and the treatment for inherited genetic disorders
  • study human migration patterns from the past as different populations of humans living in different parts of the world may have developed small differences in their genome
• • DNA (deoxyribonucleic acid) is a polymer.
  • a polymer is a molecules made from many repeating monomers
  • these DNA monomers are called nucleotides
  • each nucleotide consists of a common sugar and phosphate group (which together forms the backbone of a strand of DNA) and one of four different bases attached to the sugar
• DNA contains four bases : A, T, G and C
• a sequence of three bases is the code for a particular amino acid
• the order of bases controls the order and different types of amino acids that are joined together
• an 'A' base always links to a 'T' base and a 'C' base always links with a 'G' base - this is known as complimentary base pairing
• two bases linked together are held together by a hydrogen bond
• there are 20 types of amino acids
• protein synthesis is the process of producing a protein from DNA
• is a gene is coded to make a protein it has been expressed
• protein synthesis:

  1. proteins are made in the cell cytoplasm ribosomes
  2. ribosomes use the sequence of bases contained within the DNA to make proteins
  3. a sequence of 3 bases is the code for a specific amino acid, and amino acid sequences for a specific protein
  4. DNA contains the genetic code for making a chain of amino acids, but DNA can't travel out of the nucleus - it's too big
  5. so the two strands of DNA are pulled apart from each other and the base code of each gene on the DNA is transcribed onto messenger RNA
  6. the mRNA can move out the nucleus into the cytoplasm
  7. it attaches onto ribosomes