Topic 7: Inheritance, Variation and Evolution

• reproduction

  a fundamental life process that an organism must be able to do in order to be considered 'living' - it must occur to ensure the survival of a living species which could be a plant, animal, bacteria, fungus or protist
• Sexual Reproduction:

  • 2 parents
  • Gametes are the male and female sex cells - eggs and sperm in animals and eggs and pollen in plants
  • Variation between parents and offspring caused by combination of genetic material when gametes fuse (fertilisation)
  • Cell division takes place by meiosis
• Advantages of Sexual Reproduction:

  • Creates variation between organisms which aids long term survival
  • Less likely for mutations to be passed on
  • Disease less likely to wipe out populations
  • Helps organisms to adapt to changing environments

  Disadvantages of Sexual Reproduction:

  • Slower and more complex
  • Relies on 2 sex cells; time consuming to find a mate
  • Takes time to reach sexual maturity
  • Finding a mate can make organisms vulnerable to predators
• Asexual Reproduction:

  • Requires 1 parent
  • No gametes
  • No variation as gametes don't join so genetic material isn't mixed
  • Cell division takes place by mitosis
• Advantages of Asexual Reproduction:

  • Faster
  • No need to find a mate as only 1 parent is required
  • Population can increase quickly to colonise a new environment

  Disadvantages of Asexual Reproduction:

  • A genetic mutation could kill an entire population
  • No variation
  • Organisms are generally suited to one environment
• haploid cell/gamete

  a cell with one set of chromosomes (23 chromosomes in humans), represented as n
• diploid cell

  a cell with two sets of chromosomes (46 chromosomes in humans), represented as 2n
• meiosis

  a type of cell division producing gametes (sex cells) which takes place in the sex organs (ovaries and testes), resulting in genetic variation
• Meiosis:

  • DNA replicates during interphase (longest part of cell cycle) - at this point there are 92 chromosomes in the cell
  • Meiosis begins: identical chromosomes (sister chromatids) pair up - still 92 chromosomes
  • Crossing over occurs between sister chromatids
  • Chromosomes line up again and chromatid pairs divide between 2 cells so that there are 46 chromosomes in each cell
  • Chromosomes divide again as their arms are pulled apart into 4 genetically different gametes, each containing 23 chromosomes
• Reproduction of Fungi:

  Asexual:

  • Toadstools contain spores which allow asexual reproduction
  • Hyphae act as gametes and divide by mitosis to form spores
  • These spores germinate to form a new fungus
  • Moulds that rot our foods also reproduce asexually

  Sexual:

  • Hyphae from two different fungi join
  • Nuclei fuse to form zygote
  • Meiosis produces two genetically different spores which germinate to form a new fungus
  • Happens when conditions are too dry
• Reproduction of Malaria:

  1. Mosquito bites an infected human and picks up gametes from their blood
  2. Gametes reproduce sexually in mosquito and fuse to form a parasite
  3. Mosquito bites another human and injects the parasite
  4. Parasite infects liver and red blood cells where they reproduce asexually to make clones of themselves
  5. Some parasites differentiate into gametes through meiosis, which are picked up by a different mosquito to continue the cycle
• DNA (Deoxyribonucleic Acid):

  • Found in the nucleus of cells
  • 46 individual strands in each cell
  • Double helix: made up of two strands that twist into a helical structure
  • The double helix can then be coiled and condensed into a chromosome
  • Contains instructions to control inherited characteristics through protein synthesis
  • Polymer
  • Chemical that all of our genetic information is made of
• Nucleotides:

  • 3 parts: base, sugar and phosphate
  • Base pairs are either A and T (adenine and thymine) bonded with 2 hydrogen bonds OR C and G (cytosine and guanine) bonded with 3 hydrogen bonds
• Genes:

  • Small sections of DNA
  • Where the coded information determining inherited characteristics is stored
  • Each strand of DNA contains thousands of genes
  • Carry a specific sequence of bases that code for a particular order of amino acids
  • The sequence of amino acids then determines which protein is made
  • Proteins then control the functioning of specialised cells where they are made
• Steps of Protein Synthesis:

  1. Desired gene on DNA strand is copied into RNA in the nucleus - this is called transcription
  2. RNA leaves the nucleus, enters the cytoplasm and joins a ribosome where protein synthesis occurs - this is called translation
  3. Carrier molecules bring specific amino acids to add to the growing protein chain
  4. This continues until the particular correct sequence for the amino acids is made so a stop colon is reached
• Differences between DNA and RNA:

  • DNA can't leave the nucleus but RNA can
  • DNA is in a double stranded helix shape whereas RNA has a single strand
  • RNA uses the base 'U' and the base pair 'AU' instead of 'T'
  • RNA is also known as mRNA and the m stands for messenger
• Base Pairs, Amino Acids and Proteins:

  • There are 26 possible sequences of amino acids
  • 3 base pairs = 1 codon = 1 amino acid
  • mRNA is read in 3s and acts as a template as it is a copy of the gene found in the DNA
  • A single strand of amino acids is the primary structure of a protein
  • This then determines the protein's shape to allow it to carry out its specific function
• Coding vs. Non-Coding DNA:

  • When a gene codes for a protein it is said to be expressed
  • All of the coding genes can be put together as RNA
  • Most DNA does not code for proteins but helps to switch genes on and off
  • Each gene can code for many different proteins: it depends on how much of the DNA is switched on or off
  • This explains why our cells can synthesise so many different chemicals with so few genes
• mutation

  a sudden change in the DNA base pair sequence during meiosis or mitosis when DNA is replicated
• 3 types of mutation:

  1. Enzyme changes slightly but the active site stays the same shape so it works normally
  2. Active site changes shape so the substrate no longer fits and the enzyme no longer works
  3. Active site changes shape so that a different substrate fits and a different reaction is catalysed
• Chromosomes:

  • Strands of DNA
  • 46 chromosomes in each human cell except haploids - 2 of each type, 1 from each parent - 23 pairs
  • Structured in a tight coil
  • 23rd pair are sex chromosomes: women have 2 X chromosomes whereas men have 1 X and 1 Y chromosome
• genome

  the entire set of genetic information in an organism
• What is the Human Genome?

  • Project where scientists discovered all of the code to make up the genetic information of humans
  • Allows us to study genes that may be related to certain diseases eg. inherited diseases caused by a specific gene that are inherited from your parents
  • Helps doctors to choose better treatment for patients and allows scientists to develop better treatments
  • Can be used to trace the migration of our ancestors, as all humans share most of the same genome but small differences between genomes show when ancestors separated
• alleles

  different versions of the same gene
• homozygous

  when the 2 alleles we have of a gene are the same
• heterozygous

  when the 2 alleles we have of a gene are different
• dominant

  the allele which is expressed in a heterozygous gene
• recessive

  the allele which isn't expressed in a heterozygous gene and can only be expressed if the gene is homozygous
• genotype

  collection of alleles that we have
• phenotype

  characteristics you have as a result of your genotype
• What increases the risk of mutations?

  • Carcinogens - harmful chemicals such as in cigarette smoke
  • Ionising radiation eg. gamma rays, X-rays
• substitution mutation

  where one base is changed for another random base, changing the amino acid that is coded for
• insertion mutation

  where an extra base is inserted into a sequence, which is more dangerous than a substitution mutation as all of the bases are shifted along and changed
• deletion mutation

  where one base is deleted from the sequence, shifting all of the bases and changing them
• inherited disorders

  a group of conditions that are inherited from a person's parents as they are passed on in alleles
• Polydactyly:

  • Person born with extra fingers or toes (doesn't cause any problems)
  • Caused by dominant allele
• Cystic Fibrosis:

  • Caused by recessive allele (this makes it a recessive condition)
  • Disorder of cell membranes causing thick mucus to be released in lots of parts of the body, specifically airways of lungs and pancreas
  • 1 in 25 people are carriers as they have the heterozygous allele
• Embryo Screening:

  • Allows embryos' genes to be looked at before IVF to determine if they will have any genetic disorders

  Advantages:

  • Reduces amount of suffering as a result of illness
  • Saves money - treating disorders are expensive

  Disadvantage:

  • Ableism - implies that people with disabilities are less desirable
  • People may use embryo screening to create 'designer babies' in the future
• Gregor Mendel:

  • 'Founding father' of genetics
  • Austrian monk and scientist who lived in 1800s
  • When conducting his work, Mendel already knew that we could cross breed plants to get the most desirable offspring but didn't know why
  • Experimented with pea plants and studied how certain traits were passed on through generations - an example was breeding pea plants with yellow pods and pea plants with green pods, which revealed that the yellow colour was the dominant gene while green was recessive
  • He did the same experiment with plant heights and the colour of flowers
• variation

  genetic differences between organisms