Topic 3: Genetics

• Asexual reproduction:

  • rapid reproductive cycle (advantage)
  • only one parent needed so resources aren't wasted for a mate (advantage)
  • no genetic variation so disease/mutations are passed on forever(disadvantage)
• Sexual Reproduction:

  • has genetic variation so organisms can adapt using natural selection (advantage)
  • resources like time and energy are needed to produce fewer offspring (disadvantage)
  • two parents are required therefore increasing chances of extinction (disadvantage)
• Meiosis creates 4 haploid daughter cells:

  • chromosomes copy and pair into 23 pairs
  • cell division happens twice to create gametes (sperm/egg cells)

  Gametes need to be haploid so they can combine to form a zygote (diploid) at fertilisation
• Genome - the complete set of DNA/genetic material in an organism

  Gene - short length of DNA found on chromosomes that codes for a particular characteristic or protein
• DNA is found in the nucleus of eukaryotic cells. A molecule of DNA has a structure of:

  • two long strings coiled together to form a double helix
  • helix is held together by complementary base pairs
  • 4 bases paired are adenine and thymine/guanine and cytosine
  • held by hydrogen bonds (2 for adenine/thymine and 3 for guanine/cytosine
  • slight electrical charges in bases create the forces of attraction to form a complementary pair
  • each strand is a nucleotide (base sugar phosphate backbone) where a base is attached to a sugar attached to a phosphate group
• DNA Fruit Extraction Practical: (1)

  Safety - wear eye protection (goggles) and gloves

  1. Dissolve 10g salt in 100cm3 water in a large beaker (salt clumps/binds DNA)
  2. Add 10cm3 washing-up liquid and stir gently so mixture doesn't foam as salt dissolves (detergent breaks down cell and nucleus membrane to release DNA)
  3. Mash 50g fruit and put into empty beaker (crushing fruit breaks down cell walls)
  4. Add earlier solution and slowly stir for 1 minute
  5. Place beaker in 60'C water bath for 15 minutes (optimum temperature to increate rate of reaction)
• DNA Fruit Extraction Practical: (2)

  6. Filter mixture and collect filtrate in small beaker (solution with DNA clumps)

  7. Measure 10cm3 filtrate and pour into boiling tube

  8. Add 2 drops protease enzyme solution (breaks down enzymes that would've broken down DNA which we can't collect)

  9. Tilt boiling tube and pour ice cold ethanol slowly so it runs down tube side and stop when amount is equal to the filtrate (separates DNA and solution)

  10. Leave tube for a few minutes for DNA to precipitate into ethanol (insoluble so it will be visible)
• Proteins are made in ribosomes. It's shape is dependent on the amino acid chain sequence which comes from the non-coding region of the DNA it originates. The process is called protein synthesis which has the 2 stages transcription then translation
• Transcription - DNA bases being used to make RNA strand in the nucleus

  • Enzyme called RNA polymerase attaches to DNA in front of a non-coding region gene
  • Hetice enzyme unzips DNA before polymerase attaches
  • Enzyme separates DNA strands
  • Enzyme moves along template strand, adding complementary DNA nucleotides where uracil replaces thymine
  • Nucleotides linked to form strand of messenger mRNA which leaves nucleus through nuclear pores
  • DNA strands join back together
• Translation (1) - polypeptide is created in ribosome then folds into a protein

  • Newly formed mRNA leaves nucleus with message for protein synthesis for ribosome
  • mRNA strand attaches to ribosome cytoplasm
  • tRNA recognises and binds corresponding codon in ribosome transferred from mRNA
  • tRNA transfers appropriate amino acid to end of growing amino acid chain
• Translation (2) - polypeptide is creates in ribosome then folds into a protein

  • tRNA reads off mRNA in codons to create anticodons which are complementary
  • tRNA molecule with complementary anticodon lines up and carries specific amino acid
  • As ribosome moves along, amino acids from tRNA strand join together forming polypeptide chain
  • Polypeptide folds into protein/enzyme with specific shape (some have multiple polypeptides)
• Mutation - a change in a gene/chromosome that can lead to a change in the phenotype as a different amino acid being coded changes the polypeptide function
• Mutations can be random, genetically passed down, caused by environmental factors (evolution) which can change phenotype and when DNA isn't replicated properly in cell division
• Genetic variants (mutations) cause genes to exist in different forms called alleles.

  Mutations in the non-coding region may cause RNA polymerase to not bind well which reduces transcription and overall protein activity (different amino acid order means differently shaped protein).
• Gene transcription is regulated by limiting the amount of mRNA produced from singular genes so multiple genes are used
• Mendel observed how many characteristics were present or absent but not blended together in pea plants:

  • fertilised short female pea plant with tall male pollen then isolated seeds to prevent contamination
  • first generation contained only tall plants
  • second generation contained some short plants

  Conclusion - inherited factors (alleles) control variation of characteristics
• Mendel was ignored until chromosomes were discovered as scientists as the time believed it contradicted Darwin's theory of evolution and didn't explain other variations like eye colour
• Alleles are variations of genes. Alleles effect inherited characteristics depending on if they are dominant or recessive which may lead to an organism carrying multiple traits.

  This is possible as recessive alleles don't show in phenotype when there is a dominant allele in the genotype.
• Genotype - the alleles an organism has

  Phenotype - what the organism looks like because of the organisation of alleles

  Dominant - if the allele is present it will always be expressed in the phenotype

  Recessive - both alleles are needed to be expressed in the phenotype

  Homozygous - both alleles for a genotype are the same (AA or aa)

  Heterozygous - the alleles for the genotype are different (Aa)
• Punnett squares are used to work out the possible combination of alleles in the parents' offspring. This also shows how sex is an equal chance
• Everyone has one of the 4 blood types: A, B, AB or O

  • O is recessive to A and B which are both dominant alleles
  • a person with the genotype AB will show phenotype for both as they are codominant
• Sex-linked genetic disorders like red-green colour blindness are inherited through one of the sex chromosomes.

  If the X chromosome codes disorder, men always have it as there isn't an allele in the Y chromosome to mask the effect
• A polygenetic trait is a characteristic, like height/skin colour, that is influenced by multiple genes
• Mapping the human genome:

  • create medicine which can sequence the DNA in cancer cells (advantage)
  • find specific cures (advantage)
  • find causes for genetic mutations (advantage)
  • discrimination in workplaces by possible inherited disorders (disadvantage)
• Variation is only possible due to mutations creating alleles which make each individual unique in their hair/skin colour, height, shape, behaviour and susceptibility to disease
• Mutations that effect phenotype include sickle cell anaemia and freckles.

  Many mutations have no effect because small changes in DNA sequences don't change how cells function
• Genetic variation is caused by mutations and the mic of characteristics caused by sexual reproduction while environmental variation includes factors like climate, diet and accidents.
• Characteristics are effected by both genetic and environmental factors like height as it depends on a person's diet as well as genes
• Genetic Variation - differences inherited from parents (eye/hair/skin)

  Environmental Variation - differences caused by the environment lived in (scars/tattoos)

  Continuous Variation - characteristics that changes in a range of values (bell-shaped graph for height)

  Discontinuous Variation - grouped values (bar charts for blood type)