B6 - Inheritance, Variation and Evolution

B6 - Inheritance, Variation and Evolution

1. DNA

   • A chemical called deoxyribonucleic acid
   • In the nucleus of every cell
   • It forms long structures called chromosomes, which are made of genes
     • They are inherited from your parents
     • Each gene codes for a specific protein which is determined by the order amino acids
       • And the order of the different bases determines the amino acid
     • A whole set of genetic material is called a genome
   • A natural polymer made of nucleotides
     • Each nucleotide is made of a phosphate, sugar and base
     • The different bases A and T always pair up and C and G always pair up
   • Proteins are synthesised at the ribesomes
     • Messenger RNA carries a copy of the code to the ribesome
       • Then carrier molecules bring amino acids to the ribesome according to the code

2. Mutations

   • A random change in an organism's DNA is a mutation
   • The chance of a mutation increases when exposed to radiation or certain substances
   • Most mutations have very little or no effect on the protein
     • Though it can prevent a protein carrying out its function
   • There are different types of mutations
     • Insertion is when a new base is inserted into the DNA sequence
       • This changes the way the groups are read and codes for different amino acids
       • It causes a knock-on effect on bases after the inserted one too
     • Deletion is when a bases is taken out of the sequence
       • It is like an insertion in that it changes the whole sequence
     • A substitution is when a base is swapped for another

3. Reproduction

   • Sexual reproduction is when two organisms genetic information is combined
     • Gametes (produced through meiosis) fuse together
     • The offspring inherits characteristics from both parents because there is a mixture of genetics
     • This provides variation in the population
     • Advantages of sexual reproduction
       • Variation increases the chances of surviving a change in the environment
       • Natural selection can occur because individuals with favourable characteristic can breed and pass them on
       • Selective breeding can be used to choose favourable characteristics
   • Asexual reproduction is when one organism clones itself
     • Mitosis produces genetically identical offspring to the one parent
     • There is no genetic variation
     • Advantages of asexual reproduction
       • Only requires one parent
       • Uses less energy
       • Much faster
       • Many identical offspring can be produced in favourable conditions
   • When cells divide by meiosis it produces gametes with half the normal number of chromosomes
     • 1. The cell first duplicates its genetic information forming two armed chromosomes
     • 2. These line up in pairs and are pulled apart into two cells
     • 3. In the second division the arms of the chromosomes are pulled apart
     • 4. All four cells have only a single set of chromosomes and are all different

4. Inherited Disorders

   • Cystic fibrosis is caused by a recessive allele
     • It causes cell membrane to produce a lot of sticky mucus
     • There is a 1 in 4 chance of a child having it if the parents are both carriers
   • Polydactyly is caused by a dominant allele
     • It causes a person to be born with extra fingers or toes
     • It can be inherited even if only one parent carries the allele
   • Embryos can be screened to see if an embryo will inherit a genetic disorder

5. Gregor Mendel

   • Mendel grew peas to investigate how characteristics were inherited
     • His research became the foundation of modern genetics
   • His discoveries
     • He was able to see that some characteristics are dominant over others
     • He discovered that 'hereditary units' re passed through generations unchanged
     • One 'unit' is passed on to the offspring from each parent
   • However his work was not seen as important at the time
     • Scientists did not have to background knowledge of genes or DNA to support it
     • It was published in a lesser known journal
     • Mendel was not part of university or lab at the time - it was carried out at a monastary
     • He was very mathematical in his approach

6. Variation and Evolution

   • Differences within a species is called variation
   • Variation is caused by differences in genetics and the environment
   • Sexual reproduction leads to genetic variation
     • This is through the combining of the mother and father's genetics
     • Mutations introduce variation
   • Survival of the fittest is a theory of evolution put forward by Charles Darwin
     • He concluded that the individuals most suited to the environment would be most likely to survive
       • These individuals would reproduce and pass on their desirable characteristics
       • Over time animals would evolve and change to be suited to their environment
     • Not everyone agreed with Darwin
       • It went against religion
       • He couldn't explain how new characteristics appeared or how they were passed on
       • Not many other scientists had investigated the subject

7. Selective Breeding, Genetic Engineering and Cloning

   • Selective breeding is when human artificially select animals or plants to breed
     • They do this to achieve specific characteristics
       • If they want cows that produce more milk or plants with bigger flowers
     • They can continually select the offspring with the characteristics you want to breed together
       • In each generation the characteristic should get stronger
     • However it reduces the gene pool making them more vulnerable to a change in the environment
   • Genetic engineering is taking the desirable gene(s) from one organisms and inserting them into another
     • The gene is cut using enzymes and inserted into a vector and the vector is put into the organism
     • It can be used to have bacteria make insulin or crops be resistant to disease
     • However it can be controversial because we do not yet know the long-term side effects of GMO
   • Cloning Plants
     • Plants can be clones from a tissue culture when just a few cells are mixed with a growth medium and hormones to stimulate rapid growth
     • Plants can be cloned from cuttings
   • Cloning animals
     • Embryo transplants can create many identical offspring by splitting an embryo and implanting them into surrogate mothers
     • Adult cell cloning turns takes the genome from an adult and putting it in an empty egg cell
       • This is then stimulated to grow and is implanted into a mother
     • Cloning can be very controversial and still causes problems from the cloned animals

8. Fossils

   • Fossils are remains of animals from thousands of years ago
   • They can be formed when the remains of an animal is slowly replaced by minerals
   • Casts or impressions can be left from burrows or roots
   • If something required for decay is missing the organism can be preserved in ice or amber or peat bogs (for example)
   • Fossils can support the theory of evolution by showing how organisms have changed
   • However, because most early life forms were soft-bodied they didn't leave any fossils so scientists don't know how life came into being

9. Speciation

   • Individuals within a species are able to successfully breed to produce fertile offspring
   • Speciation is the development of new species
   • When two populations of a species are isolated and evolution occur separately this can lead to two separate species forming
     • Because the environments are different, different alleles will be favourable and be passed on