Biology topic 6

• Chromosome


  Threadlike structures made of DNA molecules that contain the genes.

  Come in pairs
• Gene


  A segment of DNA on a chromosome that codes for a specific protein.
• Genome


  all of an organism's genetic material.
• DNA structure


  Double Helix.

  Nucleotides

  
• DNA: nucleotides


  thymine (T), adenine (A), guanine (G), Cytosine (C).

  Work in pairs: AT , GC - these are called complimentary base pairs.
• Mutations


  a random error in gene replication that leads to a change in DNA
• sexual reproduction


  involve the fusion of a male and female gamete, offspring's contain a mixture of parents genes

  - 2 parents

  - meiosis

  - half number of chromosomes
• Asexual reproduction


  Process by which a single parent reproduces by itself.

  - genetically identical offspring.

  - mitosis

  - clone

  e.g. bacteria
• Meiosis


  Cell division that produces reproductive cells in sexually reproducing organisms.

  - 4 genetically different daughter cells

  
• Meiosis


  1) Chromosomes are duplicated

  2) the (maternal and paternal) chromosomes pair up in the centre of cell

  3) sections of DNA are switched between maternal and paternal chromosomes.

  4) Chromosomes move to opposite sides of the cell

  5) DIVISION 1

  6) DIVISION 2 - 4 genetically different daughter cells.
• Sexual reproduction pros


  - genetic diversity

  - variation - which increases survival rate of a species

  - more likely to successfully breed

  - can use selective breeding
• asexual reproduction pros


  - Only needs one parent

  - less energy

  - faster

  - identical offspring's are produced
• gender chromosomes


  Female: XX

  Male: XY
• Punnett square


  A chart that shows all the possible combinations of alleles that can result from a genetic cross

  
• Genotype


  the combination of alleles that an organisms possesses for a specific gene
• Phenotype


  the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.
• Heterozygous


  An organism that has two different alleles for a trait

  
• Homozygous


  An organism that has two identical alleles for a trait

  
• Inherited disorder: Cystic Fibrosis

  - caused by a RECESSIVE ALLELE

  - body produces a lot of thick/sticky mucus

  - needs 2 recessive genes
• Inherited disorder: Polydactyly


  - caused by a DOMINANT ALLELE

  - baby born with extra fingers or toes

  - Only needs 1 dominant gene
• Embryonic Screening


  Genetic analysis of an embryo before birth, in order to check that the embryo doesn't carry any genetic disorders.
• Embryonic Screening: pros


  - help to stop suffering

  - treating disorders costs a lot of money.

  - laws to stop it from going too far
• Embryonic screening: cons


  - implies ppl with genetic disorders are 'undesirable'

  - may come a time where people want to choose the most 'desirable' embryo

  - is expensive
• Mendel


  Father of Genetics; responsible of the Law of Inheritance.

  Studied pea plants
• Mendel's 3 conclusions


  1) characteristics in plants are determined by hereditary units

  2) hereditary units are passed from parents, 1 unit from each

  3) hereditary units can be dominant or recessive.
• Variation


  differences in physical traits of an individual from the group to which it belongs (the species)
• Genetic Variation


  Differences among individuals in the composition of their genes or other DNA segments

  e.g. eye colour, skin tone, height
• variation based on the environment

  the environment an organism lives and grows in causes differences between organisms of the same species.

  e.g. scars, height
• characteristics: environmental and genetic


  many characteristics are determined by a combination of environmental and genetic factors.

  e.g. weight, height, skin colour, academic skills
• Variation: Mutations


  mutations make changes to the sequence of bases in DNA.

  most mutations have no effect thought some can have a small effect.

  Its rare (but happens) to change the organisms phenotype
• Evolution


  The gradual change in a species over a period of time
• The Theory of Evolution:


  'all of today's species have evolved from simple life forms' that first developed over 3 billion years ago.
• Natural selection


  Process by which individuals that are better suited to their environment survive and reproduce most successfully; also called survival of the fittest.

  
• Extinction


  the disappearance of a species - when there are no remaining individuals of that species alive
• Reasons for extinction


  - change of environment very quickly

  - new predators

  - new diseases

  - cant compete with a new species for food

  - a catastrophic event e.g. volcanic eruption.
• Darwin's Opposition


  - went against religious beliefs about how the earth was created.

  - he couldn't explain how these useful characteristics developed or how they were passed on.

  - wasn't enough evidence to convince many scientists
• Selective breeding


  The process of selecting a few organisms with desired traits to serve as parents of the next generation
• Selective breeding steps


  1. a human/farmer determines the desired trait

  2. Interbreeds parents who show the desired trait

  3. select the offspring with the best form of the trait and interbreeds these offsprings

  4. continue the process until the population regulary produces the gene/desired trait
• selective breeding: pros


  - get an offspring with (a) desired trait(s)

  - improved quality of food

  - can add new varieties of species

  - can help to eliminate diseases
• selective breeding: cons


  - reduction in gene pool

  - can cause health problems

  - less chance of new disease resistant alleles