IB Biology : Genetics

• The genome is the complete set of genetic material found within an organism.
• Mutations are changes that occur in DNA sequences, which can lead to new traits or variations in existing ones.
• Describe the Hershey and Chase experiment.

  radioactive isotopes used to label viruses/bacteriophages/phages

  b. proteins labelled with radioactive sulphur/35S and DNA labelled with radioactive

  phosphorous/32P

  c. phage infects bacterium

  d. only viral DNA enters bacterium «viral coat/capsid/shell do not»

  e. parts of phage remaining outside bacterial cell are removed

  
  

      bacteria are separated from phage parts «by centrifuge»

  f. bacteria contain the labelled/radioactive DNA
• Genome : the individual entire set of DNA in an organism, including all genes and all non-coding DNA
• Process of Meiosis 


  Meiosis I: Prophase I, Metaphase I, Anaphase I, Telophase I and Cytokinesis. Meiosis II: Prophase II , Metaphase II, Anaphase II, Telophase II and Cytokinesis
• Meiosis I: Prophase I


  1. Chromosomes condense

  2. Homologous chromosomes pair up

  3. Crossing over occurs, where portions of chromatids may exchange between homologous chromosomes, increasing genetic variation

  4. Nuclear envelope breaks down

  5. Spindle fibres begin to form
• Meiosis I: Metaphase I

  1. Fibres align at the metaphase plate

  2. Spindle fibres attach to the centromere of each homologous chromosome

  3. Random Orientation of homologous chromsome occurs (Different combinations of chromosomes can be produced)

  • Independent Assortment: Random orientation reduces the chance of daughter cells having 50/50 genes from maternal and paternal chromosomes )
• Meiosis I: Anaphase I


  1. Homologous chromosomes are pulled apart by the spindle fibers and move to opposite poles of the cell

  2. This reductional division halves the chromosome number
• Meiosis I: Telophase I and Cytokinesis


  1. Chromosomes may decondense

  2. Nuclear envelope may reform

  3. Cell divides into two haploid cells

  4. Each cell has one chromosome from each homologous pair
• Meiosis II: Prophase II


  1. Chromosomes condense again

  2. Nuclear envelope breaks down if it had reformed

  3. Spindle fibers start to form
• Meiosis II: Metaphase II

  1. Chromosomes align at the metaphase plate

  2. Spindle fibers attach to the centromeres of sister chromatids
• Meiosis II: Anaphase II


  Sister chromatids are pulled apart and move to opposite poles of the cell
• Meiosis II: Telophase II and Cytokinesis

  1. Chromosomes decondense

  2. Nuclear envelopes may reform

  3. Cells divide

  4. This results in four genetically distinct haploid cells
• Fertilisation 


  the fusion of gametes to form a zygote with a unique combination of alleles from each parent.
• Gene composed of DNA that codes for a polypeptide, linked by a peptide bond
• chromosome 


  A chromosome is a long, organised structure of DNA and associated proteins (histones) that store genetic information.
• Humans gave 46 chromosomes in each cell, arranged in 23 pairs
• • Chromosomes contain many genes as well as non-coding regions of DNA that play roles in the regulation of gene expression and the protection of the chromosome's integrity.
• locus:  the location of a gene in a chromosome is known as its locus

  • Each gene occupies a specific locus so that the gene for a particular characteristic is always found at the same position on a particular chromosome. 
• Allele: an alternate (variation) of a gene
• genotype

  Homo: 2 alleles at a locus are identical 

  Hetero: 2 alleles at a locus are different 
• Single nucleotide polymorphisms (SNPs),

  occurs when one or a few nucleotides in a gene locus are changed - leading to the formation of new alleles. 
• Mutation 


  a gene mutation is a change in the sequence of base pairs in a DNA molecule: resulting in a new allele.
• Organism and Genome size (millions of bp)

  T2 phage 0.18

  Escherichia coli  5

  D. melanogaster  140

  Homo sapiens  3,000

  Paris japonica  150,000
• define the terms gene and allele and explain how they differ

  A gene is a heritable factor and is composed of DNA. A gene codes for a specific characteristic, polypeptide.

  An allele is a variation of a gene and occupies the same gene locus ( position of a chromosome). Alleles differ by one/ small number of base pairs. Alleles provide different variations for a gene e.g hair colour
• gene 


  the specific sequence within the chromosome that codes for a particular protein or polypeptides and controls specific characteristics
• EXAMPLES of non-coding DNA: Introns, Structural telomeres