Chap 16: Inheritance

• Definitions

  Dominant: An allele that is expressed in homozygotes and heterozygotes

  Allele: A different version of a gene

  Locus: Position of gene on chromosome

  Homozygous: 2 identical alleles of a gene

  Bivalent: a pair of chromosomes 

  Chiasmata: point where crossing over takes place

  Sex-linked: allele/gene carried on the X chromosome
• Explain why meiosis is necessary in the life cycle of sexually reproducing organisms.

  • To half the chromosomes from 46 to 23
  • To make haploid gametes
  • So during fertilisation, restores diploid number
  • To prevent chromosomes from doubling
• Meiosis I: reduction division resulting in 2 daughter nuclei (haploid) half the number of chromosomes of the parent nucleus 
• Prophase I:

  • Chromosomes condense and become visible
  • Homologous chromosomes are very close together, allowing crossing over with non-sister chromatids to occur
  • Centrioles migrate to opposite poles and the spindle fiber is formed
  • Nuclear envelope breaks down and nucleolus disappears
• Metaphase I:

  • Bilavents line up along the equator of the spindle
  • Spindle fibers are attached to the centromere
• Anaphase I:

  • Spindle fibers shorten and pull chromosomes to opposite poles
  • Centromere moves
  • Homologous chromosomes separate/pulled apart
• Telophase I:

  • Homologous chromosomes arrive at opposite poles
  • Spindle fiber starts to break down
  • Nuclear membrane forms around the two groups of chromosomes and nucleolus reforms
• Cytokinesis: division of cytoplasm occurs

  Animal cell

  • CSM pinches inwards
  • Creating a cleavage furrow in the middle of the cell which contracts,
  • Diving the cytoplasm in half

  Plant cell

  • Vesicle from Golgi apparatus gather along the equator of the spindle 
  • Vesicles fuse with each other to form new CSM
  • Layers of cellulose are laid upon cell membrane - form primary + secondary walls of cell
• Meiosis II behaves like mitosis and results in a total of four haploid nuclei
• Prophase II:

  • Nuclear envelope breaks down and chromosomes condense
  • Nucleolus disappears
• Metaphase II:

  • Chromosomes line up in a single file along the equator of the spindle
• Anaphase II:

  • Centromere divides
  • Spindle fibres pull sister chromatids apart to opposite poles
• Telophase II:

  • Nuclear membrane forms around each group of chromosomes
• Cytokinesis:

  • Cytoplasm divides as new cell membranes are formed, creating 4 haploid cells
• Describe how crossing over during meiosis leads to genetic variation

  • Non-sister chromatids
  • Exchange or swap alleles
  • At the chiasmata
  • This gives new allele combinations
• Describe how independent assortment during meiosis leads to genetic variation

  • Random alignment of bivalent on the equator during meiosis I
  • Different alleles of genes on different chromosome may end up in any combination in gametes
• Describe process of fusion of gametes

  • Meiosis creates genetic variation: crossing over and independent assortment
  • Gametes carries different alleles
  • During fertilisation, any male gamete can fuse with any female gamete to form a zygote
  • Random fertilisation → genetic variation between zygotes, each will have unique combination of alleles
• Explain how mutation of TYR gene can result in albinism

  1. Base substitution/deletion/insertion or frameshift
  2. Change in primary/secondary/tertiary structure of polypeptide or change in shape of active site
  3. Ref. to stop codon
  4. No tyrosinase produced
  5. Tyrosine is not converted to DOPA
  6. Melanin is not formed
• TYR gene - Albinism

  •  TYR gene codes for tyrosinase 
  • Albinism - lack of pigment melanin in skin, hair, eyes → pink eyes (poor vision) + white hair
  • On chromosome 11
  • Tyrosine → DOPA → dopaquinone → melanin 
•  HBB - sickle cell anaemia

  • HBB codes for B-globin → haemoglobin
  • Change in HBB: Glutamic acid (CAC) → valine (CTC) lead to distorted B-globin
  • HBB on chromosome 11
  • Sickle cell anaemia - half moon shaped RBCs, unable to store much O2, clump together
•  F8 gene - haemophilia 

  • F8 codes for factor VII → coagulating agent for blood clotting
  • Sex-linked allele on X chromosome
  • Haemophilia - lack of normal factor VII prevents normal blood clotting
• HTT - Huntington’s disease

  • HTT gene codes for huntingtin - on chromosome 4
  • Repeated triplet CAG
  • Abnormal allele is dominant
  • Hungtington’s disease - neurological disease, unable to walk, think and talk
• Structural gene

  • Codes for a protein that has a function within the cell 
  • E.g. F8 gene codes for Factor VII
  • Codes for proteins associated with rRNA and tRNA
• Regulatory gene

  • Codes for a protein that helps to control the expression of another gene
  • Regulatory protein
  • Prokaryotes → repressor protein
  • Eukaryotes → transcription factor
• Explain why enzyme D is described as inducible.

  • Not made all the time
  • Gene switched on/protein made when needed
  • Triggered by a change
  • Concentration increases when galactose is present
• Gene control: Lac Operon 

  When lactose is present

  1. Regulatory gene LacI codes for a repressor protein 

  • Has 2 binding sites
  • Binds to operator and lactose 

  1. Repressor protein binds to the operator and prevent transcription 

  • RNA polymerase cannot attach to the promoter

  1. When lactose binds to repressor protein → shape of repressor protein distorts and no longer binds to the operator

  • Unblocking the promoter → increases transcription
• Gene control: Lac Operon

  When lactose is absent

  1. Due to repressor protein, RNA polymerase is unable to bind to the promoter region
  2. Transcription of structural genes (LacZ, LacY, LacA) does not take place
  3. No structural protein synthesis → decreases transcription

  • LacZ does not synthesise lactase
  • LacY does not synthesise lactose permease
  • LacZ does not synthesise transacetylase 
• Function of transcription factors (TF) in gene expression in eukaryotes

  1. TF can form part of protein complex
  2. TF bind to DNA/promoter/enhancer
  3. So RNA polymerase binds to promoter
  4. Transcription begins - mRNA synthesised - gene expressed or switched on
  5. Or TF binds to DNA - no transcription/no mRNA synthesised - gene not expressed or switched off
  6. Can activate genes in correct order/time/amount
  7. Allow response to environmental stimuli
  8. Cell signalling → response to hormones
  9. Regulate cell cycle → role in cell cycle checkpoints/apoptosis
• Control of Gibberellin synthesis and outline how gibberellin stimulates elongation

  1. Dominant allele (Le) codes for functional enzyme
  2. Enzyme produces activate gibberellin 
  3. DELLA protein inhibits transcription factor/prevents transcription
  4. Gibberellin binds to receptor 
  5. Causes destruction of DELLA protein
  6. Transcription factor (PIF) binds to DNA/promoter
  7. Gene is switched on or expressed - transcription occurs
  8. Cell walls loosen
  9. Cells can expand when water enters - cell elongation
  10. Ref. to expansins/interactions with auxins
• How Le/le and gibberellin affects growth

  • Short /dwarf plants are homozygous recessive (le le)
  • le/recessive allele codes for non functional enzyme
  • le/recessive allele gives inactive gibberellin
  • DELLA proteins not broken down
  • Transcription factor cannot bind to promoter
  • No cell growth/cell elongation