Module 3 - Genetic variation and heredity in cells

• Meiosis

  • Reduces the chromosome number by half
  • Produces Haploid gametes
  • The total amount and content of genetic information is maintained through each generation
  • Occurs in specialised diploid gonad cells
• Asexual reproduction

  • One parent cell
  • Genetically identical
  • By mitosis or binary fission
• Sexual reproduction


  • Two parents
  • Genetically different
  • Gametes and fertilisation
• Virus

  • Particles of Nucleic Acid coated with Protein (sometimes lipids too)
  • Reproduce by infecting specific host cells
  • Genes Packaged in protein coats
• How do viruses infect specific host cells?

  • Attachment of virus to receptor (using specificity)
  • Injection of nucleic acid >OR endocytosis >OR fusion of viral envelope with membrane
• Contrast the reproduction of a virus with that of a living cell

  Virus:

  • Requires a host cell
  • Replicates itself through the cell, destroying the cell in the process

  Living cell:

  • Mitosis/cytokenesis

  > Replication/Division

  • Limited nutrient availability
• Describe the main structural components of viruses and explain what processes are used to make them

  Structure:

  • Particles of nucleic acid coated with protein (sometimes lipids too) which reproduce by infecting specific host cells
  • "genes packed in protein coats"

  Viral Nucleic acid:

  • To store genetic information

  Types:

  > RNA or DNA

  > Double-stranded or single-stranded

  • Each virus has its own specific type of nucleic acid
  • Viruses can be double or single-stranded RNA or DNA

  Capsid:

  • Protein coat of a virus that surrounds the nucleic acid

  
• Bacteriophage


  Viruses that can affect bacteria
• Lytic reproductive cycle of a bacteriophage


  1. Tail fibres of the phage stick to receptor sites of host bacterium (e.g. E. coli)

  2. Phage injects DNA into the host cell leaving empty protein coat behind

  3. DNA of host cell is destroyed and host cell's enzymes and nucleotides are used to replicate the phage DNA

  4. Host cell's enzymes and ribosomes transcribe phage genes and translate them into phage proteins

  5. Phage parts accumulate and assemble to form phages

  6. Phage enzymes digest cell wall and cell ruptures

  7. ~200 phage cells will spill out

  8. Each phage cell will infect a new cell
• Viral Reproduction Process


  1. Entry

  2. Replication

  3. Transcription and Translation

  4. Self-assembly
• Viral General Reproduction and Reproduction Strategies

  • Attachment of virus to receptor (basis of specificity, targets specific cells and sometimes specific species)
  • Injection of nucleic acid or endocytosis or fusion of viral envelope with membrane
  • Progeny particles are assembled from the modules (parts)
  • DNA virus takes over the cell reproduction system
  • Viruses self-assemble, they will come together automatically once all the pieces are there
  • Cells double the DNA and split into 2 daughter cells vs having enough modules to make 100 or more viruses in one cell at a time

  
• HIV


  • Covered with an envelope
  • Forms glycoproteins on the envelope to recognise and bind to receptor proteins on the host cell
• Protein coat


  Surrounds the genetic material (2x single-stranded DNA)
• Reverse transcriptase enzyme


  Enables virus to make DNA from RNA template
• Reproductive cycle of a retrovirus


  1. RNA molecules of HIV enter the host cell

  2. Virus fuses with cell's membrane

  3. Protein coat is removed by enzymes

  4. Reverse transcriptase catalyses the synthesis of a DNA strand complementary to the viral RNA strand

  5. Second DNA strand complementary to the first is synthesised

  6. New double-stranded DNA is incorporated as a provirus into the host cells chromosomal DNA

  7. Provirus is transcribed through both RNA and mRNA for protein formation and genetic material for the next generation of viruses

  8. Protein coats form around RNA and reverse transcriptase molecules

  9. Viruses bud off from the host cell, acquiring their own envelopes as they leave
• What is a retrovirus?

  A type of RNA virus that reproduces by reverse transcribing i its RNA into DNA then inserting the DNA into the host chromosome
• What are some components of viruses

  • Lipid envelope
  • Glycoproteins
  • Reverse transcriptase (retro only)
• Reproductive cycle of Retroviruses like HIV (diagrams)


  .

  
• Explain why viruses can change so quickly and the consequences for human and animal health 


  Viral genomes change/mutate very rapidly

  1. A large number of viral particles are produced
  2. RNA viruses have a very high rate of mutation (RNA polymerase do not correct mistakes)
• Where do enveloped viruses get their envelope from?

  They take some of the host's cell membrane before budding off from the host cell
• What are the different appearances of viruses 


  .

  
• Give examples of how some bacteria are beneficial to the organisms of the environment

  Bacteria can...

  • Make antibiotics
  • Clone genes
  • Fix nitrogen to make plants grow
  • Break down pollutants
  • Produce plastics
  • Produce fuel
• Bacteria cell defence against bacteriophages

  • Some bacteria cells can protect their DNA by cutting up viral DNA with restriction enzymes

  CRISPR-Cas

  Restriction enzymes

  • Stores some DNA from previous infections, and they are now immune against these bacteria
  • Makes cas protein, that will find invader DNA, so that it can cut it up

  
• Describe and distinguish the three primary mechanisms that contribute to horizontal gene transfer in bacteria

  * Bacteria evolve by horizontal gene transfer (HGT) as well as by verticle transfer of genes from parents

  * Transfer of genes from between cells that are existing at the same time

  • Transduction

  DNA transfer mediated by bacteriophage

  • Conjugation

  DNA transfer mediated by plasmids

  • Transformation

  DNA is taken up from the environment

  
• What is Transduction (as a mechanism of horizontal gene transfer)?

  1. Phage infects bacterial donor cell
  2. Phage DNA is replicated and phage proteins are synthesized
  3. Fragment of bacterial DNA with A+ allele is packaged in a phage capsid
  4. Phage containing A+ allele will infect bacterial recipient cell
  5. The incorporation of DNA from the phage creates recombinant cells

  • When phages infect and replicate in bacterial cells, errors can occur, and a phage is produced which contains bacterial DNA.
  • This DNA can then be transferred into another bacterial cell

  
• What is conjugation (as a mechanism of horizontal gene transfer)?

  conjugation- Bacterial 'sex'

  • The ability of gene transfer was shown to be acquired through the F(fertility) plasmid in the 1950s
  • Different bacteria were mixed to produce bacteria with new gene combinations + phenotypes
  • F Plasmid + Bacteria = New gene combinations

  ** Requires cell contact, replicates autonomously in bacterial cell

  
• What is the bacterial advantage of bacterial conjugation?

  • Can transfer chromosomal genes, with the insertion of the F plasmid
  • When transferred, the F plasmid also includes the original donor chromosome also transfers new DNA with similar attributes (A+,A-). Recombination, replacement
  • This occurs at a high frequency ie Hfr is a high-frequency recombination
• What is Transformation (as a mechanism of horizontal gene transfer)?


  • A process in which bacteria can take up and incorporate foreign DNA from their surroundings
  • During the process, bacterial cells become competent, or capable of taking up extracellular DNA fragments and integrating them into their genetic material
  • This allows scientists to manipulate the genetic material of bacterial cells and study the functions of specific genes or traits
• What induces a physiological change

  Can be induced by high calcium or salt concentrations
• Recombinases


  Specific enzymes required for integrating foreign DNA into bacterial cell's genome
• Transformation (as a mechanism of horizontal gene transfer)

  1. Bacterial cells undergo physiological change to competently take up DNA; This physiological change can be induced by high salt or calcium concentrations
  2. DNA fragments must be single-stranded to bind to receptors on bacterial cell surface
  3. Foreign DNA must be integrated into bacterial cell's genome using recombinases
  4. Integrated DNA can then be expressed by bacterial cells leading to the production of new proteins/other cellular changes
• Why are bacteria so diverse?

  • They can quickly reproduce
  • Have large population sizes
  • Have diverse ways of regeneration
  • lots of genetic variation

  
• Describe the properties of plasmids that allow them to mediate horizontal gene transfer

  • F plasmids contain genes that produce a pilus that allows the plasmid to be transferred to a recipient cell.
  • Plasmids are transferred through a mating bridge formed by the pilus

  Plasmid- small double-stranded DNA molecule (similar to a chromosome) that sits inside the bacterial cell, has a relatively small number of genes and the plasmid DNA is transferred through a bridge

  
• What is the difference in reproductive methods between a virus and a living cell?

  Virus:

  • Requires host cell
  • Replicates itself through the host cell and destroys it

  Living cell:

  • Mitosis/cytokenesis
  • replication/division
  • limited by nutrient availability
• Explain the rapid rate of mutation in viruses and it's consequences for human and animal health


  Viral genomes mutate rapidly

  • Large number of viral particles are produced
  • RNA viruses have a very high rate of mutation (RNA polymerase does not correct mistakes) compared to DNA
• How does horizontal gene transfer contribute to the rise of antibiotic bacteria?

  • When genes are transferred through HGT, recipient bacterial cells can acquire new traits such as antibiotic resistance
  • Antibiotic resistance is a major and growing threat to human health and horizontal gene transfer by R (resistance) plasmids is a major driver
  • HGT helps antibiotic resistance develop quickly

  
• Whats the difference between Asexual and Sexual reproduction ?

  Asexual Reproduction

  • One parent/cell
  • Genetically identical
  • By mitosis or binary fission

  Sexual Reproduction

  • Two parents
  • Genetically different
  • Gametes and Fertilisation
• Distinguish between sister homologous chromosomes and sister chromatids, draw or label chromosome switch alleles to represent these and explain what happens to them in meiosis

  • Same size and shape
  • Same genes at the same loci (centrosome)
  • Each gene at different alleles (2 parents AA, aa)

  
• Describe the role of sex chromosomes in mammalian sex determination and their behaviour in meiosis

  XY MALE XX FEMALE

  What does the Y chromosome do?

  -SRY gene is responsible for 'maleness' and code for regulatory protein TDF (testis determining factor)

  During meiosis:

  • Females (xx) pair up and separate = haploid with one x each
  • Males (xy) Pair up at the end, can have recombination or crossing over. Resulting in 4 haploid cells, only 1/2 carrying y

  ***An unusual number of sex chromosomes will result in infertility
• What does Aneuploidy mean?

  That there is and abnormal number of autosomes or sex chromosomes