Module 6: Section 1A

Question 1

Griffith discovers transformation and launches the field of molecular genetics - 1928

Answer 1

• Griffith showed that bacteria can transfer “information” between cells in a process he called transformation
• Later research revealed that this transferred material was DNA, and Griffith’s work essentially founded the field of molecular genetics

Question 2

The influenza pandemic of 1918-1919

Answer 2

• Killed between 20–40 million people, more than World War I, and left medical and scientific professionals unsure of its cause
• Its massive impact shifted scientific focus onto controlling infectious diseases, including pneumonia caused by Streptococcus pneumoniae

Question 3

Griffith’s Pneumonia Vaccine Study (Late 1920s)

Answer 3

• Griffith studied creating a pneumonia vaccine using two S. pneumoniae strains in mice:
  
  • A virulent type III-S (smooth) strain
  • A nonvirulent type II-R (rough) strain

Question 4

Griffith’s Pneumonia Vaccine Study - The experiment

Answer 4

• Heat-killed III-S bacteria were injected into mice along with live II-R bacteria; although each strain alone was harmless, the combination killed the mice
• Griffith isolated live bacteria of both types from the dead mice and concluded that some part of the III-S strain “transformed” the II-R strain into virulent bacteria

Question 5

Avery, MacLeod & McCarty (1944)

Answer 5

Avery, MacLeod, and McCarty showed that DNA is the substance responsible for bacterial transformation, overturning the belief that proteins carried genetic information

Question 6

Creation of a bacterial cell controlled by a chemically synthesized genome - 2010

Answer 6

• In 2010, a research team led by Craig Venter created a bacterial cell controlled by a completely synthetic 1.08-Mbp genome made from Mycoplasma
• They included “watermarks” to distinguish it from the natural genome, and the new strain (Mycoplasma mycoides JCVI-syn1.0) has additional differences from the wild type but remains viable

Question 7

Synthetic genome synthesis - Step 1

Answer 7

Short, overlapping stretches of DNA 1080-bp in size were synthesized

Question 8

Synthetic genome synthesis - Step 2

Answer 8

These were assembled in sets of 10 to produce 109~10kb pieces

Question 9

Synthetic genome synthesis - Step 3

Answer 9

These were assembled into 11~100kb fragments

Question 10

Synthetic genome synthesis - Step 4

Answer 10

The 11 fragments were joined to form the complete genome

Question 11

Expanding the genetic code - 2012

Answer 11

• In 2012, American scientists designed an unnatural base pair (UBP), a laboratory-created nucleotide that does not exist in nature
• In 2014, they synthesized a plasmid containing natural T-A and C-G base pairs plus UBP’s and inserted it into E. coli, where it replicated
• This research suggests UBP’s could allow new amino acids to be added to proteins for pharmaceutical or industrial applications

Question 12

CRISPR-Cas microbial adaptive immune system - 2010’s

Answer 12

• CRISPR-Cas was discovered and engineered as a tool to manipulate genes in plants and animals
• CRISPR-Cas system occurs in about half of all bacteria
• Understanding and developing CRISPR-Cas for genome editing took 20 years of research across many labs worldwide, including work by Sylvain Moineau in Canada

Question 13

What does CRISPR stand for?

Answer 13

Clustered regularly interspaced short palindromic repeat

Question 14

Immunization to phage via CRISPR-Cas

Answer 14

• CRISPR loci contain short spacer sequences from bacteriophages, are expressed from a promoter, and are flanked by cas genes that encode the immunity machinery
• When a phage infects a bacterium, its DNA is cleaved and inserted into the CRISPR array, acting as an immunization step

Question 15

Mechanism of developing immunity to phage via CRISPR-Cas

Answer 15

During later phage infections, the phage-derived spacer is transcribed and processed into crRNA, which guides the Cas nuclease complex to the matching phage DNA and cleaves it, preventing infection