BIOL 211 Lecture Notes - Lecture 5: Circular Bacterial Chromosome, Cell Division, Dna Replication

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8 Feb 2017
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Lecture 5 Notes
Overview of Cell Division
Prokaryotic/eukaryotic cells are the fundamental building blocks of organisms
All of today’s living cells were made by the division of a previous living cell (all cells
come from previous cells)
Cell division critical for reproduction in all living organisms
o Unicellular organisms: simple cell division involves one cell producing two
identical daughter cells; asexual
o Unicellular/multicellular eukaryotes: more complex type of cell division (meiosis)
is critical in producing gametes (sperm, eggs) used in sexual reproduction
In multicellular organisms, simple cell division is also important for development, growth
and regeneration (some animals can grow their limbs back)
In all types of cell division, four events must occur:
o A reproductive signal to initiate cell division
o Replication of DNA
o Segregation, or distribution of DNA into two new cells
o Cytokinesis: separation of the two new cells
Three main types of cell division:
o Prokaryotes:
Binary fission a parent cell divides to form two daughter cells that are
genetically identical to the parent cell
o Eukaryotes:
Mitotic cell division a parent cell divides to form two daughter cells that are
genetically identical to the parent cell
Meiotic cell division a parent cell divides to form four daughter cells that have
only half as many chromosomes as the parent; daughter cells are genetically
different than the parent cell
Prokaryotes: Binary Fission
Most of the genome is in a single circular bacterial chromosome (there are some smaller
rings of DNA called plasmids)
Cell divison occurs by binary fission
(A) Reproductive signals: Initiation of binary fission seems to depend mostly on cell size
o Once a prokaryotic cell reaches a certain size, binary fission starts
o Size is controlled largely by external factors (i.e. food in the environment)
o Under good conditions (high food, appropriate temperature), cells of the
bacterium E. coli can divide about once per 20-30 minutes
(B) DNA replication: the bacterial chromosome has two regions important in replication
o Ori (origin) where replication starts
Two strands of the double helix are separated and DNA replication
happens in both directions around the circular chromosome, ending at the
ter site; the two copies then separate
o Ter (termination) where replication ends
o DNA replication is really accurate; estimates of copying error frequency are
around 1 error per 109 nucleotides added
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