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Summary Genetics Chapter 10
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Chapter 10: molecular structure ofchromosomes and transposable
elements
Chromosomes = structures within living cells that contain the genetic material.
Genome = the entire complement of genetic material in an organism or species.
- Bacteria: genome is typically a single circular chromosome.
- Eukaryotes: genetic material is found in different cellular compartment. The nuclear genome in
humans includes 22 autosomes, the X chromosome, and (in males) the Y chromosome. Eukaryotes
also have mitochondrial genome. Plants have a chloroplast genome.
Chromosomal sequences facilitate four important processes:
- The synthesis of RNA and cellular proteins
- The replication of chromosomes
- The proper segregation of chromosomes
- The compaction of chromosomes so that they fit within living cells.
In this chapter we examine three features of bacterial and eukaryotic chromosomes.
- The general organization of functional sites along a chromosome.
- Transposition process in which short segments of DNA, called transposable element (TEs) are able to
move to multiple sites within chromosomes and accumulate in large numbers.
- Molecular mechanisms that make chromosomes more compact.
Organization of functional sites along bacterial chromosomes
In most species (bacterial), is the chromosomal DNA a circular molecule, though some have linear
chromosomes. Bacteria usually contain a single type of chromosome -> but more than one copy of that
chromosome may be found within one bacterial cell. A typical bacterial chromosome is a few millions base
pairs (bp) in length and has a few thousand different genes. Protein-encoding genes ( structural genes)
account for the majority of bacterial DNA. The nontranscribed regions of DNA located between adjacent
genes are termed intergenic regions.
Other sequences in chromosomal DNA influence DNA replication, gene transcription, and chromosome
structure. Example:
- Bacterial chromosomes have one origin of replication = this sequence functions as an initiation site
for the assembly of several protein required for DNA replication.
- Variety of repetitive sequences have been identified in many bacterial species. They are found in
multiple copies and are usually interspersed within the intergenic regions throughout the bacterial
chromosome. They play a roll in a variety of genetic processes: DNA folding, DNA replication, gene
regulation, genetic recombination.
, Some repetitive sequences are transposable element that can move throughout the
genome.
Structure of bacterial chromosomes
Inside a bacterial cell, a chromosome is highly compacted and found within a region of cell
known as a nucleoid. Bacteria may have one to four identical chromosomes per cell (depending on growth
conditions and phase of the cell cycle, also varies on bacterial species). Each chromosome is found within its
own distinct nucleoid in the cell. Unlike the eukaryotic nucleus, the bacterial nucleoid is
not a separate cellular compartment surrounded by a membrane. The DNA is in direct
contact with the cytoplasm of the cell.
Two nucleoids are in this bacterial cell ->
The formation of chromosomal loops helps make the bacterial chromosome more
compact
The mechanism of bacterial chromosome compaction is not entirely understood, and may
vary among different bacterial species.
- Loops that emanate from the core are called microdomains, they are typically 10,000 base pairs in
length. The length and boundaries of these microdomains are thought to be dynamic, changing in
response to environmental conditions.
Microdomains ->
- In E.coli many adjacent microdomains are further organized intro macrodomains (contain about 80
to 100 microdomains) that are about 800 to 1000 kbp in length
To form microdomains and macrodomains, bacteria use a set of DNA-binding proteins called = nucleoid-
associated proteins (NAPs) that facilitate chromosome compaction and organization. These proteins either
bend the DNA or act as bridges that cause different regions of DNA to bind to each other. NAPs also facilitate
chromosome segregation and play a roll in gene regulation.
- Examples: histone-like nucleoid structuring (H-NS) proteins and structural maintenance of
chromosomes (SMC) proteins. SMCs are also found in eukaryotes.
DNA supercoiling further compacts the bacterial chromosome
Because the two strands within DNA already coil around each other, the formation of additional coils due to
twisting forces is referred to as DNA supercoiling. The DNA within microdomains is further compacted
because of DNA supercoiling.
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