10 Molecular structure of
chromosomes and transposable
elements
Chromosomes= structures within living cells that contain the genetic material.
Genome= entire complement of genetic material in an organism/ species.
For bacteria: 1 single circular chromosome.
Nuclear genome (in humans)= 22 autosomes, the X chromosome+ (in males) the Y chromosome.
Mitochondria (eukaryotes+ chloroplasts (plants) have their own DNA/ genome.
4 important processes facilitated by chromosomal sequences:
1. Synthesis of RNA+ cellular proteins
2. Replication of chromosomes
3. Proper segregation (separation) of chromosomes
4. Compaction of chromosomes so they can fit within living cells.
10.1 Organization of functional sites along bacterial
chromosomes
General features of a bacterial chromosome (fig 10.1):
1 circular chromosomal DNA
Usually, 1 type of chromosome; more copies of the chromosome can be found.
Few million bp long
Protein-encoding genes (structural genes)= produce mRNA+ encode polypeptides.
Intergenic regions= in a chromosome; non-transcribed DNA regions between adjacent genes.
Bacterial chromosomes have 1 origin of replication= site on a chromosome that functions as an initiation
site for the assembly of several proteins required for DNA replication.
Repetitive sequences= Short DNA sequence that occurs many times within a species’ genome.
Often near centromere+ telomere-> also interspersed throughout the bacterial chromosome.
Role in DNA folding, DNA replication, gene regulation+ genetic recombination.
10.2 Structure of bacterial chromosomes
Bacterial chromosome (highly compact) found in a nucleoid= darkly staining region that contains the genetic
material of mitochondria, chloroplasts/ bacteria.
Not membrane-bound; DNA in a nucleoid is in direct contact with the cytoplasm of the cell.
The formation of chromosomal loops helps make the bacterial chromosome
more compact
Microdomain= DNA loops found in the bacterial chromosome (10 kbp long).
Macrodomains= further organization of microdomain (800-1000 kbp long+ 80-100 microdomains p/macro).
Nucleoid-associated proteins (NAPs)= set of DNA-binding proteins in bacteria that facilitate chromosome
compaction+ organization.
Bend the DNA/ act as bridges that cause different regions of DNA to bind to each other.
Facilitate chromosome segregation+ play a role in gene regulation.
DNA supercoiling further compacts the bacterial chromosome (fig 10.4)
2 DNA strands already coil around each other-> additional coils due to twisting forces; DNA supercoiling= formation
of additional coils due to twisting forces-> can be caused by underwinding+ overwinding of the DNA double helix.
Topoisomers= DNA conformations that differ regarding supercoiling (fig 10.4a, c+ e).
Fig 10.4b+ d: structurally not stable; don’t occur in living cells.
chromosomes and transposable
elements
Chromosomes= structures within living cells that contain the genetic material.
Genome= entire complement of genetic material in an organism/ species.
For bacteria: 1 single circular chromosome.
Nuclear genome (in humans)= 22 autosomes, the X chromosome+ (in males) the Y chromosome.
Mitochondria (eukaryotes+ chloroplasts (plants) have their own DNA/ genome.
4 important processes facilitated by chromosomal sequences:
1. Synthesis of RNA+ cellular proteins
2. Replication of chromosomes
3. Proper segregation (separation) of chromosomes
4. Compaction of chromosomes so they can fit within living cells.
10.1 Organization of functional sites along bacterial
chromosomes
General features of a bacterial chromosome (fig 10.1):
1 circular chromosomal DNA
Usually, 1 type of chromosome; more copies of the chromosome can be found.
Few million bp long
Protein-encoding genes (structural genes)= produce mRNA+ encode polypeptides.
Intergenic regions= in a chromosome; non-transcribed DNA regions between adjacent genes.
Bacterial chromosomes have 1 origin of replication= site on a chromosome that functions as an initiation
site for the assembly of several proteins required for DNA replication.
Repetitive sequences= Short DNA sequence that occurs many times within a species’ genome.
Often near centromere+ telomere-> also interspersed throughout the bacterial chromosome.
Role in DNA folding, DNA replication, gene regulation+ genetic recombination.
10.2 Structure of bacterial chromosomes
Bacterial chromosome (highly compact) found in a nucleoid= darkly staining region that contains the genetic
material of mitochondria, chloroplasts/ bacteria.
Not membrane-bound; DNA in a nucleoid is in direct contact with the cytoplasm of the cell.
The formation of chromosomal loops helps make the bacterial chromosome
more compact
Microdomain= DNA loops found in the bacterial chromosome (10 kbp long).
Macrodomains= further organization of microdomain (800-1000 kbp long+ 80-100 microdomains p/macro).
Nucleoid-associated proteins (NAPs)= set of DNA-binding proteins in bacteria that facilitate chromosome
compaction+ organization.
Bend the DNA/ act as bridges that cause different regions of DNA to bind to each other.
Facilitate chromosome segregation+ play a role in gene regulation.
DNA supercoiling further compacts the bacterial chromosome (fig 10.4)
2 DNA strands already coil around each other-> additional coils due to twisting forces; DNA supercoiling= formation
of additional coils due to twisting forces-> can be caused by underwinding+ overwinding of the DNA double helix.
Topoisomers= DNA conformations that differ regarding supercoiling (fig 10.4a, c+ e).
Fig 10.4b+ d: structurally not stable; don’t occur in living cells.
