Theme 1: Nucleic Acids
13.4 Organiza-on of Genomes
13.4.1 Bacterial cells package their DNA as a nucleoid
composed of many loops.
• Bacterial genomes are circular
• DNA double helix is underwound due to the enzyme topoisomerase II
which breaks the double helix, rotates the ends, and then seals the
break
◦ Underwinding creates strain on the DNA molecule which is
relieved by the formaAon of supercoils (DNA molecule coils on
itself) which allows all the base pairs to form
• Supercoils that result from underwinding are nega-ve
supercoils (most common)
• Supercoils that result from overwinding are posi-ve
supercoils
• In bacteria, the supercoils of DNA form a structure with mulAple loops
called a nucleoid which is bound together by proteins
◦ The proteins and negaAve supercoiling of DNA compress the
molecule into a compact volume
• Supercoils in the loops of DNA of the nucleoid can be relaxed by
introducing nicks in the DNA backbones by use of enzymes or chemicals
◦ BUT, each loop is supercoiled independently of the others (due to
the proteins) so, a nick will relieve the supercoils only in that loop
13.4.2 Eukaryo-c cells package their DNA as one molecule
per chromosome.
• EukaryoAc DNA is wrapped around a group of histone proteins called
nucleosomes (consists of 2 molecules that each consist of 4 different
histone proteins)
, • Histone Proteins- Rich in the amino acids lysine and arginine whose
posiAve charges neutralize the negaAve charges of the phosphates along
the backbone of each DNA strand
• Nucleosomes and associate DNA further coiled to form 30-nm
chromaAn fiber
• When the chromosomes in the nucleus condense in preparaAon for cell
division, each chromosome becomes progressively shorter and thicker
as the 30-nm fiber coils
◦ Chromosome CondensaAon: An acAve, energy-consuming process
requiring the parAcipaAon of many proteins
• Without histones, DNA spreads out in loops around a supporAng protein
structure (i.e. chromosome scaffold)
◦ Though the scaffold model may seem similar to the nucleoid
model (of bacteria), the structure evolved independently and use
different types of protein to bind the DNA and to form the folded
structure of DNA and protein
• EukaryoAc chromosome is much larger than the bacterial nucleoid
13.4.3 The human genome consists of 22 pairs of
chromosomes and two sex chromosomes.
• Orderly process of meiosis is possible because chromosomes occur in
pairs
• Homologous Chromosomes: Pairs of chromosomes that match in size
and appearance
◦ Members of each pair have the same genes arranged in the same
order along their length
◦ If the DNA duplexes in each pair of homologs were denatured,
each DNA strand could form a duplex with its complementary
strand from the other homolog
• There would be some differences in DNA sequence due to
geneAc variaAon, but not so many differences as to prevent
DNA hybridizaAon
• Chromosome Pain-ng- A technique whereby individual chromosomes
are isolated from cells in metaphase of mitosis (Best Ame because of the
availability of chemicals that prevent the spindle from forming; These
13.4 Organiza-on of Genomes
13.4.1 Bacterial cells package their DNA as a nucleoid
composed of many loops.
• Bacterial genomes are circular
• DNA double helix is underwound due to the enzyme topoisomerase II
which breaks the double helix, rotates the ends, and then seals the
break
◦ Underwinding creates strain on the DNA molecule which is
relieved by the formaAon of supercoils (DNA molecule coils on
itself) which allows all the base pairs to form
• Supercoils that result from underwinding are nega-ve
supercoils (most common)
• Supercoils that result from overwinding are posi-ve
supercoils
• In bacteria, the supercoils of DNA form a structure with mulAple loops
called a nucleoid which is bound together by proteins
◦ The proteins and negaAve supercoiling of DNA compress the
molecule into a compact volume
• Supercoils in the loops of DNA of the nucleoid can be relaxed by
introducing nicks in the DNA backbones by use of enzymes or chemicals
◦ BUT, each loop is supercoiled independently of the others (due to
the proteins) so, a nick will relieve the supercoils only in that loop
13.4.2 Eukaryo-c cells package their DNA as one molecule
per chromosome.
• EukaryoAc DNA is wrapped around a group of histone proteins called
nucleosomes (consists of 2 molecules that each consist of 4 different
histone proteins)
, • Histone Proteins- Rich in the amino acids lysine and arginine whose
posiAve charges neutralize the negaAve charges of the phosphates along
the backbone of each DNA strand
• Nucleosomes and associate DNA further coiled to form 30-nm
chromaAn fiber
• When the chromosomes in the nucleus condense in preparaAon for cell
division, each chromosome becomes progressively shorter and thicker
as the 30-nm fiber coils
◦ Chromosome CondensaAon: An acAve, energy-consuming process
requiring the parAcipaAon of many proteins
• Without histones, DNA spreads out in loops around a supporAng protein
structure (i.e. chromosome scaffold)
◦ Though the scaffold model may seem similar to the nucleoid
model (of bacteria), the structure evolved independently and use
different types of protein to bind the DNA and to form the folded
structure of DNA and protein
• EukaryoAc chromosome is much larger than the bacterial nucleoid
13.4.3 The human genome consists of 22 pairs of
chromosomes and two sex chromosomes.
• Orderly process of meiosis is possible because chromosomes occur in
pairs
• Homologous Chromosomes: Pairs of chromosomes that match in size
and appearance
◦ Members of each pair have the same genes arranged in the same
order along their length
◦ If the DNA duplexes in each pair of homologs were denatured,
each DNA strand could form a duplex with its complementary
strand from the other homolog
• There would be some differences in DNA sequence due to
geneAc variaAon, but not so many differences as to prevent
DNA hybridizaAon
• Chromosome Pain-ng- A technique whereby individual chromosomes
are isolated from cells in metaphase of mitosis (Best Ame because of the
availability of chemicals that prevent the spindle from forming; These
