DNA & Chromosomes
Chapter 5
(Please do read every single page)
Learn…
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Nucleotides
Nucleosides
DNA
RNA
Facts
• The DNA in each human cell is
enormous
• End to end - all 24 chromosomes would
stretch 1.8 metres
• Cell diameter is = WHO CARES!!!
• Nucleus diameter is = few um
• How is it squeezed in and also
controlled?
DNA
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Deoxyribonucleic acid
It is a Macromolecule
Made of Subunits - nucleotides
IT HAS DIRECTIONALITY
– dictated by the free end groups
– always written 5’ to 3’ direction for each
single strand
Three-dimensional structure
Figure 9.7
NUCLEIC ACID STRUCTURE
• DNA and RNA can be VERY VERY VERY large
macromolecules with several levels of complexity
– 1. Nucleotides form the repeating units
– 2. Nucleotides are linked to form a single strand
– 3. Two strands can interact to form a double strand for DNA this is called the double helix
– 4. The double helix is nursed and managed by P’s
• which bend and twist the DNA - eventually resulting in 3-D
structures in the form of chromosomes
Nucleotides
• The nucleotide is the repeating structural unit of
DNA and RNA
• HOW IS IT CONSTRUCTED?
• Many varieties
• Each has three components
– A phosphate group
– A pentose sugar
– A nitrogenous base
Figure 9.8
• The red atoms below are found within individual nucleotides
– However, they are removed when nucleotides join together to make
strands of DNA or RNA
A, G, C or T
Figure 9.9
A, G, C or U
The structure of nucleotides found in (a) DNA and (b) RNA
• Base + sugar  nucleoside
– Example
• Adenine + ribose = Adenosine
• Adenine + deoxyribose = Deoxyadenosine
• Base + sugar + phosphate(s)  nucleotide
– Example
• Adenosine monophosphate (AMP)
• Adenosine diphosphate (ADP)
• Adenosine triphosphate (ATP)
• Next…
SIMPLE PLAN OF A SINGLE STRANDED NUCLEOTIDE CHAIN
Just LOOK here
to determine if
this is RNA or
DNA
LEARN WHICH
ATOMS CONNECTS
THE SUBUNITS TO
EACH OTHER…
Figure 9.11
Other important nucleotides within cells
ATP
ADP
AMP
Base always
attached here
Phosphates are
attached there
Figure 9.10
5’ to 3’ how?
• Individual nucleotides are covalently linked together
by phosphodiester bonds
– A phosphate connects the 5’ carbon of one nucleotide to
the 3’ carbon of another
• Therefore the strand has directionality
– 5’ to 3’
• The phosphates and sugar molecules form the
backbone of the nucleic acid strand
– The bases project from the backbone
DNA
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Deoxyribonucleic acid
Macromolecule
Subunits - nucleotides
5’ to 3’ direction for each single strand
Two strands run in opposite directions
Minor groove
Major groove
The two strands are not equally spaced because of bond angles
P’s and the those enzymes that manage DNA interact with the
DNA mainly via the major groove.
Definitions • Genome - The complete set of INFORMATION
in an organism’s DNA
– That is the haploid complement for all organisms
• Chromosome - A single molecule of DNA
– Circular or linear
• Gene - a section of the DNA coding for the
production of a single protein*
• Karyotype - figure or picture showing the full set
of chromosomes from a cell during cell division
when they are condensed and visible...
*simplification of the current data
Human Karyotype
Human DNA facts
• Genome = 3.2 billion base pairs of DNA across 24
different distinct chromosomes (22 automomes + X
+ Y)
• Take about 9.5 years to read out loud (without
stopping) the three billion pairs of bases in one
person's genome sequence
• Biologist simply say 23 pairs of chromosomes
• For diploid cells = 6.4 billion base pairs per somatic
cell
• Each ? is a single enormous DNA double-helix chromosome
Genome sizes
• Humans do not have the most chromosomes
- Plants tend to have these
• Humans do not have the largest gemone size
- Fish tend to have these
• An ant species has just 1 chromosome
• No real association between genome size
and evolutionary complexity - human near the
middle some where
Chromosomes Structure
• During the cell cycle the structure of
chromosomes changes visibly
• Interphase state = very fine threads
• Metaphase state = distinct defined
chromosomes
• Ends of chromosomes are known as
telomeres
• Middles, where the spindles attach, as
centromeres
Nucleolus
• Within the nucleus are one or more
nucleoli.
• The nucleolus is the coming together of
genes from different chromosomes that
are involved in making ribosomes.
• An average, healthy cell can produce up
to 10 000 ribosomes per minute.
Nucleolus
DNA management
• 10,000 fold difference in size between
DNA helix stretched out and the size of
a metaphase chromosome - How?
• Answer = Proteins associated with DNA
– Histone Proteins (H1, H2a, H2b, H3, and
H4)
– DNA plus these proteins is called
Chromatin
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Nucleosome
• Complex consisting of
– 200 bp of DNA
– 8 histone proteins in core
– H1 histone stabilizing
– Core = 146 bp DNA and
two each of H2a, H2b, H3
and H4
Vary in length between 20 to 100 bp,
depending on species and cell type
Diameter of the
nucleosome
Figure 10.14
• Overall structure of connected nucleosomes resembles “beads on a string”
– This structure shortens the DNA length about seven-fold
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
10-45
Play a role in the
organization and compaction
of the chromosome
Figure 10.14
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display
10-47
Higher Order Folding
• DNA -double-stranded helix is 2 nm
thick
• Nucleosome - 11 nm thick
• 30nm chromatin fibre - 30 nm thick
• Chromatin loops - 300 nm thick
• Condensed chromatin - 700 nm thick
• Chromosome (mitotic) - 1400 nm thick
– DNA is wrapped or looped using a protein
matrix
Figure 10.21
10-63
Compaction level
in euchromatin
During interphase
most chromosomal
regions are
euchromatic
Compaction level
in heterochromatin
Figure 10.21
10-64
Heterochromatin
• Regions close to telomere and
centromere are classes as
heterochromatin based on observations
• These are not very active for
transcription
• DNA thought to be tightly packed here
• Few proteins can enter region - it it too
tight for them to get to the DNA
Chromatin Remodeling
Complexes
• Large Protein complexes which
selectively act at the nucleosome level
to unwind DNA to allow proteins access
• IMPORTANT control mechanism in the
cell.
• Transcription occurs when needed.