DNA & Chromosomes Chapter 5 (Please do read every single page)
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Transcript of DNA & Chromosomes Chapter 5 (Please do read every single page)
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
• 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
• 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
NUCLEIC ACID STRUCTURE
• 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
Nucleotides
Figure 9.9 The structure of nucleotides found in (a) DNA and (b) RNA
A, G, C or T
• 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 U
• 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…
Figure 9.11
LEARN WHICH ATOMS CONNECTS THE SUBUNITS TO EACH OTHER…
Just LOOK here to determine if this is RNA or DNA
Figure 9.10
Base always attached here
Phosphates are attached there
Other important nucleotides within cells
ATPADPAMP
• 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
5’ to 3’ how?
DNA
• Deoxyribonucleic acid
• Macromolecule
• Subunits - nucleotides
• 5’ to 3’ direction for each single strand
• Two strands run in opposite directions
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.
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.
Minor groove 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 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.
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
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
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 10-45
• Overall structure of connected nucleosomes resembles “beads on a string”
– This structure shortens the DNA length about seven-fold
Figure 10.14
Vary in length between 20 to 100 bp, depending on species and cell type
Diameter of the nucleosome
Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 10-47
Figure 10.14
Play a role in the organization and compaction
of the chromosome
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
10-64Figure 10.21
Compaction level in euchromatin
Compaction level in heterochromatin
During interphase most chromosomal
regions are euchromatic
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