DNA and Chromosomes Chapter 5

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DNA and Chromosomes Chapter 5

• The structure and function of DNA
• The structure of eucaryotic chromosomes

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DNA Before the DNA structure was resolved it was
determined that genes are located on
chromosomes Nobel Prize awarded to Watson and
Crick Rosalin Franklin X-Ray crystallography Human
genome project-determined all of the DNA
sequence but not all protein coding regions
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The nucleus stained with DAPI representing the
dividing and the non-dividing cell The
chromosomes become visible in preparation for
cell division The chromosomes cannot be clearly
identified in non-dividing cells Figure 5-1
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DNA is composed of 2 antiparallel strands One
nucleotide is composed of phosphate group,
sugar, and nitrogen containing base The backbone
is composed of phosphate and sugar groups There
are 4 types of nucleotides that differ by the
nitrogen containing base guanine, cytosine,
adenine, thymine Example Figure 5-2
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DNA double helix is held by hydrogen bonds Bulky
two-ring bases (Purines) A, G are always paired
with the single-ring bases (Pirimidines) T, C A
always pairs with T (2 hydrogen bonds) G always
pairs with C (3 hydrogen bonds) Figure 5-6
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The sugar-phosphate backbone is held at an equal
distance apart Nucleotides in the backbone are
linked by phosphodiester bonds DNA is
antiparallel 5 end always has a free phosphate
group 3 end always has a free hydroxyl
group Figure 5-7
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Human beta globin gene Coding regions (EXONS) are
in yellow Non-coding regions (INTRONS) are
white Human genome 3.2 x 109 arranged in 46
chromosomes (or 26 pairs) A chromosome is a very
long DNA molecule packed into a dense
structure Figure 5-10 5-11
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Most important function of a chromosome is to
carry genes Gene can be defined as a segment of
DNA that contains instructions for synthesizing a
protein Figure 5-15 Drosophila genome
(approximately 3) arranged in 6
segments Predictions of known genes is color
coded
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Example of closely related species of muntjac
deer Chromosomes of one species are fused, thus
the chromosome number between two species
differs However the number of genes remains the
same Figure 5-16
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DNA in chromosomes is highly compacted Histone is
a protein that is associated with DNA in
chromosomes 146 nucleotides of ds DNA are
wrapped around one histone histone protein is an
octamer and contains two of each H2A, H2B,H3,
H4 Nucleosome is a unit composed of a short
strand of DNA wrapped around a core of histone
protein Figure 5-22 5-23 5-24