Nucleic Acids - HoldenScienceEducation Can be used in DNA profiling (DNA fingerprinting) ... - Human...
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Transcript of Nucleic Acids - HoldenScienceEducation Can be used in DNA profiling (DNA fingerprinting) ... - Human...
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The Nucleotide Subunits of DNA
• Although DNA is the genetic material of living organisms and is therefore of immense importance, it is made of relatively simple subunits
BS
P
DNA Structure (an overview)
• DNA has three main components
– 1. deoxyribose (a pentose sugar)
– 2. N - base (there are four different ones)
– 3. phosphate
Nucleotide Structure
• Nucleotides are formed by the
condensation of a pentose sugar,
phosphate and one of the 4 bases
• The following illustration represents one
nucleotide
• Nucleotides are linked together by
covalent bonds called phosphodiester
linkage
Sugar-Phosphate Backbone
6
nucleotide
monomer
phosphate group
deoxyribose sugar
nitrogen base
1’
5’
2’3’
4’
CH2
Numbering the carbons on
deoxyribose:
1’ = nitrogen base
3’ = hydroxyl group
5’ = phosphate group
OH
PO4
The Bases
They are divided into two groups
Pyrimidines and purines
Pyrimidines (made of one 6 member ring)
Thymine
Cytosine
Purines (made of a 6 member ring, fused to a 5 member ring)
Adenine
Guanine
The rings are not only made of carbon (specific formulas and structures are not required for IB)
DNA Double Helix and Hydrogen Bonding
• Made of two strands of nucleotides that
are joined together by hydrogen bonding
• Hydrogen bonding occurs as a result of
complimentary base pairing
– Adenine and thymine pair up
– Cytosine and guanine pair up
– hydrogen bonding
– Hydrogen bonding always occurs between
one pyrimidine and one purine
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Building DNA Molecules• DNA nucleotides linked
covalent bond btw 3’ C of one nucleotide and the 5’ phosphate group of the other
• More nucleotides can be added to 3’ C in a similar way to form a strand of nucleotides
• DNA molecules consist of two strands of nucleotides wound together into a double helix
• Hydrogen bonds link the twostrands together
• These form between the basesof the two strands
• AT – CG This is called complementary base pairing
Antiparallel strands
The strands run opposite of each other.
The 5’ end always has the phosphate attached.
5’ 3’
3’ 5’
• The ‘backbones’ of DNA molecules are
made of alternating sugar and phosphates
• The ‘rungs on the ladder’ are made of
bases that are hydrogen bonded to each
other
#5. DNA Double Helix
•Adenine always pairs with thymine because they form two H bonds with each other
•Cytosine always pairs with guanine because they form three hydrogen bonds with each other
DNA Double Helix and Hydrogen Bonding
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•The bonds between
the phosphate group
and the deoxyribose
sugar on an
individual nucleotide
is a covalent bond –
phosphodiester
bond.
•Phosphodiester
bonds are arranged
phosphate –
oxygen – carbon.
•Bonding nucleotides
together:
occurs between 3’
OH group on one
nucleotide and 5’
phosphate group on
the other through a
condensation
reaction (release of
water)
•There will always be
a free 5’ end 3’ on
each strand of DNA
5’
3’
•Strands run antiparallel to each
other = one strand has the 5’ C on
the top, 3’ C on the bottom, and
the other is reversed
Nucleosome structure
• Nucleosome are the basic unit of chromatin
organization
• In eukaryotes DNA is associated with proteins
– (in prokaryotes the DNA is naked)
• Nucleosomes = basic beadlike unit of DNA
packing
– Made of a segment of DNA wound around a
protein core that is composed of 2 copies of
each of 4 types of histones
– 8 histones in the
core
• Nucleosomes have:
– DNA wrapped twice
around the core
– One histone holding
the nucleosome
together
– A DNA ‘linker’
continuing towards
the next
nucleosome
Nucleosome structure
• The DNA has a negatively charged
backbone (because of the phosphate
groups)
• The proteins (the histones) are positively
charged
• The DNA and proteins are
electromagnetically attracted to each other
to form chromatin
Nucleosome structure
20
NUCLEOSOME STRUCTURE:
- 8 histones, (+) charged, (protein) in the core
- 2 molecules of 4 different histones
- DNA, (-) charged, wraps around the core 2x
- 1 histone holds the 2 ends of the DNA, histone H1
- with 2 ends of linker DNA
- nucleosomes help to supercoil chromosomes and help to
regulate transcription
• Repetitive sequences-part of the non-
coding section of DNA
– Function-unknown
– Can be used in DNA profiling (DNA
fingerprinting)
Genes
• Genes=units of genetic information
(hereditary information)
• Order of nucleotides make up the genetic
code
• Genes can contain the information for one
polypeptide DNA RNA Protein
• Genes can also regulate how other genes
are expressed
• All cells of an organism contain the same
genetic information but they do not all
express the same genes
– THIS IS CELL DIFFERENTIATION
– Cells differentiate by genes that are activated
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In nuclear DNA there are three types:
1. Unique/Single-copy genes:- genes with coding functions
- essential to producing proteins
- Human Genome Project: to sequence all the coding genes, less than 2% of
chromosomes are coding genes
- Coding parts of DNA are not strung together neatly; there are noncoding
regions interspersed within between coding regions
- coding parts = EXONS; noncoding parts = INTRONS
- EXONS are allowed to EXIT the nucleus to be translated into a protein
- INTRONS must stay IN the nucleus because they don’t code for a protein
2. Highly repetitive sequences: - found in eukaryotes
- from 5%-45% of the total genome
- 5-300 base pairs per sequence
- Clustered together? = satellite DNA
- usually dispersed throughout the
genome = transposable
- Barbara McClintock; 1950
24
RNA• Usually single
strands
• Unlike DNA,
contains the
pyrimidine
base uracil in
place of
thymine
• Contains
ribose sugar
rather than
deoxyribose
sugar
• Three types
are key players
in protein
synthesis
Assignment (in your notebook)
• 1. Draw the structure of ribose and number the carbons
• 2. Draw a schematic representation of a nucleotide. Label
the sugar, base and phosphate.
• 3. What are the complimentary base pairs to a DNA strand
that has the following order A T A C C T G A A T?
• 4. Draw a schematic representation of an unwound DNA
double helix using the base pairs from your answer in
question 3.
– Include the number of hydrogen bonds between each
base pair. Be sure to label all of the bases and the 5’ and
3’ ends of the structure.