From DNA to Chromosomes
Cool Facts about DNA
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Each person has enough DNA to stretch from here to the Sun and back 300 times!
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A human genome (a haploid set of chromosomes in a gamete – sex cell) contains approximately 3
billion base pairs of DNA, packaged into 23 chromosomes. However, since most cells are somatic
cells (body cells) which are diploid with 23 pairs of chromosomes, there are approximately 6
billion base pairs of DNA/cell!
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Each base pair (Thymine-Adenine; and Cytosine-Guanine) is only 0.34 nanometers long
(1/billionth of a meter), which means that each haploid cell has 2 meters of DNA.
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A human body has approximately 50 trillion cells, which is about 100 trillion meters of
DNA/human!
What is DNA?
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DNA gets packed together to create chromatin, which get packed together to create a
chromatid, which join together to form a chromosome.
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DNA stands for “deoxyribonucleic acid” and is a molecule that carries genetic information.
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DNA is a double helix. The two DNA strands are known as polynucleotides since they are
composed of simpler units called nucleotides.
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Each nucleotide is composed of:
A) a nitrogen-containing nucleobase (Thymine-Adenine; and Cytosine-Guanine)
B) a monosaccharide sugar called deoxyribose
C) a phosphate group.
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The nucleotides are joined together by a covalent bond (hydrogen bond) between the sugar of
one nucleotide and the phosphate of the next – resulting in an alternating sugar-phosphate
backbone. The base pairs T-A are joined together by 2 hydrogen bonds, while the base pairs
C-G are joined together by 3 hydrogen pairs.
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Purines and Pyrimidines are nitrogenous bases that make up the two different kinds of nucleotide
bases in DNA and RNA.
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The two-carbon nitrogen ring bases (adenine and guanine) are purines, while the onecarbon nitrogen ring bases (thymine and cytosine) are pyrimidines.
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Purines contain two carbon-nitrogen rings and four nitrogen atoms, while pyrimidines
contain one carbon-nitrogen ring and two nitrogen atoms.
How does all that DNA fit into one chromosome?
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Certain proteins compact chromosomal DNA into the microscopic space of the eukaryotic nucleus.
These proteins are called histones, and the resulting DNA-protein complex is called a chromatin.
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Histones are the proteins within the nucleus that provide the energy required to fold DNA so
that the chromatins can be packaged into a smaller volume than DNA alone.
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Histones are a family of small, positively charged proteins termed: H1, H2A, H2B, H3, and H4.
DNA is negatively charged due to the phosphate groups in its phosphate sugar backbone, so
histones bind with DNA very tightly.
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A chromatin is a complex of DNA-proteins that forms chromosomes within the nucleus of a
eukaryotic cell. Chromatin exist in 2 forms: A) Euchromatin (which are less condensed and can
be transcribed) and B) Heterochromatin (which are highly condensed and typically do not
transcribe).
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Chromatins look like beads on a string. These beads are called nucleosomes. Each nucleosome is
composed of DNA wrapped around 9 histones (proteins).
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Each chromosome consists of one continuous thread-like molecule of DNA coiled tightly around
proteins and contains a portion of the 6.4 x 109 base pairs.
How does DNA Replicate?
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DNA polymerase is an enzyme that creates DNA molecules by assembling nucleotides, the
building blocks of DNA. These enzymes are essential to DNA replication and usually work in pairs
to create two identical DNA strands from a single original DNA molecule. During this process,
DNA polymerase “reads” the existing DNA strands to create two new strands that match the
existing ones.
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Every time a cell divides, DNA polymerase is required to help duplicate the cell’s DNA, so that a
copy of the original DNA molecule can be passed to each of the daughter cells. In this way,
genetic information is transmitted from generation to generation.
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Before replication can take place, an enzyme called helicase unwinds the DNA molecule from its
tightly woven form. This opens up or “unzips” the double-stranded DNA to give two single strands
of DNA that can be used as templates for replication.
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When creating DNA, DNA polymerase can add free nucleotides only to the 3' end of the newly
forming strand. This results in elongation of the newly forming strand in a 5'-3' direction.
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No known DNA polymerase is able to begin a new chain; it can only add a nucleotide onto a preexisting 3'-OH group, and therefore needs a primer at which it can add the first nucleotide. In
DNA replication, the first two bases are always RNA, and are synthesized by another enzyme
called primase.
It is important to note that the directionality of the newly forming strand (the daughter strand)
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is opposite to the direction in which DNA polymerase moves along the template strand. Remember
that DNA polymerase requires a free 3' OH group for initiation of synthesis, it can synthesize in
only one direction by extending the 3' end of the pre-existing nucleotide chain. Therefore, DNA
polymerase moves along the template strand in a 3'-5' direction, and the daughter strand is
formed in a 5'-3' direction. This difference enables the resultant double-strand DNA formed to
be composed of two DNA strands that are antiparallel to each other.
 The 5' and 3' mean "five prime" and "three prime", which in turn indicate the carbon numbers in
the DNA's sugar backbone. The 5' carbon has a phosphate group attached to it and the 3' carbon
a hydroxyl group. This asymmetry gives a DNA strand a "direction". For example, DNA
polymerase works in a 5' -> 3' direction, that is, it adds nucleotides to the 3' end of the molecule
(the -OH group), thus advancing to that direction.
What is Protein Synthesis?
transcription
translation
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Protein synthesis requires two steps: DNA → → → mRNA → → → Protein
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It takes three DNA bases to create one amino acid. These three bases are called a codon.
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There are 3 types of RNA:
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Messenger RNA (mRNA): It contains genetic information; it is a copy of a portion of
DNA. It carries genetic information from the gene (DNA) out of the nucleus, into the
cytoplasm of the cell where it is translated to produce protein.
Ribosomal RNA (rRNA): This type of RNA is a structural component of the ribosomes.
It does not contain a genetic message.
 Transfer RNA (tRNA): It transfers RNA functions to transport amino acids to the
ribosomes during protein synthesis.
Transcription is the synthesis of RNA from a DNA template. Only one strand of DNA is copied.
After transcription, the DNA strands rejoin. Some of the RNA produced by transcription is not
used for protein synthesis. These RNA molecules have other functions in the cell.
Steps involved in transcription: The enzyme RNA polymerase is responsible for
creating RNA by copying the template strand of DNA. Before transcription can begin in
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eukaryotes, proteins called transcription factors must bind to a region of the DNA
called the promoter. The promoter identifies the start of a gene, which strand is to be
copied, and the direction that it is to be copied. RNA polymerase binds to the
transcription factors and the promoter. RNA polymerase unwinds the DNA. RNA
polymerase arranges nucleotides that are complimentary to the DNA strand being copied.
RNA contains uracil instead of thymine. The direction of synthesis is 5' to 3'. The
strand of RNA that is initially produced by transcription is called a primary transcript.
In eukaryotic cells, primary transcripts that are to be translated into protein are
modified. These transcripts are called precursor mRNA (or pre-mRNA). The 5' end and
the 3' end each contain nucleotides that are not translated into protein. These two
regions are called the 5' UTR (untranslated region) and the 3' UTR.
Translation: Is the process where ribosomes synthesize proteins using the mature mRNA
transcript produced during transcription.