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Introduction to DNA

Its discovery, its structure, where it's found, and how its replicated
by

Miss Schwinge

on 3 October 2014

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Transcript of Introduction to DNA

DNA
99.9%
of human DNA is
exactly the same
, only .1% gives
each individual their own personal fingerprint.
This means that
only .1%
codes for proteins that express our
differences.
the blueprint of life
DNA stands for
deoxyribonucleic acid
,
and is the
hereditary material
in humans
and almost all other
organisms
There is a copy of our
entire DNA sequence
in almost
every cell of our body, with the exception of
red blood cells
If unwound and tied together, the
strands of DNA in
one cell
would
stretch almost
6
feet but would
be only 50 trillionths of an inch wide.
If you could unwrap all of the DNA you have in
all of your cells
, you could reach the
moon and back
6,000 times!
DNA is found
inside
a special area of the cell called the
nucleus
(which we'll learn all about in our cells unit). Because the cell is very small, and because organisms have
many DNA molecules per cell
, each DNA molecule must be
very tightly packaged
(remember, that's 6,000 moon trips worth of DNA you're trying to cram into a cell).


The order, or sequence, of these bases determines what biological instructions are contained in a strand of DNA
(for example, the sequence ATCGTT might instruct for blue eyes, while ATCGCT might instruct for brown).

The nucleotides can be joined together
in any order
, which means that any sequence of bases is possible
DNA is made of
chemical building blocks
called
nucleotides
.

These
building blocks are made of a
5-carbon sugar (deoxyribose)
a
phosphate group
and a
nitrogenous base
.
There are
four
types of

nitrogen bases
that form the
"ladder rungs" of DNA.

These nitrogen bases are:
- adenine (A)
- thymine (T)
- guanine (G)

and
- cytosine (C)
.
VERY IMPORTANT NOTE:
In DNA,
adenine (A)
ALWAYS bonds with
thymine (T)
and
guanine (G)
ALWAYS bonds with
cytosine (C)
!
Think of them as soulmates, they
cannot bond with any other base (letter)
A + T
4ever
C <3 G
See how adenine and thymine are always together?
Same with guanine and cytosine
To Review So Far:
DNA contains the biological instructions that make each species and organism unique

DNA is made up of:
a 5-carbon sugar called deoxyribose
a phosphate group
and a nitrogenous base (either A, T, C, or G)

In DNA adenine always bonds with thymine, and guanine always bonds with cytosine, but they can be strung together in any sequence
To form a strand of DNA, nucleotides are
linked into chains, with alternating phosphate and sugar groups
(this means first one, then the other, then back to the first again).

These phosphate and sugar groups make up the
backbone
of a DNA chain
A Brief Early History of DNA
1928
Frederick Griffith
Frederick Griffith discovered that he could
transform harmless bacteria
into ones that can
cause disease
1944
Oswald Avery
Avery discovered that the
nucleic acid DNA (not protein) stores and transmits the genetic information
from one generation of an organism to the next
1952
Rosalind Franklin
By using
x-ray crystallography
, Franklin produced the
clearest X-ray images of crystallized DNA ever
(shown below). From these X-rays, Franklin determined that
DNA's structure
depended on an
external backbone with bases inside,
and identified the location of the phosphate sugar
...This is where things get scandalous
1953
Watson & Crick
Watson and Crick are often touted as the "discoverers" of DNA's double helix shape. However, it was
Rosalind Franklin
who
discovered DNA's helical form
with her x-ray, and it was Watson and Crick who "borrowed" it without her permission, later denied it, and then published their findings without giving her any credit; which gained them a Nobel Prize.

Even now, even in your text book, she is merely granted the status of being "important" in the discovery of the double helix
Aside from being thieving cads,
Watson and Crick
did discover that
hydrogen bonds were what kept the two strands of DNA together
, and that hydrogen bonds could only occur with
CERTAIN base pairing
Ok, so we now know how DNA was discovered and what it is made of, but where is it found?

This packaged form of the DNA is called a
chromosome
.
Found within the nucleus (the cell's control center), a
chromosome consists of DNA tightly wrapped around proteins
which packed together forms a substance called
chromatin
. Chromatin consists of
DNA that is tightly coiled around proteins called histones.
Chromosomes are found in pair
s
called sister chromatids
(individually they are called
chromatids
), and they are often depicted in an
"X" shape
, drawn together at the middle by a
centromere
.
Tinier than a grain of salt
, each human chromosome contains
millions of letters of DNA
. Each chromosome is like a chapter of a book - brought together, they make up an organism's entire genome and tell a full genetic story.
Chromosome
DNA has a
helical (spiral) structure
, which is why it is referred to as a
double helix
.
The Structure of
DNA
Telomere
We have a total of
46 chromosomes
. We get
23
of them from our
biological mother
, and
23
from our
biological father.
Sister chromatids
are
identical
, which means that
both sides of the chromosome are exactly the same.
Each strand
of the DNA double helix
has all the information needed to reconstruct the other half
through the mechanism of base paring (A with T, G with C).

Because
each strand can be used to make the other strand
, the strands are said to be
complementary
. Being complementary means that
if you separate the two strands
, the rules of base pairing would allow you to
reconstruct the base sequence of the other strand
.
When the
structure of the double helix
was discovered, it led to the
following discovery of how DNA could be copied, or replicated
.
In humans and other multicellular organisms, DNA replication occurs at
hundreds of places in the chromosome
, and proceeds in
both directions

of the double stranded helix
until each chromosome is
completely copied
.
The
places
where double helix
separation and replication occur
are called
replication forks.
Replicating DNA
Before a cell divides, it
duplicates its DNA
in a
copying process called replication
. This process makes sure that each resulting cell will have a
complete set of DNA
molecules
During DNA replication, the DNA molecule
separates into two strands
, and then produces
two new complementary strands
following the rules of
base pairing
.
Each strand
of the double helix of DNA
serves as a template, or model,

for the new strand
.
Replication Fork
Original Strand
DNA
Polymerase
New Strand
Replication Fork
Nitrogenous Bases
Original Strand
New Strand
DNA Polymerase
Steps of DNA Replication
1.) An
enzyme
called
DNA helicase unzips and unwinds the two strands of DNA
by
breaking the hydrogen bonds
holding the bases together
2.) The
two strands separate
, which allows
replication forks
to form
3.)
RNA primers are then laid down
so the enzyme
DNA polymerase can begin adding new base pairs onto the leading strand
(it cannot initiate a new strand on its own)
DNA polymerase can ONLY add new nucleotides

onto an available 3' end
. This creates a
leading strand
, which is created
continuously in the same direction
that the
replication fork
is going.
But what about the other stand? This
antiparallel
strand is called the
lagging strand
, and it is
discontinuously synthesized in the opposite direction
. In order for DNA polymerase to add nucleotides onto it,
another enzyme called DNA primase
must first
add RNA primers
to the strand for the
polymerase to attach to.
The DNA polymerase can then add
short nucleotide segments called Okazaki fragments
, but it is
not
a continuous flow like the leading strand.
6.) Each
new double helix
now consists of
one old and one new chain
. This is what we call

the
semiconservative model of DNA replication.
DNA Replication
This process is called
DNA replication
.
4.)
Another type of DNA polymerase
comes by and
replaces the RNA primers with DNA.
5.) Once the
primers have been replaced
, the
Okazaki fragments are "stitched" together

by

DNA ligase
Full transcript