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Nucleic Acids:
DNA, RNAand Genes
Mr. Lowell
Tuesday, May 23, 2017
Nucleic Acids:
DNA
and
RNA
What is DNA
Deoxyribonucleic Acid
A complex linear molecule that contains
the information for the production of
proteins that control the cell.
Its general shape is a long “double helix”
or “twisted ladder” shape
A polynucleotide - polymers of
nucleotides
Nucleotides
Three major Components:
•Nitrogen containing bases
•Purines - Adenine and Guanine (Double ring
structure)
•Pyrimidines - Cytosine, Thymine (only in
DNA), Uricil (only in RNA)
•Sugar
•Ribose in RNA or Deoxyribose in DNA
•Phosphate
DNA
Structure
6
DNA Nucleotides
DNA Nucleotides
Removed when added to DNA chain
DNA Nucleotides
Phosphate
DNA Nucleotides
Base
DNA Nucleotides
10
Nucleotides
Adenine
Thymine
Guanine
Cytosine
Nucleotides
Adenine
Guanine
Purines - Have a double Ring
Structure
Thymine
Cytosine
Pyrimidines Single Ring
Structure
Adenine
13
Guanine
14
Thymine
15
Cytosine
16
Uricil,
found in
RNA
17
18
The Sugars and their differences:
19
Deoxyribose with labeled carbon
atoms:
20
Nucleotide Pairing
In DNA
Adenine is always paired with
Thymine
Guanine is always paired with
Cytosine
DNA:
Paired
Nucleic
Acids
Adenine to
Thymine
22
DNA:
Paired
Nucleic
Acids
Cytosine to
Guanine
23
A DNA Sequence:
ACTTGGAACGATTGCCGTATGCT
TGAACCTTGCTAACGGCATACGA
Period 4 starts here
REPLICATION
OKAZAKI
FRAGMENTS
Steps in Replication
1. A section of the DNA is separated by a
protein enzyme called a HELICASE and
forms a replication fork.
Steps in Replication
2. A molecule of DNA POLYMERASE
binds to one of the strands of DNA and
begins to move in the 3’ to 5’ direction
along it. This produces a new strand of
DNA that is called the LEADING
STRAND. DNA in the leading strand is
synthesized in the 5’ to 3’ direction
which is the ONLY way new DNA can be
synthesized.
Steps in Replication
3. A second type of DNA Polymerase
binds to the other original strand of DNA.
Since the DNA on this second strand
runs the opposite direction from the
leading strand the DNA that is
synthesized on this LAGGING strand is
created in the opposite direction from the
first.
Replication
Because of the direction of travel the
polymerase the DNA is created in short
pieces called OKASAKI FRAGMENTS.
Another enzyme called a DNA LIGASE
takes these fragments and puts them
together into what is called the LAGGING
STRAND.
Period 2 starts here
Replication of DNA
24
Speed of Replication
Prokaryotes have a single
circular strand of DNA to
replicate. This takes about 40
minutes.
R
Speed of Replication
Eukaryotes if their DNA was done
by one polymerase molecule per
chromosome would take about a
month for the DNA to replicate.
Multiple polymerase latch on the
the replicating DNA simultaneously
and as a result replication in
humans takes about an hour. R
For more on Replication see:
http://www.ultranet.com/~jkimba
ll/BiologyPages/D/DNAReplicati
on.html
Split Genes
Eukaryotic genes contain large
amounts of non-functional DNA.
These sections of DNA that are not
part of the coding for a gene are
referred to as INTRONS for
intervening sequences
More on this in the RNA transcription section
Exons
These are sections of the DNA that
actually code for the proteins of a
gene.
The DNA for a single gene is often
not contiguous but rather split into
sections of exons separated by
introns
Central Dogma
RNA
Single stranded nucleic acid
Sugar in the nucleotide is
RIBOSE not deoxyribose
Uracil replaces Thymine in the
base pairs.
Memory device:
Gene Expression in Eukarotic
Cells
(For more information on gene
expression in prokaryotic cells see:
Transcription and Translation in
Prokaryotes by Dr. John W. Kimball)
Types of RNA
Messenger RNA
mRNA
Ribosomal RNA
rRNA
Transfer RNA
tRNA
Small Nuclear RNA
snRNA
Small Nucleolar RNA
snoRNA
Micro RNA
miRNA
RNA is produced by a process
known as :
Transcription
Transcription Steps
1. Protein Transcription factors bind to a
DNA strand
2. An RNA Polymerase binds to the
transcription factors
3. The DNA is opened up
Transcription
4. The polymerase moves down one of
the DNA strands in the 3’ to 5’ direction.
5. It assembles RIBONUCLEOTIDES into
stand of RNA
6. These nucleotides are inserted using
rules for similar to DNA EXCEPT that in
place of Thymine the nucleotide URICIL
is used
Transcription
At some point the end of the RNA
molecule is reached and the
polymerase and the RNA strand are
released from the DNA into the
nucleus.
The DNA then goes back to its stable
double helix form.
32
AND then...
The transcribed RNA may be fully
functional or it may need to be
processed before it can be
functional.
mRNA must be processed and the
unneeded intron areas removed.
More on Transcription at:
http://www.ultranet.com/~jkimba
ll/BiologyPages/T/Transcription.
html
Translation
Central
Dogma
Graphic
R
Transfer
RNA
tRNA
Anticodon
Messenger RNA
Long strands of RNA that contain the
codes for the proteins that will be
produced by a gene.
These codes are in sets of 3
nucleotides and are called CODONS.
Translation Steps:
The light or
smaller sub unit
of the ribosome
binds to the
mRNA strand.
Translation Steps
It moves along the strand until it
reaches a “start” codon - AUG. At
this point it is joined by the large or
Heavy subunit and a special tRNA
that is known as the initiator tRNA
that in eukarotic cells carries the
Methionine amino acid
Steps continued
There are three sites in the large
ribosomal unit called the
A SITE, the P SITE, and the EXIT
SITE
were tRNA bind temporarily, deliver
their amino acid and then bind it to
the growing polypeptide chain.
Steps continued
tRNA enter at the A site binding to
a codon with their anticodon. They
then slide over to the P site where
their amino acid is bound to the
polypeptide chain and finally they
slide into the Exit site and are
release back out into the
cytoplasm
Steps continued
Steps continued
The process continues until a stop
codon is reached.
UAA
UGA
UAG
Steps continued
Steps continued
Most of the graphics in this
presentation come from:
http://www.sbc.edu.hk/bio/pic4we
b/dna/page_01.htm
or
http://www.nbif.org/products/clip
art/clipart.php#ca-molecular
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