Download How does every cell get a copy of DNA?

How does every cell get a
copy of DNA?
Before a cell divides, it has to copy
its DNA so the new cell can have a
copy. Why?
Remember that DNA Replication
happens during the S phase of
INTERPHASE, when the cell is
growing.
Also remember
that DNA runs
opposite
directions on
either side (one
side is upside
down). This is
called an
antiparallel
pattern.
To start, the DNA unwinds with
the help of the enzyme helicase.
Then, the DNA ladder
splits in two. The
hydrogen bonds that
hold the bases
together are broken.
The A’s separate from
the T’s and the G’s
separate from the
C’s. It is like the DNA
“unzips”.
This unzipped area
is called a
replication fork.
Typically DNA
unzips into a bubble
with a fork at either
end.
Once the DNA is unzipped, pre-made
nitrogenous bases (ATG & C) that are
loose in the nucleus come and “fill in”
each split half of the DNA like a puzzle.
This process is aided by the
enzyme DNA polymerase.
Where there is a
nucleotide with a
Guanine base on
the DNA, a
nucleotide with a
Cytosine base is
filled in by DNA
polymerase.
Where there is a
nucleotide with a
Thymine base on
the DNA, a
nucleotide with an
Adenine base is
filled in by DNA
polymerase.
When there is:
DNA Polymerase
will fill in:
A
T
C
G
G
C
T
A
Once all of the bases
are filled in, you have
two complete, perfect
copies of DNA. It is
perfect because each
unzipped half of DNA
provides a pattern, or
template, to fill in the
other half.
In the two resulting copies of DNA,
each is half original DNA, and half
new DNA. So, we say that DNA
replication is semi-conservative.
X
What Does it Actually Do?
DNA controls living things
because it holds the genetic code.
The instructions
in the genetic
code are used to
build proteins.
Remember that proteins are the
tools that cells need to do their
jobs.
Proteins make
up many
structures
in a cell!!
(your fingernails
and hair are also
made of protein).
Enzymes that make important
chemical reactions in a cell go
faster are also proteins.
Proteins are
made up of
chains of
AMINO ACIDS
Transcription is
the first step to
making proteins.
It involves
making a copy of
the genetic code.
The genetic code is
made up of the order of
the nitrogen bases
(ATGC) in the steps of
the DNA ladder.
Every three bases make up a code
that stands for a certain
AMINO ACID.
This group of 3 bases is called a
CODON.
So, the code on the DNA decides
what order the amino acids are put
together in and what kind of
protein is made.
3 Nitrogen
Bases
3 Nitrogen
Bases
Codon
Codon
for
forone
one
amino
aminoacid
acid
Codon
for second
amino acid
Codon
for third
amino acid
Codon
for fourth
amino acid
Codon
for fifth
amino acid
For example, the Codon CTG codes for
the amino acid Aspartic acid.
The Codon T T C codes for the amino
acid Lysine.
The process that
turns the DNA
code into an
actual protein is
called….
PROTEIN
SYNTHESIS.
Varying the code
in the DNA
changes the
order of Amino
Acids in the
protein, which
makes a
different protein
with different
properties.
For example,
Changing this
one Amino
Acid changes
blood cells:
The RBC on the right has sickle cell
anemia due to a 1 AA change
Nor mal Red Blood Cell
RBC with Sickle Cell Anemia
Protein Synthesis uses a
molecule like DNA called RNA.
• RNA is single stranded
–it is only made of one half of the
ladder
• RNA has a different nitrogen base
–URACIL (U) instead of Thymine (T)
• So, RNA’s bases are AUGC.
DoubleStranded
SingleStranded
There are 3 important kinds of RNA
• mRNA (messenger RNA)
• tRNA (transfer RNA)
• rRNA (ribosomal RNA)
To start protein
synthesis, the DNA
unzips, like if it were
going to copy itself.
Instead of copying
DNA, though, a copy
of MESSENGER RNA
(mRNA) is made
using the DNA
template.
The enzyme RNA polymerase helps
build mRNA just as DNA
polymerase helped build DNA.
Where DNA has:
A
T
G
C
This RNA
Base will be
filled in:
U
A
C
G
Certain sections of
DNA code called
promoters tell the
RNA polymerase
where to attach to
DNA and start
making mRNA.
These act as a start
signal for transcription.
Thus, the
genetic code
is re-written
(transcribed)
in mRNA.
The mRNA has extra “junk” code in
it called introns.
These introns are cut out and the
good code (called exons) are put
together and their ends are capped.
The mRNA is now complete.
Introns
Removed!
When the mRNA is made, it leaves
the nucleus and heads for a
ribosome (WHERE PROTEINS
ARE MADE).
In translation, the mRNA is read
and the code is used to actually
build a protein.
The ribosome attaches to the mRNA and
runs down it like a train on a track.
It stops every three bases (once per codon),
and a transfer RNA (tRNA) attaches.
AA Chain
RNA
(Protein)
Ribosome
AA Chain
RNA
(Protein)
Ribosome
TRANSFER RNA is just three bases
long, but it has and extra “arm” that
grabs onto certain amino acids.
Only certain tRNAs with just the right
three bases will grab onto certain
amino acids.
The three bases on tRNA are
called an anti-codon.
At this end,
an Amino
Acid is
carried
Anti-Codon
matches a
sequence
on the RNA
(CUU)
The ribosome puts the mRNA and
tRNA with its amino acid together
temporarily.
Only the right tRNA with the right
anti-codon and the right amino acid
will fit onto each mRNA codon.
So, a tRNA’s job is to carry an
amino acid to its mRNA codon.
Once everybody is on board, the
ribosome moves three more
bases down, and another tRNA
attaches with another amino acid.
The two amino acids on the two
tRNAs on the ribosome on the
mRNA stick to each other.
Now that two amino acids are
stuck together with a peptide bond,
the amino acid chain that makes a
protein has begun to form.
The ribosome keeps moving and
attaching more tRNAs and more
amino acids to the chain.
Once the tRNAs have
done their job and their
amino acid has joined the
protein chain, they leave
to get another amino acid.
When the ribosome gets to
the end of the mRNA, it finds
a stop codon that tells it the
protein is complete.
The ribosome then lets go of
the mRNA and the protein lets
go of the ribosome.
Many ribosomes may read a
strand of mRNA at once.
From here the protein enters
the Endoplasmic Reticulum
(ER) and Golgi Complex to
be finalized, packaged in
vesicles and sent off to go
do its job in the cell, or to be
sent out of the cell in
exocytosis.