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

C

G

T

T

G

C

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 for one

amino acid

Codon for one

amino 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.

Double-Stranded

Single-Stranded

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.

This RNA Where DNA has: Base will be

filled in:

A UT A

G C 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.

RNA

AA Chain (Protein)

Ribosome

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.

RNA

AA Chain (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.

Anti-Codon matches a sequence on the RNA (CUU)

At this end, an Amino Acid is carried

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.