Waching daily Aug 29 2018

Have you ever thought about what life is?

Is it like one of the discoverers

of the structure of DNA

James Watson said

it's just a matter of exquisitely organized

physics and chemistry?

Watson also believed that

DNA is the code of life

What does it really mean?

To understand Watson's idea

we have to know the structure of DNA

and the decoding mechanism

Then we can understand

how DNA affects the functions of cells

the communication among cells

and the formation of

biological structures and characteristics

Thereby

we can have a deeper understanding of

the interaction between organisms

and the environment

Let's use eukaryotic cells

which are cells with true nuclei

for explanation

Chromosomes in the nucleus consist of

DNA molecules and proteins

The DNA molecules are made of

two DNA strands

which form the double-helix structure

There are four nucleotide bases

on each DNA strand

which are represented by the four letters

A, T, C, and G

The sequence of these four bases

are called DNA sequence

The DNA sequences on the two DNA strands

are complementary

Base A pairs with base T

while base C pairs with base G

One DNA molecule can have

as many as 200,000,000 base pairs

forming a very long DNA sequence

in which the biological information

is hidden

So how could these biological information

be expressed?

To answer this question

we have to know the decoding mechanism

of the DNA sequence

that means the mechanism of gene expression

Gene is a segment of DNA sequence

carrying genetic information

Through the processes of

transcription and translation

the hidden genetic information is expressed

First

in transcription

the two DNA strands are unzipped

One of the DNA strands

serves as the template

for the synthesis

of a complementary mRNA strand

Please note that

mRNA has the base U instead of the base T

The mRNA is processed

and cut to become a mature mRNA

and then leave the nucleus

The biological information

in the DNA sequence

has been transcribed accurately

into the mRNA sequence

At the second step of gene expression

translation

The mRNA binds with ribosomes in the cell

A step-by-step translation begins to reveal

the genetic information

hidden in the mRNA strands

Three RNA bases are read as one coding unit

Two coding units are paired with

two complementary tRNA molecules

which carry two specific

amino acids to the ribosome

The two adjacent amino acids are then bound

and the first tRNA molecule is discharged

Then the ribosome starts

to decode the next coding unit

until the entire translation process

is completed

In this way

the information in the mRNA sequence

is translated into amino acid chains

which fold into 3D-structured proteins

with specific biological functions

Let's look at this example

In the haemoglobin gene

The biological information

in the DNA sequence

is transcribed into the mRNA sequence

and then translated

into haemoglobin proteins

which are responsible for

carrying oxygen in red blood cells

When a person carries

an abnormal haemoglobin gene

such as having a mutation

in the DNA sequence

the hidden biological information

is different

After transcription and translation

the amino acid chains may be different

Thus

abnormal haemoglobin proteins are formed

which cause the formation of

sickle-shaped red blood cells

This reduces the

oxygen-carrying capacity of red blood cells

and may result in

shortening one's life span

Up to here

do you agree that DNA is the code of life?

DNA decoding is an

exquisitely organized process

involving only

physical processes and chemical reactions

So do you agree with Watson that

life is just a matter of

exquisitely organized physics and chemistry?