5.4 DNA and Heredity
How does the strucutre of DNA allow for its replication?
We studied two key features of life on Earth that are likely to be crucial to life anywhere else as well: the structural units of cells and the metabolic processes that keep cells alive.
The third feature that is needed for life is the DNA.
It encodes hereditary information that allows organisms to reproduce.
It's molecular structure is a double helix.
On the two backbones there are little strands that link bases. There are 4 bases.: A G T C
A(adenine) G(Guanine) T(Thymine) C(Cytosine)
The key to DNA’s ability to be duplicated by cellular machinery lies in the way the four DNA bases pair up to link the two strands: T can pair up only with A, while C can pair up only
with G. This is supposed to be complementary bases
The process by which DNA is copied, called DNA replication, is illustrated in the Figure:
Although this looks simple, it is actually complex and a dozen special enzymes are involved in various steps.
This complexity is one reason why errors sometimes occur in DNA replication, which is crucial in evolution.
DNA probably formed from RNA.
How is heredity encoded in DNA?
Not only having the ability to replicate, DNA also determines the structure and functions of cells within any living organism.
Technology can rapidly determine sequences in any strand of DNA.
Within a large DNA molecule, isolated sequences of DNA bases represent the instructions for a variety of cell functions.
The instructions representing any individual function—such as the instructions for building a single protein—make up what we call a gene.
A gene is the basic functional unit of an organism’s heredity—a single gene consists of a sequence of DNA bases (or RNA bases, in some viruses) that provides the instructions
for a single cell function.
In eukarya, a lot of DNA does not carry instruction for any particular cell function. This is called noncoding DNA
They make up for 95% of the total DNA in humans.
They represent evolutionary artifacts, DNA that once had functions in ancestral cells but aren't needed anymore in our bodies.
The complete sequence of DNA bases in an organism, so all the organisms genes and its noncoding DNA, is called an organism's genome.
Different things have different number of genes.
We humans have around 20 to 25 thousand genes in its sequence a 3 billion DNA bases.
We're not even complex.
Every member of a particular species have the same basic genome, but has variations among individuals.
With a few exceptions every cell in a living organism contains the same set of genes.
To read a sequence we have to break the sequence into words, and theres a set of rules for reading them, called the genetic code.
There are 64 words spelled out with different genetic codes. This is because there are three DNA bases, and 4 bases, so 4x4x4
The codes for most amino acids really depend on the first 2 bases in the three base genetic words
The code is the same in all living organisms on Earth.
This common language of the genetic code is further evidence for a common ancestor of all life on Earth.
RNA plays an important role in the DNA replication because it carries out genetic instructions, like a messenger that delivers the gene.
It copies and pastes the DNA instructions for use in another part of the cell and goes to a part called the ribosome and assembles amino acids, effectively translates the genetic
instructions, and create an actual protein from amino acids.
How does Life Evolve?
DNA allowed us to confirm Darwin's theory of evolution through natural selection.
He is right, we found out that DNA molecules gradually changed through time. We now know how this change occurred.
DNA replication proceeds with remarkable speed and accuracy. Some microbes can copy their complete genomes in a matter of minutes, and copying the complete threebillion
base sequence in human DNA takes a human cell only a few hours.
In terms of accuracy, the copy ing process generally occurs with less than one error per billion bases copied. Nevertheless, errors sometimes occur.
Any change in the base sequence of an organism’s DNA is called a mutation.
Mutations can affect proteins in a variety of ways.
In some cases, such a change will alter a protein only slightly, hardly affecting its functionality. But in other cases, the change can be much more dramatic.
Mutations that add or delete a base within a gene tend to have the most dramatic effects on protein structure. It kind of shifts where all the spacing between the code happens like this:
"The cat ate the rat" will turn into "ThZ eca tat eth era t"
Usually this is leathal but sometimes they survive and the mutation will be copied every time it's DNA is replicated. If that cell is the one that is passed down when the animal has
offspring, the baby will have the