- [Presenter] This
video covers section 1.5
of the AP Biology Curriculum
which focuses on the
structure and function
of biological macromolecules.
In this video, we will take a close look
at the specific structures
of nucleic acids,
proteins and carbohydrates
and see how those structures
relate directly to function.
This video is meant to be a quick overview
to help you better understand the topics
and prepare for the AP test.
Let's get started.
Here is a quick overview of
the topics we will be covering.
Feel free to skip forward
to any of these sections
if you just need a quick review.
Let's start with nucleic acids.
Nucleic acids are directional molecules.
This means that they can
only be formed one way
with a hydroxyl group exposed on one end
and a phosphate group
exposed on the other.
Each strand of DNA is a separate molecule
and each strand has a hydroxyl
group exposed on one end
and a phosphate group
exposed on the other.
We call these ends the five prime end
and the three prime end.
You can remember the difference
because hydroxyl groups
at the three prime end are much smaller
than the phosphate groups
at the five-prime end.
DNA polymerase, the molecule responsible
for attaching new nucleotides
to a growing sequence
can only function in the three prime
to five prime direction.
When it is time to duplicate
the DNA within cells,
the two old strands of DNA are separated
and DNA polymerase moves in to
start adding new nucleotides.
DNA polymerase builds a new strand
that corresponds to the template
by adding new nucleotides
that are complimentary
to the template strand.
Multiple DNA polymerase
molecules work at the same time,
moving in opposite directions
on the two template strands.
The double helix structure of DNA
is formed through a
relatively simple mechanism,
hydrogen bonding.
Let's take a look at how this works.
If we look at the
complimentary nucleotides,
adenine and thymine, we can
see that two hydrogen bonds
are formed between the nucleotides.
Adenine has a slightly
positive amino group
which easily forms a hydrogen bond
with thymine's slightly negative oxygen.
The nitrogen on adenine
is slightly negative,
allowing another hydrogen bond to form
with thymine slightly positive nitrogen.
If we look at the
complimentary relationship
between guanine and cytosine,
we see a similar relationship.
Everywhere that guanine has
a slightly positive charge,
cytosine has a corresponding
negative charge.
In this case, three hydrogen
bonds can be formed.
If guanine tried to form
hydrogen bonds with thymine,
positive charges would meet
other positive charges.
This would cause the two nucleotides
to be repelled, disrupting the structure
of the DNA double helix.
This is how DNA repair
enzymes can easily find
and replace nucleotides that
are incorrect in the sequence.
Between the sugar phosphate backbone
and these hydrogen bonds,
DNA takes on a double helix
structure within the cell.
The two strands run anti
parallel to each other.
In other words, one strand
runs in the three prime
to five prime direction while
the opposite strand runs
in the five prime to
three prime direction.
This double helix typically
has a major groove
and a minor groove as
it wraps around itself.
This structure protects
the nucleotide sequence
and allows DNA to be stored
in massive units known as chromosomes.
Similar to the directionality
of DNA molecules,
proteins are also directional molecules.
Each amino acid has a
carboxyl group at one end
and an amino group at the other end.
This directionality makes it possible
for ribosomes to create
a chain of amino acids.
Let's see how this
process works in detail.
First, the ribosome grabs onto a piece
of messenger RNA, mRNA for short.
Floating around the ribosome
are many loose transfer RNA molecules.
These tRNAs have three nucleotides exposed
and hold a specific amino acid.
The tRNAs move into the E site.
If they form hydrogen bonds
with the nucleotides exposed
on the mRNAs, they can
move into the P site.
As they transfer from
the P site to the A site,
a dehydration reaction is encouraged
and a new peptide bond is formed.
The new covalent bond is formed
between the carboxyl group
on the growing peptide chain
and the amino group on the new amino acid.
This leaves another
carboxyl group exposed,
allowing another amino acid to be added
in the same direction.
This is important because
it means that amino acids
can only be added in one direction.
The amino terminus on the first amino acid
can not be added to, meaning that peptides
can only be made in the
order that mRNA dictates.
This ensures that the DNA code
can be perfectly translated
into functional protein molecules.
You can now pause the video
and take the quiz below.
There is another quiz
at the end of this video
and you can find all the
answers to these questions
in the quick test prep link below.
Proteins are very complex molecules,
thanks to the 20 plus amino acids
that can be used to construct them.
Each amino acid has a different art group
which confers both physical
and chemical properties
to a molecule.
The primary structure of a molecule
is simply the order of amino
acids within the molecule.
This order is dictated
by the codons in mRNA
which were transcribed directly
from the codon sequence in DNA.
Therefore, the primary
structure of a protein
is determined solely by
the order of nucleotides
in a DNA molecule.
However, as soon as this primary structure
is created interactions
between amino acids
in the chain start to
create secondary structure.
Secondary structure is the simplest level
of three dimensional
structure in a protein.
There are several common
motifs in secondary structure.
The two most common are beta
sheets and alpha helices.
A beta sheet is formed when
a protein strand folds back
on itself and creates hydrogen bonds.
This creates a flat
structure much like a ribbon.
By contrast, an alpha helix is formed
when peptides next to
each other in the chain
form hydrogen bonds,
creating a helix structure
that creates a rod like 3D shape.
The tertiary structure
of proteins is formed
by interactions between
different secondary structures.
In this protein, both alpha
helices and beta sheets interact
to fold the molecule
into a specific shape.
In general, tertiary structures
are formed by hydrogen
bonding, polar interactions
and attractions between
hydrophobic parts of the molecule.
This also means that these interactions
can be disrupted when the
conditions in the cell
are not just right.
For instance, if the temperature rises
or the pH has changed,
this can lead to
denaturation of a protein.
The protein will lose its
tertiary structure and unfold.
Though the primary and secondary structure
remains unchanged, the
protein will not be functional
in these conditions.
However, if the congested conditions
are changed back by
lowering the temperature
or buffering the pH, the
protein will renature
and become functional once again.
This is a major reason
why cells and organisms
have mechanisms for
controlling the physical
and chemical conditions within.
Well, many smaller polypeptides
serve functional roles
by themselves others are
simply part of large complexes.
When two or more
polypeptides come together
into a larger structure,
this is known as quaternary structure.
Like tertiary structure, the
quaternary structure formed
between two different peptide chains
is created through many
different interactions,
hydrogen bonds, hydrophobic interactions
and the physical shape of each chain.
These interactions are made possible
by all the structural levels below,
primary, secondary and tertiary.
When we look at the
electron cloud structure
of two subunits of a large
quartenary structure,
we can see that they
fit together perfectly.
Any departures in the primary structure
could lead to a change of shape,
making it impossible for these
subunits to fit together.
Therefore, the primary
structure of a protein
is ultimately responsible for
creating functional proteins.
Minor changes in the primary structure
can create many changes in the
functionality of a protein.
Proteins may work faster,
slower or not at all.
Whether or not these are
beneficial changes to the organism,
depends entirely on the protein,
what it does for the cell
and how these cellular changes
affect the organism's ability
to survive and reproduce.
In other words, a nonfunctional
protein does not always mean
that an organism is worse off.
Carbohydrates are also
directional molecules.
Glucose, the most common monosaccharide
has a large carbon group that
extends off of the top side.
Molecules in a
polysaccharide, bond together
with glycosidic bonds.
Using the same orientation
as the molecule before them,
these chains are also directional.
This allows them to be deconstructed
from the end of the molecule
back to the beginning.
However, there are some differences
in the structure of these large polymers
that allow different
functions within a cell.
Storage molecules like starch and glycogen
have a structure that is easily compressed
into a small space.
The reason that cells
store energy as starch
or glycogen is because
they take up less water
within a cell.
If glucose molecules were
simply packed within the cell,
each glucose molecule would be surrounded
by many water molecules.
By binding the glucose molecules together
into starch or glycogen,
the amount of water needed
to store these molecules
is greatly reduced.
By contrast, fiber
molecules like cellulose
have a very linear structure.
This allows cross-links to be formed
between each polysaccharide,
creating a rigid structure
that is very hard to break.
Cellulose is used to
create cell walls in plants
so it needs to be very hard to break.
The linear structure means
that they are not as good
as packing energy into a small space
but that is not their function in plants.
You can now pause the video again
and answer the following questions.
You can find answers to all the questions
in this video at the quick
test prep link below.
If you are studying for the AP test,
be sure to check out all
the other resources we have.
They can get you up to speed
on any AP biology topics.
Thanks for watching.
We hope this video was
useful and informative.
If you enjoyed it, please like the video
and subscribe to the
Biology Dictionary Channel.
Leave us comments if you have any feedback
or just want to say thanks.
Good luck.
