Full transcript
0:00[Music]
0:02in science prefix mono means one are
0:06single and the prefix poly means more
0:09than one are many a polymer is a long
0:12chain of smaller chemical units called
0:14monomers the polymer shown here is made
0:17up of ten monomers but polymers can
0:19consist of thousands or even millions of
0:21monomers in biology polymers consisting
0:25of monomers called nucleotides are very
0:28important these are called poly
0:30nucleotides two very important poly
0:33nucleotides are DNA and RNA here we'll
0:38concentrate on DNA which is an
0:41abbreviation for deoxyribonucleic acid
0:44now we'll take a more detailed look at
0:47the nucleotides that make up DNA
0:51this shows the chemical structure of one
0:54nucleotide of the poly nucleotide DNA
0:57now we'll concentrate on one part on the
1:00left side of this molecule
1:05this is called the phosphate group it
1:07consists of a phosphorus atom surrounded
1:10by oxygen atoms hydrogen atoms can bond
1:13to the oxygen atoms with a negative
1:15charge and sometimes an OHA shown up
1:18here rather than an ol negative now will
1:22return to the whole nucleotide this time
1:25we'll concentrate on this part of the
1:27nucleotide
1:30this is a five-carbon sugar called
1:32deoxyribose
1:34when deoxyribose is by itself and not
1:37part of a larger molecule it has o h
1:40groups attached as shown here when
1:43looking at this type of structural
1:45formula it's important to realize that
1:47in organic chemistry carbon atoms are by
1:50far the most common atoms and molecules
1:52so to simplify structures we can just
1:55show the structures with straight lines
1:57and it is assumed that each point of
2:00intersection of the straight lines
2:01represents a carbon atom as shown by the
2:04arrows on here so if we had depicted all
2:08five carbon atoms with the letter C the
2:11structure would look like this so from
2:13now on when you see a chemical structure
2:16with just straight lines assume that
2:18every point of intersection represents a
2:20carbon atom
2:22the carbon atoms in deoxyribose are
2:24numbered in a very specific way this
2:27carbon atom is designated one prime
2:32this is the carbon atom on deoxyribose
2:34that's connected to the group on the
2:36right side of the nucleotide
2:39we proceed in a clockwise direction and
2:42the next carbon atom is called to prime
2:45this one is called three prime the three
2:49prime carbon is important to remember
2:51because it's the carbon atom and a
2:53nucleotide that bonds to other
2:55nucleotides as we'll see later it's the
2:58one near the bottom
3:01this carbon atom is called for Prime
3:05this is the fifth carbon on deoxyribose
3:08called five-prime it is the carbon atom
3:11outside of the five membered ring this
3:15five prime carbon is also important as
3:17it is the carbon atom in deoxyribose
3:20that's bonded to the phosphate group in
3:22the nucleotide looking at the whole
3:25nucleotide again this time we'll
3:27concentrate on the group on the right
3:32this is what we call a base and because
3:36it contains a number of nitrogen atoms
3:38we can also call it a nitrogenous base
3:42if this was a single molecule rather
3:45than being part of a nucleotide it would
3:47have a hydrogen atom here and it would
3:50be called adenine
3:53so to summarize our nucleotide consists
3:55of three major parts a phosphate group a
3:58sugar called deoxyribose and a
4:01nitrogenous base remember the phosphate
4:04group is connected to the five prime
4:06carbon of deoxyribose and the base is
4:09connected to its one prime carbon
4:13the bass shown here in this nucleotide
4:15is adenine adenine is only one of four
4:19bases found in the nucleotides of DNA
4:22the foreign nitrogenous bases are
4:25adenine guanine thymine and cytosine
4:30note that the groups with the N and the
4:33two H's can also be depicted simply as
4:36NH two which we'll see in other models
4:38the two bases consisting of five and six
4:42membered rings joined together are
4:44called purines these are adenine and
4:47guanine the two basis consisting of only
4:50six membered rings are called pyramidal
4:52these include thymine and cytosine if we
4:56look more closely at adenine we see that
4:59this hydrogen atom is attached directly
5:01to a nitrogen atom hydrogen atoms that
5:05are attached directly to a nitrogen or
5:07oxygen atom have a relatively high
5:10partial positive charge this is
5:13indicated by the lowercase Greek letter
5:15called Delta with a plus sign shown in
5:19purple above the eight now we'll look at
5:21this nitrogen atom colored blue nitrogen
5:25atoms and compounds tend to carry a
5:27partial negative charge indicated by the
5:30green Delta minus written below the n
5:32here looking at the base thymine we see
5:35it also has a hydrogen atom bonded
5:37directly to a nitrogen this means it has
5:41a partial positive charge now let's look
5:44at this oxygen atom on thymine it is
5:47known that oxygen atoms and compounds
5:49carry partial negative charges this is
5:53indicated by the green Delta minus
5:55written above the blue o atom here
5:57notice that the positive H on adenine
6:00and the negative o on thymine will
6:03directly line up with each other and at
6:06the same time the positive H on thymine
6:08also lines up directly with the negative
6:11N on adenine positive and negative
6:14charges attract one another and the
6:16dashed lines represent two attractive
6:18forces between a molecule of adenine and
6:20a molecule of thymine
6:23these attractive forces tend to pull the
6:26molecules together
6:29this forms what are known as hydrogen
6:31bonds although weaker than covalent
6:34bonds there are relatively strong bonds
6:36which tend to hold the bases adenine and
6:39thymine together we've shown them here
6:42as dotted lines now we'll look at two
6:45nitrogenous bases guanine and cytosine
6:49notice the two red hydrogen atoms that
6:52guanine has and the red hydrogen atoms
6:54cytosine has that are all attached
6:56directly to nitrogen atoms this means
6:59that all three of these hydrogen atoms
7:02carry a partial positive charge the
7:04other hydrogen atoms connected to the
7:06nitrogen's also have a positive charge
7:08but will dis concern ourselves with the
7:10three that we colored red remember
7:13nitrogen and oxygen atoms and compounds
7:15carry a partial negative charge let's
7:17concentrate on these nitrogen and oxygen
7:20atoms that we've colored blue we'll use
7:22the Delta minus to show that each of
7:24these blue atoms has a partial negative
7:27charge the three dash lines show how
7:29these positive charges a negative charge
7:32is line up perfectly between guanine and
7:34cytosine these dashed lines all
7:37represent attractive forces between
7:39guanine and cytosine notice there are
7:42three this time these three attractive
7:45forces tend to pull the bases guanine
7:47and cytosine together we see that three
7:51hydrogen bonds form between the bases
7:54guanine and cytosine the exact shapes
7:57and exact positions of hydrogen oxygen
7:59and nitrogen atoms on these two bases
8:02make them fit perfectly together
8:04and form hydrogen bonds just remember
8:07that guanine always pairs up with
8:09cytosine recall that the base is adenine
8:12and thymine also fit together perfectly
8:14to form two hydrogen bonds remember the
8:18de base adenine always pairs up with the
8:20base thymine
8:22here is a diagram of a nucleotide with
8:25the base adenine notice this is written
8:27with the phosphate group on the top left
8:30also notice that the five prime carbon
8:32is above the three prime carbon here's a
8:36diagram of a nucleotide with the base
8:39guanine notice this is also written with
8:41the phosphate group on the top left
8:43again the five prime carbon is above the
8:46three prime carbon
8:48here's a nucleotide in which thymine is
8:51the base again noticed the positions of
8:53the phosphate group and the five Prime
8:55and three prime carbon atoms on the
8:57sugar you may recall in the model we
8:59used before that thymine had a ch3 group
9:03attached to its ring in this model the
9:06single solid line represents a carbon
9:08atom with three hydrogen's or a ch3
9:11group just two different ways of
9:14representing the same thing here's a
9:16nucleotide with the base cytosine now
9:20we'll consider two nucleotides the top
9:22one has the base adenine and the bottom
9:25one has the base cytosine an H from the
9:28top nucleotide and an O H from the
9:30bottom one combine to form water and the
9:33two nucleotides bond together
9:37we'll discard the water molecule now
9:40we'll take the dinucleotide we made and
9:42introduce another nucleotide below it
9:45the nucleotide on the bottom has the
9:48base thymine designated by a green tea
9:52as the top molecule loses in age and the
9:55bottom molecule loses in OAH
9:59they move together and join to form a
10:01molecule with three nucleotides
10:05we'll discard the water molecule that
10:07was formed after adding another
10:09nucleotide with guanine we end up with
10:12this poly nucleotide chain notice on
10:15this chain that a 5 prime carbon is on
10:18the top left and a 3 prime carbon is on
10:20the bottom left now we're going to add
10:22another chain of nucleotides beside this
10:25one recalling the base pairing rules
10:27remember that cytosine must pair up with
10:30guanine adenine must pair up with
10:32thymine guanine must pair up with
10:35cytosine and thymine must pair up with
10:37adenine we'll bring in a chain on the
10:40right that has the bases in this order
10:42and we'll move the chains together like
10:46this we see that the base is from the
10:49left and right strand are now aligned up
10:51perfectly to form hydrogen bonds between
10:53the pairs as shown by the orange dashed
10:56lines it is these hydrogen bonds that
10:59hold the left side and right side
11:01together what we've made is a small
11:04molecule of DNA a DNA molecule consists
11:08of two strands this is the Strand on the
11:11left and this is the Strand on the right
11:13the two strands are held together by
11:16hydrogen bonds in the center notice the
11:19Strand on the right has the 3 prime
11:22carbon on the top and the 5 prime carbon
11:24on the bottom this is just the opposite
11:28of the Strand on the left if we draw an
11:31arrow from the 5 prime carbon to the 3
11:33prime carbon on the left strand it
11:36points downward
11:38but if we draw an arrow from the five
11:40prime carbon to the three prime carbon
11:41on the right strand it is pointing
11:44upward the two strands of the DNA
11:47molecule are said to be anti parallel
11:49this means they are aligned in opposite
11:52directions as you can see this anti
11:55parallel arrangement allows the bases to
11:57line up perfectly to form hydrogen bonds
12:00remember this is a phosphate group and
12:03this is a sugar called deoxyribose
12:07so what we've outlined in dark red are
12:10what we call the phosphate sugar
12:12backbones of DNA if the molecule is
12:16compared to a ladder these would be the
12:18vertical rails of the ladder in the
12:21center of the molecule are the
12:23complementary base pairs here's the
12:25complementary base pair adenine and
12:28thymine
12:29here's another complementary base pair
12:31cytosine and guanine now we've shaded in
12:35all four complementary base pairs in
12:37this molecule if this is compared to a
12:40ladder the base pairs form the rungs of
12:42the ladder this points out both the
12:45phosphate sugar backbones and the bases
12:48in the center of the DNA molecule this
12:50particular model has four base pairs
12:53however real DNA molecules are very much
12:56longer it has been determined that all
12:58of the DNA in the 23 chromosomes of the
13:01human gamete contained about 3.2 billion
13:04base pairs and all of these are either a
13:08T or CG the structure of DNA is often
13:12depicted in a more simplified model like
13:14this
13:16here are the two antiparallel phosphate
13:19sugar back molds and the complementary
13:22base pairs are shown in the center of
13:24this model notice that adenine pairs up
13:27with thymine and cytosine pairs up with
13:29cloning notice there are two hydrogen
13:32bonds between adenine and the thymine
13:34bases we see that there are three
13:37hydrogen bonds between the bases
13:38cytosine and guanine all of the hydrogen
13:41bonds between the base pairs hold the
13:44two strands of DNA together due to all
13:47the forces between atoms and the atoms
13:50trying to form natural bond angles the
13:52two strands tend to coil together
13:56and form what is called a double helix
14:01this model also shows the double helix
14:04structure of DNA here we're looking at a
14:07section of DNA this section of DNA we've
14:10shown here represents what we call a
14:13gene a gene is a section of DNA which
14:17contains the code or the blueprint for
14:19making a protein the code is carried by
14:21the order of the colored sections or the
14:24bases shown in the center of the spiral
14:28[Music]
14:37you