1. Chapter 03
The Molecules of Cells
© 2012 Pearson Education, Inc. 1

2. 3.1 Life’s molecular diversity is based on the properties of carbon
Structural formula
Ball-and-stick model
Space-filling model
Diverse molecules found in cells are composed of carbon bonded to other carbons and atoms of other elements.
Carbon-based molecules are called organic compounds.
The reason why the carbon atom is so central to the chemistry of life is because of the number of electrons in its outermost shell.
Remember that carbon has 4 electrons in its outer shell, leaving 4 “available spots” to share electrons and form covalent bonds. By sharing electrons, carbon can bond to four other atoms and branch in up to four directions.
Methane (CH4) is one of the simplest organic compounds. Four covalent bonds link four hydrogen atoms to the carbon atom. Each of the four lines in the formula for methane represents a pair of shared electrons.
Methane is a three dimensional molecule, with the four bonds angling out towards the corners of an imaginary tetrahedron.
This shape occurs every time a carbon atom participates in four single bonds. However, these angles and overall shape will shift when the carbon atom is forming double or triple bonds.
The four single bonds of carbon point to the corners of a tetrahedron. 2

3. 3.1 Life’s molecular diversity is based on the properties of carbon
A carbon skeleton is a chain of carbon atoms that can
Vary in length
Be branched or unbranched
Have double or triple bonds
Be arranged in rings
Methane and other compounds composed of only carbon and hydrogen are called hydrocarbons.
Methane and propane are examples of hydrocarbon fuels. You can think of longer hydrocarbons that make up fat molecules as the fuel for you body’s cells.
Carbon, with attached hydrogens, can bond together in chains of various lengths.
The chain of carbon atoms in an organic molecule is called a carbon skeleton.
© 2012 Pearson Education, Inc. 3

4. 3.1 Life’s molecular diversity is based on the properties of carbon
Compounds with the same formula but different structural arrangements are called isomers.
For example, butane and isobutane have the same molecular formula, C4H10, but differ in the bonding pattern of their carbon skeleton.
1-butene and 2-butene also have the same number of atoms, but they have different three-dimensional shapes because of the location of the double bond.
Isomers can also result from different spatial arrangements of the four partners bonded to a carbon atom, and because the shape of a molecule usually helps determine the way it functions in the body, isomers of a molecule can have different effects.
© 2012 Pearson Education, Inc. 4

5. 3.2 A few chemical groups are key to the functioning of biological molecules
An organic compound has unique properties that depend upon the
size and shape of the molecule and
groups of atoms (functional groups) attached to it.
A functional group affects a biological molecule’s function in a characteristic way.
Compounds containing functional groups are hydrophilic (water-loving).
Because of the atoms present, functional groups are polar compounds, and this polarity makes compounds with functional groups hydrophilic, meaning they are soluble in water, which is a necessary characteristic for compounds that participate in biological reactions.
© 2012 Pearson Education, Inc. 5

6. hydroxyl group—consists of a hydrogen bonded to an oxygen which is bonded to the carbon skeleton. Organic compounds that contain hydroxyl groups are called alcohols
carbonyl group—a carbon linked by a double bond to an oxygen atom. If the carbonyl group is at the end of a carbon skeleton is called and aldehyde; if it is within the chain the compound is called a ketone. Sugars usually contain at least one carbonyl group and several hydroxyl groups
carboxyl group—consists of a carbon double-bonded to both an oxygen and a hydroxyl group. The carboxyl group acts as an acid by contributing a hydrogen atom to a solution. Compounds with carboxyl groups are called carboxylic acids, ex. Acetic acid
amino group—composed of a nitrogen bonded to two hydrogen atoms and the carbon skeleton. This group acts as a base because it takes a hydrogen atom from a solution. Organic compounds with an amino group are called amines.Amino acids, the building blocks of proteins, have that name because they have an amino group and a carboxyl group
phosphate group—consists of a phosphorus atom bonded to four oxygen atoms; it is usually ionized and attached to the carbon skeleton by one of its oxygen atoms. Compounds with phosphate groups are called organic phosphates and are often involved in energy transfers, as in ATP
methyl group – consists of a carbon bonded to three hydrogen atoms. Compounds with methyl groups are called methylated compounds. The addition of a methyl group to DNA, for example, affects gene expression 6

7. 3.2 A few chemical groups are key to the functioning of biological molecules
An example of similar compounds that differ only in functional groups is sex hormones.
Male and female sex hormones differ only in functional groups.
The differences cause varied molecular actions.
The result is distinguishable features of males and females.
These subtle differences result in the different actions of these molecules, which help produce the contrasting features of males and females in vertebrates.
Testosterone
Estradiol 7

8. 3.3 Cells make a huge number of large molecules from a limited set of small molecules
There are four classes of molecules important to organisms:
carbohydrates,
proteins,
lipids, and
nucleic acids.
© 2012 Pearson Education, Inc. 8

9. 3.3 Cells make a huge number of large molecules from a limited set of small molecules
The four classes of biological molecules contain very large molecules.
They are often called macromolecules because of their large size.
They are also called polymers because they are made from identical building blocks strung together.
The building blocks of polymers are called monomers.
The way a cell builds such a large molecule is by joining smaller molecules into polymer chains – poly, multiple, and meros, part
© 2012 Pearson Education, Inc. 9

10. 3.3 Cells make a huge number of large molecules from a limited set of small molecules
Monomers are linked together to form polymers through dehydration reactions, which remove water.
Polymers are broken apart by hydrolysis, the addition of water.
All biological reactions of this sort are mediated by enzymes, which speed up chemical reactions in cells.
Most of the organic molecules in your food are in the form of polymers, which must be broken down to be used by the cells.
So, during digestion, hydrolytic enzymes break apart the polymers.
© 2012 Pearson Education, Inc. 10

11. 3.3 Cells make a huge number of large molecules from a limited set of small molecules
A cell makes a large number of polymers from a small group of monomers. For example,
proteins are made from only 20 different amino acids and
DNA is built from just four kinds of nucleotides.
The monomers used to make polymers are universal.
A cell makes all its thousands of different macromolecules from a small list of about 40 to 50 common components and a few others that are rare.
The key to the diversity of polymers in natural organisms is arrangement – polymers vary in the sequence in which their monomers are strung together.
Even though monomers are universal, and we have the same amino acids than a tree or an insect, the way the monomers are arranged in polymers accounts for the uniqueness of each individual organism
© 2012 Pearson Education, Inc. 11

12. 3.4 Monosaccharides are the simplest carbohydrates
Carbohydrates range from small sugar molecules (monomers) to large polysaccharides.
Sugar monomers are monosaccharides, such as those found in honey,
glucose, and
fructose.
Monosaccharides can be hooked together to form
more complex sugars and
polysaccharides.
Carbohydrates are a class of molecules that range from the small sugar molecules that make soft drinks sweet to the large polysaccharides that we eat in potatoes and pasta, called starches.
© 2012 Pearson Education, Inc. 12

13. Glucose (an aldose) Fructose (a ketose)
Here we can see the molecular structure of glucose and fructose, showing the two functional groups that characterize sugars, a carbonyl group and several hydroxyl groups.
So, sugars are alcohols, because of the hydroxyl groups, and also, depending where the carbonyl group is, they are either an aldose or aldehyde sugar, or a ketose or a ketone sugar.
Notice that glucose and fructose have the same number of atoms, they differ only in their arrangement, which means that they are …?
This simple difference makes fructose taste sweeter than glucose
Glucose (an aldose)
Fructose (a ketose) 13

14. 3.4 Monosaccharides are the simplest carbohydrates
The carbon skeletons of monosaccharides vary in length.
Glucose and fructose are six carbons long.
Others have three to seven carbon atoms.
Monosaccharides are
the main fuels for cellular work and
used as raw materials to manufacture other organic molecules.
The most common monosaccharides are five carbon sugars, called pentoses, and six carbon sugars, called hexoses.
© 2012 Pearson Education, Inc. 14

15. Abbreviated structure6 5 4 1
In aqueous solutions many monosaccharides form rings.
The ring diagram may be abbreviated by not showing the carbon atoms at the corners of the ring and drawn with different thicknesses for the bonds, to indicate that the ring is a relatively flat structure with attached atoms extending above and below it.
To form the glucose ring, carbon 1 bonds to the oxygen attached to carbon 5. 3 2
Structural formula
Abbreviated structure
Simplified structure 15

16. 3.5 Two monosaccharides are linked to form a disaccharide
Two monosaccharides (monomers) can bond to form a disaccharide in a dehydration reaction.
The disaccharide sucrose is formed by combining
a glucose monomer and
a fructose monomer.
The disaccharide maltose is formed from two glucose monomers.
© 2012 Pearson Education, Inc. 16

17. 3.6 CONNECTION: What is high-fructose corn syrup, and is it to blame for obesity?
Sodas or fruit drinks probably contain high-fructose corn syrup (HFCS).
Fructose is sweeter than glucose.
To make HFCS, glucose atoms are rearranged to make the glucose isomer, fructose.
Almost any processed food in the market shows high-fructose corn syrup as one of the ingredients.
The reason is so popular in food industry is because is cheaper than sucrose and easier to mix into drinks and processed foods.
Like the name indicates, it contains corn syrup, which comes from the starch in corn.
Industrial processing hydrolyzes starch, a polymer, into its component monomers, glucose, producing corn syrup.
However, glucose by itself is not as sweet as sucrose, and fructose is actually sweeter than glucose and sucrose.
In 1970 a process was developed that uses an enzyme that can rearrange the atoms of glucose to give the sweeter isomer, fructose.
So, HFCS came to be, by mixing regular corn syrup with fructose, which produces a mix of about 55% fructose and 45% glucose.
© 2012 Pearson Education, Inc. 17

18. 3.6 CONNECTION: What is high-fructose corn syrup, and is it to blame for obesity?
High-fructose corn syrup (HFCS) is
used to sweeten many beverages and
may be associated with weight gain.
Good health is promoted by
a diverse diet of proteins, fats, vitamins, minerals, and complex carbohydrates and
exercise.
Maybe you have heard reports linking high-fructose corn syrup to the “obesity pandemic”.
The reason why is because in the same time period that HFCS consumption tripled, the incidence of obesity doubled.
And this increase in HFCS intake is associated to consumption of drinks that are sweetened with HFCS, such as soft drinks.
There is still research undergoing and there is no definitive proof that HFCS is responsible for the increased incidence of diseases associated with increased weight.
However, there is a consensus among researchers and nutritionists that overconsumption of sugar and HFCS along with fats and decreased physical activity contribute to weight gain.
© 2012 Pearson Education, Inc. 18

19. 3.7 Polysaccharides are long chains of sugar units
macromolecules and
polymers composed of thousands of monosaccharides.
Polysaccharides may function as
storage molecules or
structural compounds.
Linked together by dehydration reactions.
© 2012 Pearson Education, Inc. 19

20. 3.7 Polysaccharides are long chains of sugar units
Starch granules in potato tuber cells
Starch
Glucose monomer
Glycogen granules in muscle tissue
Glycogen
Three common types of polysaccharides are starch, glycogen, and cellulose.
Starch is a storage polysaccharide found in plants, is made up entirely of glucose monomers, and the chain coils into a helical shape and is packed into starch granules.
These granules serve as carbohydrate “banks” that the plant can access at any time to withdraw glucose for energy or building materials.
Humans and many animals possess enzymes that can hydrolyze the starch polymer into its glucose monomers.
For example, if you place an unsalted cracker in your mouths, holding it there while it mixes well with saliva, you might soon notice that a sweeter taste begins to emerge. The salivary enzyme amylase begins the digestion of starches into disaccharides, which may be degraded further by other enzymes. These disaccharides are the source of the sweet taste.
Another common polysaccharide is glycogen, which is used by animals to store glucose.
Glycogen is more highly branched than starch, and most of it is stored as granules in the liver and muscle cells.
Cellulose, the most abundant organic compound on Earth, is a major component of plant’s cell walls, and it is also a polymer of glucose.
However, its monomers are linked in a different orientation: arranged parallel to each other, cellulose molecules are joined by hydrogen bonds, forming cable-like microfibrils.
Animals cannot hydrolyze the glucose linkages in cellulose because we lack the necessary enzyme. However, when we eat this insoluble fiber it helps to our digestive system health.
Some animals, such as cows and termites, have microorganisms (bacteria) in their digestive system that allow them to access the glucose in cellulose because they produce the necessary enzyme.
Chitin is another structural polysaccharide used by insects and crustaceans to build their exoskeleton, and is also found in the cell walls of fungi.
Cellulose microfibrils in a plant cell wall
Cellulose
Hydrogen bonds
Cellulose molecules 20

21. 3.7 Polysaccharides are long chains of sugar units
Polysaccharides are usually hydrophilic (water-loving).
Bath towels are
often made of cotton, which is mostly cellulose, and
water absorbent.
Because of the many hydroxyl groups attached to the sugar monomers, almost all carbohydrates are hydrophilic.
Which is why cotton bath towels are the best ones to dry – cotton is mostly cellulose and it absorbs water readily because of its water-loving nature – which is due to what?: hydroxyl groups (-OH) in cellulose.
© 2012 Pearson Education, Inc. 21

22. 3.8 Fats are lipids that are mostly energy-storage molecules
are water insoluble (hydrophobic, or water-fearing) compounds,
consist mainly of carbon and hydrogen atoms linked by nonpolar covalent bonds.
are important in long-term energy storage,
contain twice as much energy as a polysaccharide
Ducks, for example, spread oils on their feathers to make them repel water, which helps them stay afloat.
The main function of fats is long-term energy storage and it also helps to cushion vital organs and to isolate the body.
One gram of fat stores more than twice the energy found in a gram of polysaccharide, which is advantageous for moving organisms such as us.
The downside is that it takes more effort to burn off excess fat.
© 2012 Pearson Education, Inc. 22

23. 3.8 Fats are lipids that are mostly energy-storage molecules
Lipids differ from carbohydrates, proteins, and nucleic acids in that they are
not huge molecules and
not built from monomers.
Lipids vary a great deal in
structure and
function.
© 2012 Pearson Education, Inc. 23

24. 3.8 Fats are lipids that are mostly energy-storage molecules
We will consider three types of lipids:
fats,
phospholipids, and
steroids.
A fat is a large lipid made from two kinds of smaller molecules,
glycerol and
fatty acids.
A fatty acid can link to glycerol by a dehydration reaction.
A fat contains one glycerol linked to three fatty acids.
Fats are often called triglycerides because of their structure.
© 2012 Pearson Education, Inc. 24

25. 3.8 Fats are lipids that are mostly energy-storage molecules
Some fatty acids contain one or more double bonds, forming unsaturated fatty acids that have one fewer hydrogen atom on each carbon of the double bond.
Double bonds cause kinks or bends in the carbon chain, and prevent them from packing together tightly and solidifying at room temperature.
Fats with the maximum number of hydrogens are called saturated fatty acids.
© 2012 Pearson Education, Inc. 25

26. 3.8 Fats are lipids that are mostly energy-storage molecules
Unsaturated fats include corn and olive oils.
Most animal fats are saturated fats.
Hydrogenated vegetable oils (such as those in margarine) are unsaturated fats that have been converted to saturated fats by adding hydrogen.
This hydrogenation creates trans fats associated with health risks.
© 2012 Pearson Education, Inc. 26

27. 3.9 Phospholipids and steroids are important lipids with a variety of functions
Phospholipids are the major component of all cells
Phospholipids are structurally similar to fats.
Fats contain three fatty acids attached to glycerol.
Phospholipids contain two fatty acids attached to glycerol.
Cells as we know them could not exist without phospholipids since these molecules are the major component of cell membranes.
They are structurally similar to fats, but they contain only two fatty acids attached to glycerol.
© 2012 Pearson Education, Inc. 27

28. Symbol for phospholipid
Phosphate group
Glycerol
Water
Hydrophilic heads
Hydrophobic tails
Symbol for phospholipid
Water
A negatively charged phosphate group is attached to glycerol’s third carbon.
The hydrophilic and hydrophobic ends of multiple molecules assemble in a bilayer of phospholipids to form a membrane.
The hydrophobic tails of the fatty acid cluster in the center and the hydrophilic phosphate heads face the watery environment on either side of the membrane.
The reason why the tails are depicted with one bending is because one of the two fatty acids has a double bonds that gives the molecule this shape.
If you were to add phospholipids to water they would either form a membrane surrounding water or they would form a micelle, with the lipid tails completely excluding water from the center. 28

29. 3.9 Phospholipids and steroids are important lipids with a variety of functions
Steroids are lipids in which the carbon skeleton contains four fused rings.
Cholesterol is a
common component in animal cell membranes and
starting material for making steroids, including sex hormones.
It is also used by animals cells as starting material for making other steroids, including sex hormones, like testosterone and estradiol
The body naturally produces cholesterol, and we also ingest it in our diets, that’s why if you eat foods with too much cholesterol your blood will have to much of it, leading to atherosclerosis, which is basically formation of plaques of cholesterol in your arteries, blocking the blood flow.
© 2012 Pearson Education, Inc. 29

30. 3.10 CONNECTION: Anabolic steroids pose health risks
are synthetic variants of testosterone,
can cause a buildup of muscle and bone mass, and
are often prescribed to treat general anemia and some diseases that destroy body muscle.
The word anabolic refers to anabolism, which is the natural process of substance building by the body.
Testosterone causes a general buildup of muscle and bone mass in males during puberty and maintains masculine traits throughout life.
Because anabolic steroids structurally resemble testosterone, they also mimic some of its effects.
© 2012 Pearson Education, Inc. 30

31. 3.10 CONNECTION: Anabolic steroids pose health risks
Anabolic steroids are abused by some athletes with serious consequences, including
violent mood swings,
depression,
liver damage,
cancer,
high cholesterol, and
high blood pressure.
Many athletes use steroids to “artificially” build up their muscles quickly and “enhance” their performance.
Additionally, use of steroids makes the body reduce its output of natural male sex hormones, which can cause shrunken testicles, reduced sex drive, infertility, and breast enlargement in men.
In women, it can lead to menstrual cycle disruption and development of masculine characteristics.
In teens, bones may stop growing, stunting growth.
Also, although the muscles grow really quick, the joints and ligaments continue normal development, leading to severe injuries.
And a the deadliest “side effect” is that many steroid abusers share needles, sky rocketing the probabilities of acquiring HIV.
© 2012 Pearson Education, Inc. 31

32. 3.11 Proteins are made from amino acids linked by peptide bonds
involved in nearly every dynamic function in your body and
very diverse, with tens of thousands of different proteins, each with a specific structure and function, in the human body.
Proteins are composed of differing arrangements of a common set of just 20 amino acid monomers.
A protein is a polymer of amino acids.
© 2012 Pearson Education, Inc. 32

33. 3.11 Proteins are made from amino acids linked by peptide bonds
Amino group
Carboxyl group
All amino acids have an amino group and a carboxyl group.
These functional groups are covalently bonded to a central carbon atom, called the alpha carbon, which is also covalently bonded to a hydrogen atom and a chemical group usually symbolized by R.
The R group, or side chain, is what differs in each amino acid.
Glycine, the simplest amino acid, has a hydrogen atom.
In other amino acids the R group can cosist of one or more carbon atoms with various functional groups attached. 33

34. 3.11 Proteins are made from amino acids linked by peptide bonds
Hydrophobic
Hydrophilic
The composition and structure of the R group will determine the particular properties of each of the 20 different amino acids .
And depending on these, they will be hydrophobic or hydrophilic.
Leucine, for example, has an R group that is nonpolar, making it hydrophobic.
Serine, on the other hand, has a hydroxyl acid group in its side chain, making it polar, hydrophilic.
Aspartic acid is also polar because of it’s carboxylic group in the R group, and it is negativelly charged at the pH of the cell.
Other amino acids have side chains with amino groups and are positively charged at the cell’s pH.
As a matter of fact, all the amino and carboxyl groups of amino acids are usually ionized at cellular pH, because of the watery environment of the cell.
Amino acids with polar and charged R groups help proteins dissolve in the aqueous solutions inside cells.
Leucine (Leu)
Serine (Ser)
Aspartic acid (Asp) 34

35. 3.11 Proteins are made from amino acids linked by peptide bonds
Carboxyl group
Amino group
Dehydration reaction
Amino acid
Amino acid
Dipeptide
Amino acid monomers are linked together in a dehydration reaction, joining carboxyl group of one amino acid to the amino group of the next amino acid, and creating a peptide bond.
Additional amino acids can be added by the same process to create a chain of amino acids called a polypeptide.
The product of the first reaction then is called a dipeptide, made from two amino acids.
For the inverse reaction to occur hydrolysis takes place, with a molecule of water being added back to break each peptide bond in the polypeptide chain.
Most polypeptides are 100 amino acids long, some are 1000 or more, and each polypeptide has a unique sequence of amino acids.
However, a long polypeptide chain of specific amino acids is still not a functioning protein
It needs to be precisely coiled, twisted, and folded into a unique three-dimensional shape 35

36. 3.12 A protein’s specific shape determines its function
Probably the most important role for proteins is as enzymes, proteins that
serve as metabolic catalysts and
regulate the chemical reactions within cells.
Enzymes speed up chemical reactions!
© 2012 Pearson Education, Inc. 36

37. Other proteins are also important.1. 2. 3. 4. 6.
Structural proteins provide associations between body parts.
Contractile proteins are found within muscle.
Defensive proteins include antibodies of the immune system.
Signal proteins are best exemplified by hormones and other chemical messengers.
Receptor proteins transmit signals into cells.
Transport proteins carry oxygen.
Storage proteins serve as a source of amino acids for developing embryos or storage of important compounds, such as iron. 5. 7. 37

38. Groove
Groove
A polypeptide chain contains hundreds or thousands of amino acids linked by peptide bonds.
The amino acid sequence causes the polypeptide to assume a particular shape.
The shape of a protein determines its specific function.
Here, for example, you can see models of lysozyme, an enzyme found in your sweat, tears, and saliva.
Lysozyme’s general shape is called globular.
The yellow bridges are bonds between sulfur atoms present in some of the amino acid’s side chains, and they help stabilize and maintain the protein’s shape.
Most enzymes and many proteins are globular.
One exception are structural proteins, which are usually long and thin and are called fibrous proteins. 38

39. 3.12 A protein’s specific shape determines its function
If a protein’s shape is altered, it can no longer function.
In the process of denaturation, a polypeptide chain
unfolds,
loses its shape, and
loses its function.
Proteins can be denatured by changes in salt concentration, pH, or by high heat.
Nearly all proteins must recognize and bind to some other molecule to function.
Lysozyme, for example, must bind to specific molecules on bacterial cell surfaces to be able to destroy them.
These surface molecules fit into the groove.
When we cook meat, we’re basically causing the proteins to denature.
One of the reasons why extremely high fever is so dangerous is that some proteins in your body, including brain cells, become denatured and cease to function.
Under proper cellular conditions, a newly synthesized protein chain will spontaneously fold into its functional shape.
© 2012 Pearson Education, Inc. 39

40. Beta pleated sheet Alpha helix
Four Levels of Protein Structure
Primary structure
Amino acids
Amino acids
Secondary structure
Hydrogen bond
Beta pleated sheet
Alpha helix
Tertiary structure
Transthyretin polypeptide
The primary structure of a protein is its unique sequence of amino acids, which is determined by inherited genetic information.
Here we have transthyretin, a protein that transports vitamin A and one of the thyroid hormones throughout your body - a complete molecule of transthyretin has four identical polypeptide chains of 127 amino acids. each, the three letter abbreviations here represents the specific amino acids that make up the chain.
For the protein to function it must have the correct amino acids arranged in a precise order, and even a slight change in primary structure may affect its overall shape hindering its ability to function.
For example, a single amino acid change in hemoglobin causes sickle-cell anemia, a serious blood disorder that results in decreased oxygenation of organs and tissues.
In the second level of protein structure, parts of the polypeptide coil or fold into local patterns called secondary structure.
If the polypeptide chain coils it forms a structure called an alpha helix, and if it undergoes a certain kind of folding it forms a structure called beta pleated sheets.
These structures are maintained by regularly spaced hydrogen bonds between hydrogen atoms and oxygen atoms along the backbone of the polypeptide chain, here represented by a row of dots.
The tertiary structure of a polypeptide refers to its three-dimensional shape, which results from interactions between the R groups of the various amino acids.
For example, transthyretin and other proteins found in aqueous solutions are folded so that the hydrophobic R groups are on the inside of the molecule and the hydrophilic R groups on the outside, exposed to water. Also, hydrogen bonding between polar side chains and ionic bonding of some of the charged R groups add stabilization. And covalent bonds between sulfur atoms present in some amino acid’s side chains also reinforce the protein’s shape.
Many proteins consist of two or more polypeptide chains aggregated into one functional macromolecule, this is what is called the protein’s quaternary structure, which results from the associations of the polypeptides, known as subunits. Here, you can see a complete molecule of transthyretin with its four identical globular subunits
Quaternary structure
Transthyretin, with four identical polypeptides 40

41. 3.14 DNA and RNA are the two types of nucleic acids
Gene
DNA
Transcription
Nucleic acids
RNA
The amino acid sequence of a polypeptide is programmed by a discrete unit of inheritance known as a gene.
Genes consist of DNA(deoxyribonucleic acid), a type of nucleic acid. Make note that the “NA” in DNA and RNA stands for nucleic acid.
DNA is inherited from an organism’s parents.
A characteristic that is unique to DNA is that it provides directions for its own replication.
Thus, as a cell divides, its genetic instructions are passed to each daughter cell.
These instructions program all of a cell’s activities by directing the synthesis of proteins.
However, the genes present in DNA do not build proteins directly, they work through an intermediary, the ribonucleic acid or RNA
So, a gene directs the synthesis of an RNA molecule, which is called transcription.
The RNA then interacts with the protein-building machinery of the cell and the genes instructions, written in “nucleic acid language”, are translated into protein language producing the amino acid sequence of a polypeptide.
Additional to protein coding RNAs, other types of RNA molecules are also transcribed from the DNA which play many other roles in the cell.
Translation
Protein
Amino acid 41

42. Nitrogenous base (adenine)
3.15 Nucleic acids are polymers of nucleotides
Nitrogenous base (adenine)
The monomers that make up nucleic acids are called nucleotides.
Each nucleotide contains three parts:
At the center of the nucleotide is a five-carbon sugar – in DNA this sugar is deoxyribose, while in RNA the sugar is ribose.
Linked to one side of the sugar in both types of nucleotides is a negatively charged phosphate group.
Linked to the other side is a nitrogenous base, a molecular structure containing nitrogen and carbon.
Phosphate group
Sugar 42

43. 3.15 Nucleic acids are polymers of nucleotides
DNA nitrogenous bases are
adenine (A),
thymine (T),
cytosine (C), and
guanine (G).
RNA also has A, C, and G, but instead of T, it has uracil (U).
© 2012 Pearson Education, Inc. 43

44. 3.15 Nucleic acids are polymers of nucleotides
A nucleic acid polymer, a polynucleotide, forms from the nucleotide monomers when the phosphate of one nucleotide bonds to the sugar of the next nucleotide by dehydration reactions, and by producing a repeating sugar-phosphate backbone with protruding nitrogenous bases.
© 2012 Pearson Education, Inc. 44

45. Sugar-phosphate backbone
Nucleotide A T C G C G T T A C C G A T
Base pair G A T
As a result, a repeating sugar-phosphate backbone polymer is formed, with protruding nitrogenous bases.
In DNA, the two polynucleotide strands wrap around each other forming a double helix.
The nitrogenous bases protrude from the two sugar-phosphate backbones and pair in the center of the helix: A always pairs with T and C always pairs with G.
The two chains are hold together by hydrogen bonds between the paired bases. Collectivelly these hydrogen bonds zip the two strands together into a very stable double helix.
Because of the base pairing rule, the two strands are said to be complementary - If you have one you can predict the other.
This is the key to how a cell makes two identical copies of each of its DNA molecules every time it divides.
RNA usually consists of a single polynucleotide strand, but the same base-pairing rules account for the precise transcription of information from DNA to RNA. G C T A T A T T A
Sugar-phosphate backbone 45

46. 3.16 EVOLUTION CONNECTION: Lactose tolerance is a recent event in human evolution
The majority of people
stop producing the enzyme lactase in early childhood and
do not easily digest the milk sugar lactose.
Lactose tolerance represents a
relatively recent mutation in the human genome and
survival advantage for human cultures with milk and dairy products available year-round.
In the US as many as 80% of african americans and native americans and 90% of asian americans are lactase-defficient once they reach their teenage years.
On the other hand, americans of northern european descent make up one of the few groups in which lactase production continues into adulthood.
Researchers were curious about the genetic and evolutionary basis for the regional distribution of lactose tolerance.
In 2002 a group of scientists completed a study of the genes of 196 lactose-intolerant adults of african, asian, and european descent, and they determined that lactose intolerance is actually the human norm, while lactose tolerance represents a relatively recent mutation in the human genome.
In northern-europe’s relatively cold climate only one harvest a year is possible, making animals the main source of food for early humans in that region.
Cattle was first domesticated in northern-europe about 9000 years ago.
With milk and other dairy products at hand year-round, natural selection would have favored anyone with a mutation that kept the lactase genes switched on.
© 2012 Pearson Education, Inc. 46

47. 3.16 EVOLUTION CONNECTION: Lactose tolerance is a recent event in human evolution
Researchers identified three mutations that keep the lactase gene permanently turned on.
The mutations appear to have occurred
about 7,000 years ago and
at the same time as the domestication of cattle in these regions.
This was also found in a different study of 43 ethnic groups in east africa in 2006.
The mutations found in this study were shown to be different from the european mutations, and the mutations appear to have occurred around 7000 years ago, the same time that archaeological evidence shows the domestication of cattle in these african regions.
© 2012 Pearson Education, Inc. 47