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© 2012 Pearson Education, Inc.

Introductory Chemistry

Fourth Edition

Nivaldo J. Tro

Chapter 19

Biochemistry

Dr. Sylvia Esjornson

Southwestern Oklahoma State University

Weatherford, OK



19.1 The Human

Genome Project

• The similarities between parents and their children are caused by genes, inheritable blueprints for making organisms.

• The structure at the bottom of this image is DNA, the molecular basis of genetic information.


19.1 The Human Genome Project

• A 15-year project to map the human genome, which contains all of the genetic material of a human being.

• The Human Genome Project was possible because of decades of research in biochemistry, the study of the chemical substances and processes that occur in plants, animals, and microorganisms.

• The mapping of the human genome revealed that humans have 20,000–25,000 genes.

• Understanding single nucleotide polymorphisms, differences from one individual to another, can help identify individuals who are susceptible to certain diseases.

• An understanding of gene function can lead to smart drug design.


19.1 The Human Genome Project

• Human genes can provide the blueprint for certain types of drugs.

• Interferon, a drug taken by people with multiple sclerosis, is a complex compound normally found in humans.

• The blueprint for making interferon is in the human genome.

• Scientists have been able to take this blueprint out of human cells and put it into bacteria, which then synthesize the needed drug.

• The drug is harvested from bacteria, purified, and given to patients.


19.2 The Cell and Its Main Chemical

Components

• The cell is the smallest structural unit of living organisms that has the properties traditionally associated with life.

• A cell can be an independent living organism or a building block of a more complex organism.

• Some cells contain a nucleus, the part of the cell that contains genetic material.

• The perimeter of the cell is bound by a cell membrane that holds the contents of the cell together.

• The region between the nucleus and the cell membrane is called the cytoplasm.

• The cytoplasm contains a number of specialized structures that carry out much of the cell’s work.



Components

• The cell is the

smallest

structural unit

of living

organisms.

• The primary

genetic

material is

stored in the

nucleus.



Components

The main chemical components of the cell

can be divided into four classes:

• carbohydrates

• lipids

• proteins

• nucleic acids


19.3 Carbohydrates: Sugar, Starch, and

Fiber

• Carbohydrates are the primary molecules responsible for short-term energy storage in living organisms.

• Carbohydrates form the main structural components of plants.

• Carbohydrates often have the general formula (CH2O)n.

• Structurally, carbohydrates are aldehydes or ketones containing multiple −OH groups.


Glucose (C6H12O6), Fructose (C6H12O6), and

Galactose (C6H12O6)

• Glucose is an aldehyde (it contains the −CHO group) with −OH groups on most of the carbon atoms.

• The many −OH groups make glucose soluble in water and blood, which is important in glucose’s role as the primary fuel of cells.

• Glucose is easily transported in the bloodstream and is soluble within the aqueous interior of a cell.


Glucose is an example of a monosaccharide, a

carbohydrate that cannot be broken down into simpler

carbohydrates. Monosaccharides such as glucose

rearrange in aqueous solution to form ring structures.


Glucose, fructose, and galactose are examples of

hexoses, six-carbon sugars. The most common

monosaccharides in living organisms are pentoses

and hexoses. Fructose and galactose form rings that

are isomers of glucose.


Glucose (C6H12O6), Fructose (C6H12O6), and

Galactose (C6H12O6)

• Fructose, also known as fruit sugar, is a hexose found in many fruits and vegetables and is a major component of honey.

• Galactose, also known as brain sugar, is a hexose usually found combined with other monosaccharides in substances such as lactose.

• Galactose is also present within the brain and nervous system of most animals.


Monosaccharides Combine to Form

Disaccharides

• Two monosaccharides can react, eliminating water to form a carbon–oxygen–carbon bond called a glycosidic linkage that connects the two rings. The resulting compound is a disaccharide, a carbohydrate that can be decomposed into two simpler carbohydrates.

• The link between individual monosaccharides is broken during digestion, allowing the individual monosaccharides to pass through the intestinal wall and enter the bloodstream.


Glucose and fructose condense to form sucrose.


Monosaccharides can link together to form

polysaccharides, long, chainlike molecules

composed of many monosaccharide units.

Polysaccharides are a type of polymer—chemical

compounds composed of repeating structural units in a

long chain.


19.3 Carbohydrates: Sugar, Starch, and Fiber

• Monosaccharides and disaccharides are simple sugars or simple carbohydrates.

• Polysaccharides are complex carbohydrates.

• Some common polysacchharides include starch and cellulose, both of which are composed of repeating glucose units.

• A third kind of polysaccharide is glycogen. Glycogen has a structure similar to starch, but the chain is highly branched. In animals, excess glucose in the blood is stored as glycogen until it is needed.


Difference between Starch and Cellulose

• The difference between starch and cellulose is the link between the glucose units.

• In starch, the oxygen atom joining neighboring glucose units points down (as conventionally drawn) relative to the planes of the rings, a configuration called an alpha linkage.

• In cellulose, the oxygen atoms are roughly parallel with the planes of the rings but pointing slightly up (as conventionally drawn), a configuration called a beta linkage.

• This difference in linkage causes the differences in the properties of starch and cellulose.


Difference between Starch and Cellulose

• Starch is common in potatoes and grains. It is a soft, pliable substance that we can easily chew and swallow.

• During digestion, the links between individual glucose units are broken, allowing glucose molecules to pass through the intestinal wall and into the bloodstream.

• Cellulose—also known as fiber—is a stiffer and more rigid substance. Cellulose is the main structural component of plants.

• The bonding in cellulose makes it indigestible by humans.

• When we eat cellulose, it passes right through the intestine undigested.


19.4 Lipids

• Lipids are chemical components of the cell that are insoluble in water but soluble in nonpolar solvents.

• Lipids include fatty acids, fats, oils, phospholipids, glycolipids, and steroids.

• Insolubility in water makes lipids an ideal structural component of cell membranes.

• Lipids are used for long-term energy storage and for insulation.


Lipids: Fatty Acids

• One class of lipids is the fatty acids,

carboxylic acids with long hydrocarbon tails.

The general structure for a fatty acid is:

where R represents a hydrocarbon chain

containing 3 to 19 carbon atoms.


Fatty acids may be saturated or unsaturated.

• Myristic acid is an example of a saturated fatty acid; its formula is CH3(CH2)12COOH. Its carbon chain has no double bonds.

• Myristic acid occurs in butterfat and in coconut oil.

• Other fatty acids—called monounsaturated or polyunsaturated fatty acids—have one or more double bonds in their carbon chains.

• Oleic acid is an example of a monounsaturated fatty acid; its formula is CH3(CH2)7CH:CH(CH2)7COOH.

• Oleic acid occurs in olive oil, peanut oil, and human fat.


Fatty acids may be saturated or unsaturated.


Fatty acids differ only in their R group. The long hydrocarbon

tails of fatty acids make them insoluble in water.


Lipids: Fats and Oils

• Fats and oils are

triglycerides,

triesters composed of

glycerol linked to

three fatty acids, as

shown in the block

diagram.


Triglycerides form by the reaction of glycerol with

three fatty acids. The bonds that join the glycerol

to the fatty acids are called ester linkages.


Tristearin—the main component of beef fat—is

formed from the reaction of glycerol and three

stearic acid molecules.


Lipids: Fats and Oils

• If the fatty acids in a triglyceride are saturated, the triglyceride is called a saturated fat and tends to be solid at room temperature.

• Lard and many animal fats are examples of saturated fat.

• If the fatty acids in a triglyceride are unsaturated, the triglyceride is called an unsaturated fat, or an oil, and tends to be liquid at room temperature.

• Canola oil, olive oil, and most other vegetable oils are examples of unsaturated fats.


Other Lipids: Phospholipids The basic structure is the

same as triglycerides, except that one of the fatty acid

groups is replaced with a phosphate group.

• Unlike a fatty acid, which is nonpolar, a phosphate group is polar and often has another polar group attached to it.

• The phospholipid molecule has a polar section and a nonpolar section.


In the structure of a phosphatidylcholine, a

phospholipid found in the cell membranes,

the polar part of the molecule is hydrophilic

(has a strong affinity for water), while the

nonpolar part is hydrophobic (avoids water).


Other Lipids: Phospholipids and Glycolipids

• Glycolipids have similar structures and properties to phospholipids.

• The nonpolar section of a glycolipid is composed of a fatty acid chain and a hydrocarbon chain.

• The polar section is a sugar molecule such as glucose.

• Phospholipids and glycolipids are often schematically represented as a circle with two long tails.

• The structure of phospholipids and glycolipids is ideal for constructing cell membranes; the polar parts interact with the aqueous environments of the cell and the nonpolar parts interact with each other.

• Lipid bilayer membranes encapsulate cells and many cellular structures.


The circle represents the polar

hydrophilic part of the

molecule, and the tails

represent the nonpolar

hydrophobic parts.

Cell membranes are

composed of lipid bilayers, in

which phospholipids or

glycolipids form a double layer.

In this bilayer, the polar heads

of the molecules point outward

and the nonpolar tails point

inward.


Other Lipids:

Steroids are

lipids that

contain the

four-ring

structure

shown here.

Some common

steroids

include

cholesterol,

testosterone,

and estrogen.


Other Lipids: Steroids

• Cholesterol serves many important functions in the body.

• Like phospholipids and glycolipids, cholesterol is part of cell membranes.

• Cholesterol serves as a precursor for the body to synthesize other steroids such as testosterone, a principal male hormone, and estrogen, a principal female hormone.

• Hormones are chemical messengers that regulate many body processes, such as growth and metabolism. They are secreted by specialized tissues and transported in the blood.


Chemistry and Health

Dietary Fats

• Most of the fats and oils in our diet are triglycerides.

• During digestion, triglycerides are broken down into fatty acids, glycerol, monoglycerides, and diglycerides.

• These products pass through the intestinal wall and then reassemble into triglycerides before they are absorbed into the blood. This process is slower than the digestion of other food types, and eating fats and oils gives a lasting feeling of fullness.

• The Food and Drug Administration (FDA) recommends that fats and oils compose less than 30% of total caloric intake. The FDA also recommends that no more than one-third of those fats (10% of total caloric intake) should be saturated fats.

• A diet high in saturated fats increases the risk of artery blockages that can lead to stroke and heart attack. Monounsaturated fats may help protect against these threats.


19.5 Proteins

• From a biochemical perspective, proteins have a broad definition.

• Within living organisms, proteins do much of the work of maintaining life.

• Most of the chemical reactions that occur in living organisms are catalyzed or enabled by proteins.

• Proteins that act as catalysts are called enzymes. Without enzymes, life would be impossible.

• Proteins are the structural components of muscle, skin, and cartilage.

• Proteins transport oxygen in the blood, act as antibodies to fight disease, and function as hormones to regulate metabolic processes.


What are proteins?

• Proteins are polymers of amino acids.

• Amino acids are molecules containing an amine group, a carboxylic acid group, and an R group (also called a side chain). The general structure of an amino acid is:

• In a protein, an R group does not necessarily mean a pure alkyl group.

• Amino acids differ from each other only in their R groups.


What are proteins?

• The R groups, or side chains, of different amino acids can be very different chemically.

• Alanine has a nonpolar side chain (—CH3) while serine has a polar one (—CH2OH).

• Aspartic acid has an acidic side chain

(—CH2COOH), while lysine has a basic one

((—CH2)4NH2).

• When amino acids are strung together to make a protein, these differences determine the structure and properties of the protein.


20

Common

Amino

Acids


• Amino acids link together because the amine end of one amino acid reacts with the carboxylic acid end of another amino acid.

• The resulting bond is a peptide bond, and the resulting molecule is called a dipeptide. Short chains of amino acids are called polypeptides.

• Functional proteins contain hundreds or even thousands of amino acids joined by peptide bonds.


19.6 Protein Structure

• In proteins, amino acids interact with one another, causing the protein chain to twist and fold in a very specific way.

• The exact shape that a protein takes depends on the types of amino acids and their sequence in the protein chain.

• Different amino acids and different sequences result in different shapes.

• Insulin is a protein that recognizes muscle cells because their surfaces contain insulin receptors, molecules that fit a specific portion of the insulin protein. If insulin were a different shape, it would not latch onto insulin receptors on muscle cells and therefore would not do its job.

• The shape, or conformation, of proteins is crucial to their function.

• There are four levels of protein structure analysis: primary structure, secondary structure, tertiary structure, and quaternary structure.


Protein structure

(a) Primary structure is the amino acid sequence.

(b) Secondary structure refers to small-scale repeating patterns such as the helix or the pleated sheet.

(c) Tertiary structure refers to the large-scale bends and folds of the protein.

(d) Quaternary structure is the arrangement of individual polypeptide chains.


Proteins: Primary Structure

• The primary protein structure is the sequence of amino acids in its chain. Primary structure is maintained by the covalent peptide bonds between individual amino acids.

• For example, one section of the insulin protein has the sequence

Gly-Ile-Val-Glu-Gln-Cys-Cys-Ala-Ser-Val-Cys.

• Each three-letter abbreviation represents an amino acid.

• The first amino acid sequences for proteins were determined in the 1950s.

• Today, the amino acid sequences for thousands of proteins are known.

• Changes in the amino acid sequence of a protein, even minor ones, can have devastating effects on the function of a protein.


Proteins: Primary Structure Hemoglobin is composed of

four protein chains, each containing 146 amino acid units. The

substitution of glutamic acid for valine in just one position on two

of these chains results in sickle-cell anemia, in which red blood

cells take on a sickle shape that ultimately leads to damage of

major organs.


Secondary Protein Structure: The Alpha-Helix

The structure is maintained by hydrogen-bonding interactions between NH

and CO groups along the peptide backbone of the coiled protein strand.


Secondary Protein

Structure:

The beta-pleated

sheet is maintained

by interactions

between the

peptide backbones

of neighboring

protein strands.

In this structure,

the chain is

extended (as

opposed to coiled)

and forms a zigzag

pattern like an

accordion pleat.


Secondary Protein Structure

• Some proteins—such as keratin, which composes hair—have the α-helix pattern throughout their entire chain.

• Some proteins—such as silk—have the β-pleated sheet structure throughout their entire chain.

• Since its protein chains in the β-pleated sheet are fully extended, silk is inelastic.

• Many proteins have some sections that are β-pleated sheet, other sections that are α-helix, and still other sections that have less regular patterns called random coils.


Everyday Chemistry

Why Hair Gets Longer When It Is Wet

• Hair is composed of a protein called keratin. The secondary structure of keratin is α-helix throughout, meaning that the protein has a wound-up helical structure. This structure is maintained by hydrogen bonding.

• Individual hair fibers are composed of several strands of keratin coiled around each other.

• When hair is dry, the keratin protein is tightly coiled, resulting in the normal length of dry hair.

• When hair becomes wet, water molecules interfere with the hydrogen bonding that maintains the α-helix structure.

• The result is the relaxation of the α-helix structure and the lengthening of the hair fiber. Completely wet hair is 10 to 12% longer than dry hair.


Tertiary Protein Structure

TERTIARY STRUCTURE consists of the large-scale bends and folds due to interactions between the R groups of amino acids that are separated by large distances in the linear sequence of the protein chain.

These interactions include:

• hydrogen bonds

• disulfide linkages (covalent bonds between sulfur atoms on different R groups)

• hydrophobic interactions (attractions between large nonpolar groups)

• salt bridges (acid–base interactions between acidic and basic groups)


Tertiary Protein Structure

• Proteins with structural functions—such as keratin, which composes hair, or collagen, which composes tendons and much of the skin—tend to have tertiary structures in which coiled amino acid chains align parallel to one another, forming long, water-insoluble fibers.

• These kinds of proteins are called fibrous proteins.

• Proteins with nonstructural functions—such as hemoglobin, which carries oxygen, or lysozyme, which fights infections—tend to have tertiary structures in which amino acid chains fold in on themselves, forming water-soluble globules that can travel through the bloodstream.

• These kinds of proteins are called globular proteins.


Quaternary Protein Structure

• Many proteins are composed of more than one amino acid chain.

• Recall that hemoglobin is composed of four amino acid chains—each chain is called a subunit.

• The quaternary protein structure describes how these subunits fit together.

• The same kinds of interactions between amino acids maintain quaternary structure and tertiary structure.


Interactions that create tertiary and quaternary structure include hydrogen

bonds, disulfide linkages, hydrophobic interactions, and salt bridges.


To summarize protein structure:

• Primary structure is simply the amino acid sequence. It is maintained by the peptide bonds that hold amino acids together.

• Secondary structure refers to the small-scale repeating patterns often found in proteins. These are maintained by interactions between the peptide backbones of amino acids that are close together in the chain sequence or adjacent to each other on neighboring chains.

• Tertiary structure refers to the large-scale twists and folds within the protein. These are maintained by interactions between the R groups of amino acids that are separated by long distances in the chain sequence.

• Quaternary structure refers to the arrangement of chains (or subunits) in proteins. It is maintained by interactions between amino acids on the individual chains.


19.7 Nucleic Acids: Molecular Blueprints

• What ensures that proteins have the correct amino acid sequence? The answer lies in nucleic acids.

• Nucleic acids contain a chemical code that specifies the correct amino acid sequences for proteins.

• Nucleic acids can be divided into two types: deoxyribonucleic acid, or DNA, which exists primarily in the nucleus of the cell; and ribonucleic acid, or RNA, which is found throughout the entire interior of the cell.

• Like proteins, nucleic acids are polymers.


The individual units composing nucleic acids are

nucleotides. Each nucleotide has three parts: a

phosphate, a sugar, and a base. In DNA, the sugar is

deoxyribose, while in RNA the sugar is ribose.


Components of DNA

DNA is a polymer of nucleotides.

Each nucleotide has three parts: a sugar group, a phosphate group, and a base.

Nucleotides are joined by phosphate linkages.


Every nucleotide in DNA has the

same phosphate and sugar, but

can have one of four different

bases.

In DNA, the four bases are

adenine (A), cytosine (C),

guanine (G), and thymine (T).

In RNA, the sugar is different,

and the base uracil (U) replaces

thymine.



• The order of bases in a nucleic acid chain specifies the order of amino acids in a protein.

• Since there are only four bases and about 20 different amino acids to be specified, a single base cannot code for a single amino acid.

• It takes a sequence of three bases—called a codon—to code for one amino acid.

• The genetic code—the understanding of which amino acid is coded for by which specific codon—was discovered in 1961.

• It is nearly universal– the same codons specify the same amino acids in nearly all organisms.

• In DNA the sequence AGT codes for the amino acid serine and the sequence TGA codes for the amino acid threonine.

• In a rat, a bacterium, or a human, the code is the same.



Codons A sequence of three nucleotides with

their associated bases is called a codon.

Each codon codes for one amino acid.



• A gene is a sequence of codons within a DNA molecule that codes for a single protein.

• Because proteins vary in size from 50 to thousands of amino acids, genes vary in length from 50 to thousands of codons.

• Each codon is like a three-letter word that specifies one amino acid.

• String the correct number of codons together in the correct sequence, and you have a gene, the instructions for the amino acid sequence in a protein.

• Genes are contained in structures called chromosomes—46 in humans—within the nuclei of cells.


Organization of

the genetic

material:

Chromosomes

Genes

Codons

Nucleotides


19.8 DNA Structure

• The ability of DNA to copy itself is related to its structure.

• DNA is stored in the nucleus as a double-stranded helix.

• The bases on each DNA strand are directed toward the interior of the helix, where they hydrogen-bond to bases on the other strand.

• The hydrogen bonding between bases is not random.


19.8 DNA Structure

• Each base is complementary—capable of precise pairing—with only one other base.

• Adenine (A) hydrogen-bonds only with thymine (T), and cytosine (C) hydrogen-bonds only with guanine (G).


DNA REPLICATION

When a cell is about to divide, the DNA within its nucleus unwinds and the hydrogen bonds joining the complementary bases break, forming two single parent strands.

With the help of enzymes, a daughter strand complementary to each parent strand—with the correct complementary bases in the correct order—is formed.

The hydrogen bonds between the strands then re-form, resulting in two complete copies of the original DNA, one for each daughter cell.


19.7 Nucleic Acids: Molecular

Blueprints Protein Synthesis

• Humans and animals must synthesize the proteins they need to survive from the dietary proteins that they eat.

• Dietary protein is split into its constituent amino acids during digestion.

• These amino acids are reconstructed into the correct proteins—those needed by the particular organism—in the organism’s cells.

• Nucleic acids direct the process.




• When a cell needs to make a particular protein, the gene—the section of the DNA that codes for that specific protein—unravels.

• The segment of DNA corresponding to the gene acts as a template for the synthesis of a complementary copy of that gene in the form of another kind of nucleic acid, messenger RNA (or mRNA).

• The mRNA moves out of the cell’s nucleus to a cell structure within the cytoplasm called a ribosome.

• At the ribosome, protein synthesis occurs.

• The mRNA chain that codes for the protein moves through the ribosome.

• As the ribosome “reads” each codon, the corresponding amino acid is brought into place and a peptide bond forms with the previous amino acid.

• As the mRNA moves through the ribosome, the protein (or polypeptide) is formed.


• Protein Synthesis The mRNA strand that

codes for a protein moves through the ribosome.

• At each codon, the correct amino acid is brought

into place and bonds with the previous amino

acid.




To summarize:

• DNA contains the code for the sequence of amino acids in proteins.

• A codon—three nucleotides with their bases—codes for one amino acid.

• DNA strands are composed of four bases, each of which is complementary—capable of precise pairing—with only one other base.

• A gene—a sequence of codons—codes for one protein.

• Chromosomes are molecules of DNA found in the nuclei of cells. Humans have 46 chromosomes.

• When a cell divides, each daughter cell receives a complete copy of the DNA—all 46 chromosomes in humans—within the parent cell’s nucleus.

• When a cell synthesizes a protein, the base sequence of the gene that codes for that protein is transferred to mRNA. The mRNA then moves out to a ribosome, where the amino acids are linked in the correct sequence to synthesize the protein.

• The general sequence is DNA RNA protein.



Drugs for Diabetes

• Diabetes is a disease in which a person’s body does not make enough insulin, the substance that promotes the absorption of sugar from the blood. Consequently, diabetics have high blood sugar levels, which can—over time—lead to a number of complications, including kidney failure, heart attacks, strokes, blindness, and nerve damage.

• One treatment for diabetes is the injection of insulin, which can help manage blood sugar levels and reduce the risk of these complications.

• Insulin is a human protein and cannot be easily synthesized in the laboratory.

• For many years, the primary source was animals, particularly pigs and cattle. Although animal insulin worked to lower blood sugar levels, some patients could not tolerate it.



Drugs for Diabetes

• Today, diabetics inject human insulin.

• Scientists were able to remove the gene for insulin from a sample of healthy human cells.

• They inserted that gene into bacteria, which incorporated the gene into their genome.

• When the bacteria reproduced, they passed on exact copies of the gene to their offspring. The result was a colony of bacteria that all contained the human insulin gene.

• The chemical machinery within the bacteria expressed the gene—meaning the bacteria synthesized the human insulin that the gene codes for.

• Today insulin made in this way is harvested from the cell cultures and bottled for distribution to diabetics.


Chapter 19 in Review

• The Cell: The main chemical components of the cell can be divided into four categories: carbohydrates, lipids, proteins, nucleic acids.

• Carbohydrates are aldehydes or ketones containing multiple —OH groups. Monosaccharides include glucose and fructose. Disaccharides, such as sucrose and lactose, are two monosaccharides linked together by glycoside linkages. Polysaccharides include starch and cellulose. Polysaccharides are also called complex carbohydrates.

• Lipids are chemical components of the cell that are insoluble in water but soluble in nonpolar solvents. Important lipids include fatty acids, triglycerides, phospholipids, glycolipids, and steroids.

• Proteins are polymers of amino acids. Amino acids are molecules composed of an amine group on one end and a carboxylic acid on the other. Between these two groups is a central carbon atom that has an R group attached. Amino acids link together by means of peptide bonds. Functional proteins are composed of hundreds or thousands of amino acids.



Protein Structure:

• Primary protein structure is the linear amino acid sequence in the protein chain. It is maintained by the peptide bonds.

• Secondary structure refers to the small-scale repeating patterns found in proteins. These are maintained by interactions between the peptide backbones of amino acids that are close together in the chain sequence or on neighboring chains.

• Tertiary structure refers to the large-scale twists and folds within the protein. These are maintained by interactions between R groups of amino acids that are separated by long distances in the chain sequence.

• Quaternary structure refers to the arrangement of chains in proteins. Quaternary structure is maintained by interactions between amino acids on the individual chains.



• Nucleic Acids, DNA Replication, and Protein Synthesis: Nucleic acids, including DNA and RNA, are polymers of nucleotides.

• In DNA, each nucleotide contains one of four bases: adenine (A), cytosine (C), thymine (T), and guanine (G). The order of these bases contains a code that specifies the amino acid sequence in proteins.

• A codon, a sequence of three bases, codes for an amino acid.

• A gene, a sequence of hundreds to thousands of codons, codes for a protein. Genes are contained in cellular structures called chromosomes.

• Complete copies of DNA are transferred from parent cells to daughter cells via DNA replication.

• In this process, the two complementary strands of DNA within a cell unravel and two new strands that complement the original strands are synthesized. In this way, two complete copies of the DNA are made, one for each daughter cell.

• When a cell synthesizes a protein, the base sequence of the gene that codes for that protein is transferred to mRNA. The mRNA then moves out to a ribosome, where the amino acids are linked in the correct sequence to synthesize the protein.

• The general sequence is: DNA RNA protein.

Chapter 19 Biochemistry - acceleratedchem.weebly.comacceleratedchem.weebly.com/uploads/4/3/8/8/43886037/19_lecture.pdf · •A 15-year project to map the human genome, which ... 12

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