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Saladin A&P, 8e Extended Chapter Outline

Chapter 4

Genetics and Cellular Function

I. DNA and RNA—The Nucleic Acids (pp. 112–117)

A. Johann Friedrich Miescher found, in 1869, that cell nuclei contained a substance that he called

nuclein, which we now know to be deoxyribonucleic acid (DNA). (p. 112)

B. DNA and other nucleic acids are composed of nucleotides, each of which is made up of a sugar, a

phosphate group, and a nitrogenous base. (pp. 112–114) (Fig. 4.1a)

1. Three bases, cytosine (C), thymine (T), and uracil, are pyrimidines; two bases,

adenine (A) and guanine (G) are purines. (Fig. 4.1b)

2. The structure of DNA resembles a spiral staircase; each sidepiece is a backbone

composed of phosphate groups alternating with the sugar deoxyribose. (Fig. 4.2)

3. The nitrogenous bases form the steplike connections between the two backbones.

a. Adenine and thymine form two hydrogen bonds with each other; guanine and

cytosine form three hydrogen bonds. (Fig. 4.2b)

b. The A–T and C–G combinations are called base pairs.

c. The law of complementary base pairing allows prediction of the sequence of one

strand if the other strand’s sequence is known.

4. The essential function of DNA is to code for the proteins a cell synthesizes.

Insight 4.1 Discovery of the Double Helix (Fig. 4.3)

C. DNA is complexed with proteins to form a fine filamentous material called chromatin. (pp.

114–116)

1. Human chromatin consists of 46 long filaments called chromosomes.

2. The DNA is about 2 m in length if fully unwound but is packed into the nucleus.

3. Viewed with electron microscopy, the chromatin has an appearance like beads on a string

and is divided into segments called nucleosomes. (Fig. 4.4a)

a. Each nucleosome consists of a core particle and a short segment of linker

DNA.

b. The core particle is a disc-shaped cluster of eight proteins called histones, with

the DNA wound around it like ribbon around a spool. (Fig. 4.4b)

4. Nucleosomes are arranged in a zigzag pattern, folding the chromatin like an accordion.

5. The folded strand is then packed into irregular loops and coils that make up the

chromosome.

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6. Each chromosome is packed into its own region of the nucleus called a chromosome

territory, permeated with channels.

7. When a cell is preparing to divide, it copies its entire nuclear DNA. Each chromosome is

made up of two sister chromatids that then coil further. These chromatids are visible in the light

microscope.

a. In early cell division, a chromosome is compacted to its greatest extent.

b. Sister chromatids are joined at a centromere, which has a kinetochore on each side.

(Fig. 4.5)

D. Ribonucleic acid (RNA) is produced in three types: messenger RNA (mRNA), ribosomal RNA

(rRNA), and transfer RNA (tRNA). (p. 116)

1. RNA and DNA have significant differences although both are nucleic acids.

(Table 4.1)

a. RNA is much smaller than DNA, ranging from 70 to 90 bases in tRNA to over

10,000 bases in the largest mRNA; DNA averages more than 100 million base pairs.

b. RNA is a single nucleotide chain, not a double helix.

c. The sugar in RNA is ribose, not deoxyribose; and thymine is replaced by uracil.

(Fig. 4.1b)

2. The essential function of RNA is to interpret the code in DNA and direct the synthesis of

proteins.

II. Genes and Their Action (pp. 117–125)

A. The classical concept of a gene was as an abstract “unit of heredity” by which a trait passes to

offspring. (p. 117)

B. We now define a gene as an information-containing segment of DNA that codes for the production

of a molecule of RNA, which in most cases goes on to play a role in protein synthesis. (p. 117)

C. The 46 human chromosomes come in two sets of 23 each, one set from each parent. All the

DNA in one 23-chromosome set is called the genome. (p. 118)

1. The human genome consists of about 3.1 billion nucleotide pairs.

2. The Human Genome Project (HGP) revealed the base sequence of more than 99% of the

genome.

3. Genomics is a new field of biology that studies a genome and how its genes and

noncoding DNA interact.

4. Revelations from the HGP include six major findings.

a. Humans have fewer genes than the 100,000 previously thought.

b. These genes generate millions of different proteins. A single gene can code for

many proteins through alternative splicing of mRNA.

c. Genes average about 3,000 bases long, but range up to 2.4 million bases.

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d. All humans are at least 99.99% genetically identical, but even this small

variation means there is a possibility to differ from one another in more than

3 million base pairs.

i. Combinations of these differences, called single-nucleotide

polymorphisms, account for all human genetic variation.

e. Some chromosomes are gene-rich, and some are gene-poor.

f. We now know that there are more than 1,400 disease-producing mutations,

which opens the door to a new branch of medical diagnosis called genomic

medicine.

Insight 4.2 Genomic Medicine

D. The genetic code is a system that enables 4 nucleotides to code for the amino acid sequences of

all proteins. (pp. 118–119)

1. The minimum code to symbolize 20 amino acids is 3 nucleotides per amino acid.

a. Each group of 3 nucleotides coding for an amino acid is called a base triplet.

b. Messenger RNA contains a “mirror image” of each base triplet, and this

group of three bases is called a codon.

2. The possible combinations of three bases would yield a maximum of 64 codons;

therefore, some amino acids are specified by more than one codon. (Table 4.2)

3. The code also contains stop codons that end the translation to amino acids and a start

codon that also codes for methionine.

E. DNA and RNA collaborate to make proteins. (pp. 119–120)

1. The genetic code specifies which proteins a cell can make.

2. Different cells have different active genes. A given cell uses only one-third to two-

thirds of its genes.

3. When a gene is activated, an mRNA copy is made; this migrates to the cytoplasm

where its code is “read” by a ribosome.

F. Transcription is the process by which DNA is copied into RNA. (pp. 118–119)

1. RNA polymerase is the enzyme that binds to the DNA and assembles the RNA strand.

a. Certain DNA sequences (such as TATATA) instruct the RNA polymerase

where to begin.

b. The enzyme opens up the DNA helix, reads the bases in one strand, and

matches them with complementary bases.

c. At the end of the gene is a terminator sequence that signals the enzyme to

stop.

d. The RNA produced is pre-mRNA that contains “sense” portions called exons

and “nonsense” portions called introns.

i. Enzymes excise the introns and splice together the exons.

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ii. Through alternative splicing, a gene can code for more than one

protein. (Fig. 4.6)

G. Translation is the process by which the language of nucleotides is converted into the language

of amino acids. (pp. 119–122) (Fig. 4.8)

1. Three components are needed for the process.

a. The mRNA begins with a protein cap where a ribosome will begin translation.

b. tRNA is a molecule that coils on itself to form an angular L shape that has a

3-nucleotide anticodon on one end, and a binding site for one amino acid on the

other. (Fig. 4.7)

c. Ribosomes, which consist of two subunits (a large subunit and a small

subunit), are composed of rRNA and several enzymes.

i. The two subunits come together only during translation.

ii. There are three pockets, the A site, the P site, and the E site, where

tRNAs bind.

2. Translation occurs in three steps.

a. Initiation starts when mRNA passes through a nuclear pore. (Fig. 4.8-1)

i. The small subunit binds to the leader sequence near the cap.

ii. The subunit slides along the mRNA until it reaches the first start

codon AUG.

iii. Initiator tRNA with anticodon UAC pairs with the start codon and

positions itself into the P site of the joining large subunit.

iv. The assembled ribosome slides along the mRNA and reads the

codons.

b. Elongation represents sequential ribosomal activity that increases the number

of amino acids in the polypeptide chain. (Fig. 4.8-2)

i. The next tRNA arrives, carrying another amino acid; it binds to the

A site of the ribosome and its anticodon pairs with the second codon.

ii. An enzyme in the ribosome transfers the Met of the initiator tRNA to

the amino acid of the second tRNA and creates a peptide bond, giving a

dipeptide.

iii. The ribosome slides down to read the next codon, which shifts the

initiator tRNA (without the amino acid) to the E site, where it leaves

the ribosome.

iv. The second tRNA (carrying the dipeptide) shifts into the P site,

opening the A site for the third tRNA to bind. The dipeptide from the

second tRNA is now transferred to the amino acid of the third, and a

new peptide bond is formed, creating a tripeptide.

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v. The process is repeated by the sliding of the ribosome to the next

codon. The empty tRNA exiting the E site goes off to pick up another

amino acid.

vi. Some mismatches are tolerated in the third base of the codon; this is

called “wobble,” and it allows 48 different tRNAs to pair up with the

61 codons that represent amino acids.

c. Termination is the process where the ribosome reaches a stop codon.

(Fig. 4.8-3)

i. The A site binds a protein called release factor instead of tRNA. ii.

This causes the release of the protein from the ribosome; the ribosome

dissociates into two subunits that can reassemble at the same mRNA

and start the process again.

3. If protein is to be packaged into a lysosome or secreted from the cell, the ribosome

docks on the rough endoplasmic reticulum and the new protein spools off into the

cisterna of the ER instead of the cytosol. (Fig. 4.8-4)

4. One mRNA can hold 10 or 20 ribosomes in a cluster called a polyribosome, and the

mRNA is translated by all of these ribosomes at once. (Fig. 4.9)

5. Transcription and translation are sequential processes that begin with DNA and end

with a protein product. (Fig. 4.10)

H. Protein synthesis requires more than the primary sequence of amino acids. It also involves a

process that folds and coils the protein chain into secondary and tertiary structures, and in some

cases association with other protein chains (quaternary structure), or binding with other molecules.

(pp. 122–125)

1. Chaperone proteins guide a new protein strand into its proper configuration. Some of

these are called stress proteins or heat shock proteins because they are produced under

these conditions.

2. The endoplasmic reticulum (ER) and the Golgi complex have important roles in

protein processing and secretion. (Fig. 4.11)

a. A protein assembled on the ER surface moves into the ER cisterna where it is

modified by enzymes (posttranslational modification).

b. The ER pinches off a bubblelike transport vesicle coated with clathrin that

holds completed proteins. These transport vesicles may fuse and move to the

Golgi complex.

c. The cluster of vesicles fuses with the Golgi complex, releasing its contents

into the Golgi cisterna.

d. The Golgi complex modifies proteins further, often by adding carbohydrate

chains.

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e. The Golgi cisterna farthest from the ER buds off or breaks up into Golgi

vesicles.

f. Some of the Golgi vesicles become lysosomes, while others become secretory

vesicles that merge with the plasma membrane, releasing cell products by

exocytosis.

g. Proteins synthesized in this way have a variety of destinations and functions.

(Table 4.3)

I. Genes are regulated by a number of factors that turn them on or off as their products are needed.

(p.124)

1. The production of casein by mammary gland cells provides an example of gene

regulation. (Fig. 4.12)

a. The hormone prolactin binds to receptors in the plasma membrane of a

mammary gland cell.

b. The binding to receptors triggers activation of a regulatory protein in the

cytoplasm.

c. The regulatory protein moves into the nucleus and binds to DNA near the

gene for casein, a milk protein.

d. This binding enables RNA polymerase to transcribe the gene, producing

mRNA.

e. Casein mRNA moves to the cytoplasm and is translated by ribosomes on the

rough ER.

f. The Golgi complex packages casein into secretory vesicles.

g. The vesicles release casein outside the cells, and it becomes part of breast

milk.

2. Regulatory proteins can turn gene transcription on or off in a number of ways,

including attracting and positioning RNA polymerase, and coiling or uncoiling DNA in a

nucleosome to make it accessible.

J. Compounds other than proteins are synthesized in cells and are made through the activity of

enzymes, which are proteins encoded by genes. (p.125)

1. There is no gene for the steroid testosterone, but a cell of the testis enzymatically

converts cholesterol to testosterone if genes for the enzymes are activated. (Fig. 4.13)

2. DNA codes for RNA and protein synthesis, but it indirectly controls the synthesis of a

wide range of substances.

III. DNA Replication and the Cell Cycle (pp. 126–130)

A. DNA replication is necessary prior to cell division and is accomplished through complementary

base pairing. (p. 126–127) (Fig. 4.14)

1. The first step is the unwinding of the double helix from the histones.

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2. An enzyme called DNA helicase opens up a short segment of the helix, exposing the

bases. The point where the DNA is opened is called the replication fork.

3. Molecules of DNA polymerase match the exposed bases with complementary free

nucleotides.

a. The two strands are copied by separate DNA polymerase molecules moving

in opposite directions.

i. One polymerase makes a long, continuous strand; the other makes

short strands moving away from the replication fork, and these are

joined together by DNA ligase.

b. Two new daughter DNA molecules are thus synthesized from the original

parental molecule, and each daughter DNA consists of one old (conserved) helix

and one newly synthesized helix.

i. This process is therefore called semiconservative replication.

4. While DNA is synthesized in the nucleus, new histones are being synthesized in the

cytoplasm and are transported into the nucleus for combination with each new DNA helix

to make new nucleosomes.

5. Thousands of polymerase molecules work simultaneously on each DNA molecule.

B. Changes in DNA structure are called mutations and result from replication errors or

environmental factors. (p. 127)

1. DNA polymerase may make errors in copying, but there are many modes of error

correction.

2. Some mutations have no effect because they do not change the amino acid for which

they code, for example, TGG and TGC both code for threonine. (Table 4.2)

3. Since 98% of the DNA does not code for proteins, the majority of mutations do not

affect protein structure at all.

C. The cell cycle is the life cycle period from one cell division to the next. It has four main phases:

G1, S, G2, and M. (pp. 127–128) (Fig. 4.15)

1. G1, the first gap phase, is an interval between cell division and DNA replication.

a. During this time the cell synthesizes proteins, grows, and carries out its tasks

in support of the body.

b. Cells in G1 also accumulate the materials needed to replicate DNA in the next

phase.

2. S, the synthesis phase, is the period during which the cell makes a duplicate copy of its

centrioles and its entire nuclear DNA.

3. G2, the second gap phase, is a relatively brief interval during which the cell finishes

replicating its centrioles and synthesizes enzymes that control cell division; it also checks

replication for errors.

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4. M, the mitotic phase, is the period during which a cell replicates its nucleus and then

pinches in two to form two new daughter cells.

5. Phases G1, S, and G2 are collectively called interphase—the time between M phases.

6. The length of the cell cycle varies depending on cell type; skin cells divide rapidly,

bone cells slowly, and skeletal muscle and nerve cells not at all.

7. Some cells cease to divide for days, years, or the rest of one’s life; these are said to be

in G0 (G-zero) phase.

a. An inability to stop cycling and enter G0 is characteristic of cancer cells.

D. Cells divide by two mechanisms: mitosis and meiosis; meiosis is restricted to production of

eggs and sperm, and mitosis serves all the other functions of cell division. (pp. 128–130)

1. Mitosis is responsible for development of an individual from a one-celled fertilized

egg.

2. Mitosis brings about growth of all tissues and organs after birth.

3. Cells that die are replaced by mitosis.

4. Repair of damaged tissues is accomplished by mitosis.

5. Mitosis has four phases: prophase, metaphase, anaphase, and telophase. (Fig. 4.16)

a. Prophase is the period during which the chromosomes shorten and thicken,

becoming compact rods, and then align in the middle of the cell.

i. There are two chromatids per chromosome.

ii. The nuclear envelope disintegrates and the chromosomes are

released into the cytosol.

iii. The centrioles sprout elongated microtubules called spindle fibers,

which push the centrioles apart.

iv. Some spindle fibers grow toward the chromosomes and attach to the

kinetochore on each side of the centromere. (Fig. 4.5)

v. The chromosomes are tugged back and forth until they line up at the

cell’s midline.

b. Metaphase is the period during which the aligned chromosomes, anchored by

the mitotic spindle, await the signal to split apart.

c. Anaphase is the cleaving of the centromere of each chromatid pair. After

splitting apart, each chromatid is a daughter chromosome.

i. One daughter chromosome migrates to one pole, and the other

daughter chromosome to the opposite pole.

ii. Migration is achieved by motor proteins in the kinetochore crawling

along the spindle fiber.

iii. Sister chromatids/daughter chromosomes are genetically identical to

each other, and so the daughter cells that result are genetically identical.

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d. Telophase is the last phase of mitosis, during which the daughter

chromosomes are clustered on each side of the cell. A new nuclear envelope

forms and the chromosomes begin to uncoil, while the mitotic spindle vanishes.

6. Cytokinesis overlaps telophase; it is the division of the cytoplasm into two cells.

a. The motor protein myosin pulls on microfilaments of actin in the

cytoskeleton, creating a crease called the cleavage furrow.

b. The parent cell eventually pinches in two as the furrow is deepened, and the

daughter cells enter interphase.

E. The timing of cell division and its inhibition are areas of intense research. (p. 128–129)

1. Cells divide when a number of conditions are met:

a. When they grow large enough to have sufficient cytoplasm for two daughter

cells.

b. When they have replicated their DNA.

c. When they receive an adequate supply of nutrients.

d. When they are stimulated by growth factors.

e. When neighboring cells die, creating space for new cells.

2. Cells cease dividing under other conditions:

a. When they are snugly in contact with neighboring cells.

b. When growth factors are removed.

c. Contact inhibition is the cessation of cell division in response to contact with

other cells.

IV. Chromosomes and Heredity (pp. 130–134)

A. Heredity is the transmission of genetic characteristics from parent to offspring. (p. 130)

B. A karyotype is a chart of the chromosomes, arranged in pairs and by size and other physical

features. (pp. 130–131) (Fig. 4.17)

1. The two members of each chromosome pair are called homologous chromosomes;

one is inherited from the mother and one from the father.

2. Homologous chromosomes generally look alike with the exception of the sex

chromosomes (X and Y); the non-sex chromosomes are called autosomes.

3. A female has two homologous X chromosomes; a male has one X chromosome and a

smaller Y chromosome.

4. Any cell with 23 pairs of chromosomes is said to be diploid; sperm and egg cells,

however, are haploid, having half as many chromosomes.

a. Sperm and eggs, and cells that will become sperm and eggs, are called germ

cells.

b. All other cells of the body are called somatic cells.

5. At fertilization, a sperm cell and an egg cell fuse, restoring the diploid number.

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C. Each chromosome carries many genes; the location of a gene is called its locus. Homologous

chromosomes may carry different forms of a single gene, called alleles, at the same locus.

(pp. 131–132)

1. Alleles produce alternative forms of a trait.

2. Frequently, one allele is dominant, and the other allele is recessive.

a. A dominant allele masks the effect of a recessive allele. Recessive alleles are

expressed only when present on both homologous chromosomes.

b. Typically, dominant alleles code for a normal, functional protein and

recessive alleles for a nonfunctional variant.

c. An example of a dominant trait is the trait for cleft chin. (Fig. 4.18)

3. Individuals with two identical alleles at a locus are said to be homozygous for that

trait; individuals with different alleles at a locus are said to be heterozygous.

4. The alleles that an individual possesses constitute the genotype. An observable trait is

called the phenotype.

5. An allele is said to be expressed if it appears in the phenotype.

a. The only way most recessive alleles can be expressed is if an individual

inherits them from both parents.

6. Recessive traits can “skip” one or more generations.

a. A diagram called a Punnett square shows how heterozygous parents can

produce a child with a homozygous recessive trait. (Fig. 4.18b)

b. Each parent produces two types of germ cells; the phenotype of 75% of

children would be the dominant phenotype, but 25% would have the recessive

phenotype.

c. Heterozygous individuals with normal phenotypes may be carriers of

hereditary diseases in the form of recessive alleles, such as in the case of cystic

fibrosis.

d. For some hereditary diseases, tests are available, and a genetic counselor can

have tests performed and advise potential parents of risks.

D. Some genes have multiple alleles within the gene pool of the population. (p. 132)

1. The ABO blood types have two dominant alleles, IA and IB, and one recessive allele, i.

a. To have type A blood, a person must have either IAIA or IAi alleles.

b. To have type B blood, a person must have either IBIB or IBi alleles.

c. To have type AB blood, a person must have IAIB alleles.

d. To have type O blood, a person must have ii alleles.

2. The two dominant alleles for the ABO blood types are an example of alleles that are

equally dominant, or codominant.

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a. The A and B alleles code for enzymes that produce different surface

glycolipids on red blood cells.

b. In type AB, both types of glycolipids are present; in type O, neither type is

present.

3. Some alleles exhibit incomplete dominance, in which the phenotype is intermediate if

two alleles are present.

E. Polygenic or multiple-gene inheritance occurs when genes at two or more loci, or even on

different chromosomes, contribute to a single phenotypic trait. (p. 132–133) (Fig. 4.19)

1. Human eye and skin colors are examples of polygenic traits.

2. Pleiotropy occurs when a single gene produces multiple phenotypic effects.

a. Alkaptonuria is a disorder caused by a mutation on chromosome 3 that blocks

the breakdown of tyrosine, resulting in accumulation of homogenistic acid.

(Fig. 4.20)

b. Oxidation of homogenistic acid causes connective tissues to turn gray to

bluish black and causes degeneration of these tissues.

c. The urine also turns dark when exposed to air.

d. In addition to darkening, damage also occurs to joints, heart valves, and other

internal organs.

e. Sickle-cell disease is another example of pleiotropy.

F. Sex-linked traits are carried on the X or Y chromosome and therefore tend to be inherited by

one sex more than the other. (p. 133)

1. Men are more likely to exhibit red-green color blindness or hemophilia than are

women. (Fig. 4.21)

2. These phenotypes are caused by alleles on the X chromosome. A woman has two

X chromosomes, so that she may be heterozygous, whereas a man has only one X

chromosome and normally expresses any allele found there.

3. The X chromosome carries about 260 genes, most of which are not related to sex. The

Y chromosome has very few genes, so that most sex-linked traits are associated with the

X chromosome.

G. In some cases, a genotype does not produce the expected phenotype. (pp. 133–134)

1. Penetrance refers to the percentage of a population with a given genotype that actually

exhibits the predicted phenotype

a. The penetrance of the polydactyly allele may be 80%, indicating that 80% of

those having the polydactyly genotype exhibit extra digits.

2. Environmental factors play an important role in gene expression; at the very least, all

gene expression depends on nutrition. (Fig. 4.22)

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H. Dominance and recessiveness are unrelated to how common an allele is in the population. Type

O is the most common ABO blood type in North America, and polydactyly is rare. (p. 134)

I. The fast-growing field of epigenetics is the study of heritable changes that are not caused by

mutations in the DNA. (p. 134)

1. DNA methylation is a common way to alter gene expression without altering the DNA

sequence.

a. Inappropriate DNA methylation has been implicated in cancers and in

hereditary diseases such as Prader-Willi syndrome.

b. If epigenetic changes persist over multiple generations, your phenotype may

be a result of environmental influences on your grandparents, just as

environmental influences on you might affect your grandchildren.

Insight 4.3 Cancer (Fig. 4.23 and 4.24)

Cross ReferencesAdditional information on topics mentioned in Chapter 4 can be found in the chapters listed below.

Chapter 18: Sickle Cell Anemia

Chapter 27: Meiosis

Chapter 29: Hereditary defects and nonhereditary birth defects

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