1. Cells reproduce by cell division.
One cell gives rise to two or more cells, called daughter cells. (Virchow)
Each daughter cell receives a complete set of heredity information—identical to the information in the parent cell—and about half of the cytoplasm.

2. Cell division transmits hereditary information to each daughter cell.
The hereditary information in each cell is deoxyribonucleic acid (DNA).
DNA is contained in chromosomes.
A molecule of DNA consists of smaller subunits called nucleotides.

3. The eukaryotic chromosome consists of DNA bound to protein.
Human chromosomes contain a single DNA double helix that is 50 to 250 million nucleotides long, which would be about 3 inches long if the DNA were completely relaxed.

4. The nucleotides are held together by hydrogen bonding between the bases in two strands, called a double helix, which looks like a twisted ladder.
James Watson and Francis Crick combined the X-ray data with bonding theory to deduce the structure of DNA

5. DNA is composed of four nucleotides
DNA is made of chains of small subunits called nucleotides
Each nucleotide has three components
A phosphate group
A deoxyribose sugar
One of four nitrogen-containing bases
Thymine (T)
Cytosine (C)
Adenine (A)
Guanine (G)

6. The structure of DNA Fig. 8-1 phosphate nucleotide base T A A sugar C
A single strand of DNA
(b)
The double helix
Fig. 8-1

7. Hydrogen bonds between complementary bases hold two DNA strands together in a double helix (continued)
Because of their structures and the way they face each other, adenine (A) bonds only with thymine (T) and guanine (G) bonds only with cytosine (C)
Bases that bond with each other are called complementary base pairs
Thus, if one strand has the base sequence CGTTTAGCCC, the other strand must have the sequence GCAAATCGGG

8. The parental DNA is unwound Parental DNA double helix New DNA strands
are synthesized with
bases complementary
to the parental
strands
Each new double helix is composed
of one parental strand (blue) and one
new strand (red)

9. Segments of different lengths along a DNA molecule are the units of inheritance called genes.
Each gene spells out the instructions for making the proteins of the cell.
When a cell divides, it first replicates its DNA, and each copy is transferred into each daughter cell.

10. gene loci a pair of homologous chromosomes
Both chromosomes carry the same allele
of the gene at this locus; the organism
is homozygous at this locus
gene loci
This locus contains another gene for
which the organism is homozygous
Each chromosome carries a different
allele of this gene, so the organism is
heterozygous at this locus
the chromosome
from the male
parent
the chromosome
from the female
parent 10

11. Homologous chromosomes carry the same kinds of genes for the same characteristics
Genes for the same characteristic are found at the same loci on both homologous chromosomes
Genes for a characteristic found on homologous chromosomes may not be identical
Alternative versions of genes found at the same gene locus are called alleles

12. a pair of homologous chromosomes
Both chromosomes carry the same allele
of the gene at this locus; the organism
is homozygous at this locus
gene loci
This locus contains another gene for
which the organism is homozygous
Each chromosome carries a different
allele of this gene, so the organism is
heterozygous at this locus
the chromosome
from the male
parent
the chromosome
from the female
parent 12

13. An organism’s two alleles may be the same or different
Each cell carries two alleles per characteristic, one on each of the two homologous chromosomes
If both homologous chromosomes carry the same allele (gene form) at a given gene locus, the organism is homozygous at that locus
If two homologous chromosomes carry different alleles at a given locus, the organism is heterozygous at that locus (a hybrid)

14. a pair of homologous chromosomes
Both chromosomes carry the same allele
of the gene at this locus; the organism
is homozygous at this locus
gene loci
This locus contains another gene for
which the organism is homozygous
Each chromosome carries a different
allele of this gene, so the organism is
heterozygous at this locus
the chromosome
from the male
parent
the chromosome
from the female
parent 14

15. Diploid cells contain 2n chromosomes.
The number of different types of chromosomes in a species is called the haploid number and is designated n.
In humans, n = 23.
Diploid cells contain 2n chromosomes.
Humans body cells contain 2n = 46 (2 x 23) chromosomes.

16. A typical human cell has 23 pairs of chromosomes.
22 of these pairs have a similar appearance and are called autosomes.
Human cells also have a pair of sex chromosomes, which differ from each other in appearance and in genetic composition.
Females have two X chromosomes.
Males have one X and one Y chromosome.

17. Eukaryotic chromosomes usually occur in pairs.
sex chromosomes
Eukaryotic chromosomes usually occur in pairs.
An entire set of stained chromosomes from a single cell is called a karyotype.
Fig. 8-6

18. Cell division is required for growth and development.
Cell division in which the daughter cells are genetically identical to the parent cell is called Mitotic cell division.
After cell division, the daughter cells may grow and divide again, or may differentiate, becoming specialized for specific functions.
The repeating pattern of division, growth, and differentiation followed again by division is called the cell cycle.

19. Duplicated chromosomes separate during cell division.
Prior to cell division, the DNA within each chromosome is replicated.
The duplicated chromosomes then consist of two DNA double helixes and associated proteins that are attached to each other at the centromere.

20. Duplicated chromosomes separate during cell division (continued).
Each of the duplicated chromosomes attached at the centromere is called a sister chromatid.
During mitotic cell division, the sister chromatids separate and each becomes a separate chromosome that is delivered to one of the two resulting daughter cells.

21. Eukaryotic chromosomes during cell division
centromere
genes
duplicated chromosome (2 DNA double helices)
sister chromatids
(a)
A replicated chromosome consists of two sister chromatids
independent daughter chromosomes, each with one identical DNA
double helix
(b)
Sister chromatids separate during cell division
Fig. 8-5

22. Eukaryotic cell cycle

23. There are two types of division in Eukarytic cells: mitotic cell division and meiotic cell division.
Mitotic cell division may be thought of as ordinary cell division, such as occurs during development from a fertilized egg, during asexual reproduction, and in skin, liver, and the digestive tract every day.
Meiotic cell division is a specialized type of cell division required for sexual reproduction.

24. Meiotic cell division
Meiosis

25. Cell division is required for sexual and asexual reproduction.
Sexual reproduction in eukaryotic organisms occurs when offspring are produced by the fusion of gametes (sperm and eggs) from two adults.
Gametes are produced by meiotic cell division, (Meiosis which results in daughter cells with exactly half of the genetic information of their parent cells).
Fertilization of an egg by a sperm results in the restoration of the full complement of hereditary information in the offspring.

26. Mitotic cell division
Mitosis

27. Reproduction in which offspring are formed from a single parent, without having a sperm fertilize an egg, is called asexual reproduction.
Asexual reproduction produces offspring that are genetically identical to the parent.
Examples of asexual reproduction occur in bacteria, single-celled eukaryotic organisms, multicellular organisms such as Hydra, and many trees, plants, and fungi.

28. The eukaryotic cell cycle is divided into two major phases: interphase and cell division.
During interphase, the cell acquires nutrients from its environment, grows, and duplicates its chromosomes.
During cell division, one copy of each chromosome and half of the cytoplasm are parceled out into each of two daughter cells.

29. Mitotic cell division
Mitotic cell division consists of nuclear division (called mitosis) followed by cytoplasmic division (called cytokinesis) and the formation of two daughter cells.

31. Mitosis is divided into four phases.
Prophase
Metaphase
Anaphase
Telophase

32. Prophase
Fig. 8-9b–c

33. Three major events happen in prophase:
During prophase, the chromosomes condense and are captured by the spindle microtubules.
Three major events happen in prophase:
The duplicated chromosomes condense.
The spindle microtubules form.
The chromosomes are captured by the spindle.

34. The centriole pairs migrate with the spindle poles to opposite sides of the nucleus.
When the cell divides, each daughter cell receives a centriole.
Every sister chromatid has a structure called a kinetochore located at the centromere, which attaches to a spindle apparatus.

35. Metaphase
Fig. 8-9d

36. During metaphase, the chromosomes line up along the equator of the cell.
At this phase, the spindle apparatus lines up the sister chromatids at the equator, with one kinetochore facing each cell pole.

37. Anaphase
Fig. 8-9e

38. During anaphase, sister chromatids separate and move to opposite poles of the cell.
Sister chromatids separate, becoming independent daughter chromosomes.
The kinetochores pull the chromosomes poleward along the spindle microtubules.

39. Telophase
Fig. 8-9f

40. During telophase, nuclear envelopes form around both groups of chromosomes.
Telophase begins when the chromosomes reach the poles.
The spindle microtubules disintegrate and the nuclear envelop forms around each group of chromosomes.

41. Cytokinesis
Fig. 8-9g

42. Cytokinesis occurs during telophase, separating each daughter nucleus into a separate cell that then begins interphase.

43. During cytokinesis, the cytoplasm is divided between two daughter cells.
Microfilaments form a ring around the cell’s equator.
The microfilament ring contracts, pinching in the cell’s “waist.”
The waist completely pinches off, forming two daughter cells
(a)
Microfilaments contract, pinching the cell in two
(b)
Scanning electron micrograph of cytokinesis.
Fig. 8-10

44. Cytokinesis in plant cells is different than in animal cells.
In plants, carbohydrate-filled vesicles bud off the Golgi apparatus and line up along the cell’s equator between the two nuclei.
The vesicles fuse, forming a cell plate.
The carbohydrate in the vesicles become the cell wall between the two daughter cells.

46. Prokaryotic cell cycle

47. The prokaryotic cell cycle consists of a long period of growth, during which the cell duplicates its DNA.
cell division by binary fission
cell growth and DNA replication
(a)
The prokaryotic cell cycle
Fig. 8-3a

48. 1.Binary fission occurs among prokaryotes (cells that do not contain a nucleus).
2.Mitosis occurs among eukaryotes (cells that have a nucleus).
3.Binary fission does not include spindle formation (mitotic apparatus) and sister chromatids in its process making it a faster means of cellular division than mitosis.
4.Binary fission does not have the four distinct cellular phases (from G1 down to the final mitotic phase) that are seen in mitosis.

49. 10.1 What Is the Physical Basis of Inheritance?
Inheritance is the process by which the traits of organisms are passed to their offspring

50. 10.1 What Is the Physical Basis of Inheritance?
A gene is a unit of heredity that encodes information needed to produce proteins, cells, and entire organisms
Genes comprise segments of DNA ranging from a few hundred to many thousands of nucleotides in length
The location of a gene on a chromosome is called its locus (plural, loci)

51. Homologous chromosomes carry the same kinds of genes for the same characteristics
Genes for the same characteristic are found at the same loci on both homologous chromosomes
However genes for a characteristic found on homologous chromosomes may not be identical
Alternative versions of genes found at the same gene locus are called alleles

52. An organism’s two alleles may be the same or different
Each cell carries two alleles per characteristic, one on each of the two homologous chromosomes
If both homologous chromosomes carry the same allele (gene form) at a given gene locus, the organism is homozygous at that locus
If two homologous chromosomes carry different alleles at a given locus, the organism is heterozygous at that locus (a hybrid)

53. Mutations are the source of alleles
Alleles arise as mutations—changes in the nucleotide sequence in genes
If a mutation occurs in a cell that becomes a sperm or egg, it can be passed on from parent to offspring
Most mutations occurring in the DNA of an organism initially appeared in the reproductive cells

54. Gregor Mendel, an Austrian monk, discovered the common patterns of inheritance and many essential facts about genes, alleles, and the distribution of alleles in gametes and zygotes during sexual reproduction
He chose the edible pea plant for his experiments, which took place in the monastery garden
Because of their structure, pea flowers naturally self-fertilize
Pollen from the stamen of a plant transfers to the carpel of the same plant, where the sperm then fertilizes the plant’s eggs

55. Mendel was able to mate two different plants by hand (cross-fertilization)
Female parts (carpels) were dusted with pollen from other selected plants
Unlike previous researchers, Mendel chose a simple experimental design
He chose to study individual characteristics (called traits) that had unmistakably different forms, such as white versus purple flowers
He started out by studying only one trait at a time

56. 10.3 How Are Single Traits Inherited?
True-breeding organisms possess traits that remain inherited unchanged by all offspring produced by self-fertilization
Mendel’s cross-fertilization of pea plants used true-breeding organisms
Mendel cross-fertilized true-breeding, white-flowered plants with true-breeding, purple-flowered plants
The parents used in a cross are part of the parental generation (known as P)
The offspring of the P generation are members of the first filial generation (F1)
Offspring of the F1 generation are members of the F2 generation

57. Parental generation (P) First-generation offspring (F1) pollen pollen
Figure 10-4 Cross of pea plants true-breeding for white or purple flowers
pollen
Parental
generation (P)
pollen
pollen
cross-fertilize
true-breeding,
purple-flowered
plant
true-breeding,
white-flowered
plant
First-generation
offspring (F1)
all purple-flowered
plant 57

58. 10.3 How Are Single Traits Inherited?
Mendel’s flower color experiments
Mendel allowed the F1 generation to self-fertilize
The F2 was composed of 3/4 purple-flowered plants and 1/4 white-flowered plants, a ratio of 3:1
The results showed that the white trait had not disappeared in the F1 but merely was hidden
Mendel then self-fertilized the F2 generation
In the F3 generation, all the white-flowered F2 plants produced white-flowered offspring
These proved to be true-breeding

59. Figure 10-5 Self-fertilization of F1 pea plants with purple flowers
First-
generation
offspring (F1)
self-fertilize
Second-
generation
offspring (F2)
3/4 purple
1/4 white 59

60. 10.3 How Are Single Traits Inherited?
In the F3 generation, self-fertilized purple-flowered F2 plants produced two types of offspring
About 1/3 were true-breeding for purple
The other 2/3 were hybrids that produced both purple- and white-flowered offspring, again, in the ratio of 3 purple to 1 white
Therefore, the F2 generation included 1/4 true-breeding purple-flowered plants, 1/2 hybrid purple, and 1/4 true-breeding white-flowered plants

61. There are two alleles for a given gene characteristic (such as flower color)
Let P stand for the dominant purple-flowered allele: A homozygous purple-colored plant has two alleles for purple flower color (PP) and produces only P gametes
Let p stand for the recessive white-flowered allele: A homozygous white-colored plant has two alleles for white flower color (pp) and produces only p gametes

62. The particular combination of the two alleles carried by an individual is called the genotype
For example, PP or Pp
The physical expression of the genotype is known as the phenotype (for example, purple or white flowers)

63. A cross between a purple-flowered plant (PP) and a white-flowered plant (pp) produces all purple-flowered F1 offspring, with a Pp genotype
Dominant P gametes from purple-flowered plants combined with recessive p gametes from white-flowered plants to produce hybrid purple-flowered plants (Pp)

64. Simple “genetic bookkeeping” can predict genotypes and phenotypes of offspring
The Punnett square method predicts offspring genotypes and phenotypes from combinations of parental gametes
First, assign letters to the different alleles of the characteristic under consideration (uppercase for dominant, lowercase for recessive)
Determine the gametes and their fractional proportions (out of all the gametes) from both parents

65. Write the gametes from each parent, together with their fractional proportions, along each side of a 2 x 2 grid (Punnett square)
Fill in the genotypes of each pair of combined gametes in the grid, including the product of the fractions of each gamete (e.g., 1/4 PP, 1/4 Pp and 1/4 pP, and 1/4 pp)
Add together the fractions of any genotypes of the same kind (1/4 Pp + 1/4 pP = 1/2 Pp total)

66. 6. From the sums of all the different kinds of offspring genotypes, create a genotypic fraction
1/4 PP, 1/2 Pp, 1/4 pp is in the ratio 1 PP : 2 Pp : 1 pp
Based on dominant and recessive rules, determine the phenotypic fraction
A genotypic ratio of 1 PP : 2 Pp : 1 pp yields 3 purple-flowered plants : 1 white-flowered plant

67. Figure 10-8 Determining the outcome of a single-trait crossPp
self-fertilize P
eggs p
offspring
genotypes
genotypic
ratio
(1:2:1)
phenotypic
ratio
(3:1)
sperm
eggs P P P PP PP PP Pp
sperm P p Pp
purple Pp p P pP p p p pp pp
white pP pp
Punnett square of a single-trait cross
Using probabilities to determine the offspring of a
single-trait cross 67