Concept 13.2: Fertilization and meiosis alternate in sexual life cycles

Question 1

is the generation-to-generation sequence of stages in the reproductive history of an organism, from conception to production of its own offspring.

Answer 1

life cycle

Question 2

In humans, each somatic cell has

Answer 2

46 chromosomes

Question 3

During mitosis, the chromosomes become ___________ enough to be visible under a light microscope.

Answer 3

condensed

Question 4

At this point, they can be distinguished from one another by their

Answer 4

size, the position of their centromeres, and the pattern of colored bands produced by certain chromatin-binding stains.

Question 5

Careful examination of a micrograph of the 46 human chromosomes from a single cell in mitosis reveals that there are

Answer 5

two chromosomes of each of 23 types.

Question 6

This becomes clear when images of the chromosomes are arranged in pairs, starting with the

Answer 6

longest chromosomes.

Question 7

The resulting ordered display is called a

Answer 7

karyotype

Question 8

the two chromosomes of a pair have the same length, centromere position, and staining pattern: These are called

Answer 8

homologous chromosomes (or homologs)

Question 9

Both chromosomes of each pair carry

Answer 9

genes controlling the same inherited characters.

Question 10

if a gene for eye color is situated at a particular locus on a certain chromosome, then its homologous chromosome (its homolog) will also have a

Answer 10

version of the eye-color gene at the equivalent locus.

Question 11

figure 13.3 Research method

Answer 11

.

Question 12

The two chromosomes referred to as X and Y are an important exception to the general pattern of

Answer 12

homologous chromosomes in human somatic cells.

Question 13

Typically, human females have a homologous pair of X chromosomes (XX), while males have

Answer 13

one X and one Y chromosome

Question 14

Only small parts of the X and Y are

Answer 14

homologous.

Question 15

Most of the genes carried on the X chromosome do not have counterparts on the tiny Y, and the Y chromosome has genes lacking on the

Answer 15

X.

Question 16

Due to their role in sex determination, the X and Y chromosomes are called

Answer 16

sex chromosomes

Question 17

The other chromosomes are called

Answer 17

autosomes.

Question 18

The occurrence of pairs of homologous chromosomes in each human somatic cell is a consequence of our

Answer 18

sexual origins

Question 19

We inherit one chromosome of a pair from each

Answer 19

parent

Question 20

thus, the 46 chromosomes in our somatic cells are actually

Answer 20

two sets of 23 chromosomes—a maternal set (from our mother) and a paternal set (from our father).

Question 21

The number of chromosomes in a single set is represented by

Answer 21

n

Question 22

Any cell with two chromosome sets is called a ______________ and has a diploid number of chromosomes, abbreviated 2n.

Answer 22

diploid cell

Question 23

For humans, the diploid number is 46(2n=46) , the number of

Answer 23

chromosomes in our somatic cells.

Question 23

In a cell in which DNA synthesis has occurred, all the chromosomes are duplicated, and therefore each consists of

Answer 23

two identical sister chromatids, associated closely at the centromere and along the arms.

Question 24

figure 13.4, bioflix chromosomes

Question 25

Unlike somatic cells, gametes contain a single set of chromosomes. Such cells are called _________________, and each has a haploid number of chromosomes (n).

Answer 25

haploid cells

Question 26

For humans, the haploid number is

Answer 26

23(n=23)

Question 27

The set of 23 consists of the 22 autosomes plus a

Answer 27

single sex chromosome.

Question 28

An unfertilized egg contains an X chromosome; a sperm contains either an

Answer 28

X or a Y chromosome.

Question 29

Each sexually reproducing species has a characteristic

Answer 29

diploid and haploid number

Question 30

the fruit fly Drosophila melanogaster has a diploid number

Answer 30

(2n) of 8 and a haploid number (n) of 4

Question 31

for dogs, 2n is 78 and n is

Answer 31

39

Question 32

The chromosome number generally does not correlate with the

Answer 32

size or complexity of a species’ genome

Question 33

it simply reflects how many linear pieces of

Answer 33

DNA make up the genome,

Question 34

The human life cycle begins when a haploid sperm from the father fuses with a

Answer 34

haploid egg from the mother

Question 35

This union of gametes, culminating in fusion of their nuclei, is called

Answer 35

fertilization

Question 36

The resulting fertilized egg, or _________, is diploid because it contains two haploid sets of chromosomes bearing genes representing the maternal and paternal family lines

Answer 36

zygote

Question 37

As a human develops into a sexually mature adult, mitosis of the zygote and its descendant cells generates all the

Answer 37

somatic cells of the body.

Question 38

Both chromosome sets in the zygote and all the genes they carry are passed with precision to the

Answer 38

somatic cells.

Question 39

figure 13.5, animation the human life cycle

Question 40

The only cells of the human body not produced by mitosis are the gametes, which develop from specialized cells called

Answer 40

germ cells in the gonads—ovaries in females and testes in males

Question 41

gamete formation involves a type of cell division called

Answer 41

meiosis

Question 42

This type of cell division reduces the number of sets of chromosomes from two to one in the gametes, counterbalancing the doubling that occurs at

Answer 42

fertilization.

Question 43

As a result of meiosis, each human sperm and egg is

Answer 43

haploid (n=23) .

Question 44

Although the alternation of meiosis and fertilization is common to all organisms that reproduce sexually, the timing of these two events in the life cycle varies, depending on the

Answer 44

species.

Question 45

These variations can be grouped into three main types of

Answer 45

life cycles.

Question 46

n the type that occurs in humans and most other animals, gametes are the only

Answer 46

haploid cells

Question 47

occurs in germ cells during the production of gametes, which undergo no further cell division prior to fertilization.

Answer 47

Meiosis

Question 48

After fertilization, the diploid zygote divides by mitosis, producing a

Answer 48

multicellular organism that is diploid.

Question 49

figure 13.6

Question 50

Plants and some species of algae exhibit a second type of life cycle called

Answer 50

alternation of generations

Question 51

This type includes both

Answer 51

diploid and haploid stages that are multicellular

Question 52

the multicellular diploid stage is called the

Answer 52

sporophyte.

Question 53

Meiosis in the sporophyte produces haploid cells called

Answer 53

spores

Question 54

Unlike a gamete, a haploid spore doesn’t fuse with another cell but divides mitotically, generating a multicellular haploid stage called the

Answer 54

gametophyte.

Question 55

Cells of the gametophyte give rise to gametes by

Answer 55

mitosis.

Question 56

Fusion of two haploid gametes at fertilization results in a

Answer 56

diploid zygote, which develops into the next sporophyte generation.

Question 57

Therefore, in this type of life cycle, the sporophyte generation produces a gametophyte as its offspring, and the gametophyte generation produces the next

Answer 57

sporophyte generation

Question 58

The term ______________________________ fits well as a name for this type of life cycle.

Answer 58

alternation of generations

Question 59

A third type of life cycle occurs in most

Answer 59

fungi and some protists, including some algae

Question 60

After gametes fuse and form a diploid zygote, meiosis occurs without a

Answer 60

multicellular diploid offspring developing.

Question 61

Meiosis produces not gametes but haploid cells that then divide by mitosis and give rise to either

Answer 61

unicellular descendants or a haploid multicellular adult organism.

Question 62

Subsequently, the haploid organism carries out further mitoses, producing the cells that develop into

Answer 62

gametes.

Question 63

The only diploid stage found in these species is the

Answer 63

single-celled zygote.

Question 64

Note that either haploid or diploid cells can divide by mitosis, depending on the type of life cycle. Only diploid cells, however, can undergo meiosis because

Answer 64

haploid cells have only a single set of chromosomes that cannot be further reduced