The Cellular Basis of Reproduction and Inheritance : part Cfaculty.collin.edu/cdoumen/1408/1408_Ch8_9/1408_Ch8C.pdf · 1 © 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko

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© 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko

PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey

The Cellular Basis of Reproduction and Inheritance : part C

! To summarize what we have seen before

Mitosis duplicates cells (chromosomes = 2n, diploid cells) into two daughter cells with the same number of chromosomes (they remain diploid cells)

Meiosis creates 4 daughter cells (haploid cells, chromosomes = n), where each cell contains half the number of chromosomes of the original diploid cell

Mitosis vs Meiosis

© 2012 Pearson Education, Inc.

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Mitosis where 2n=4


! Can you name the different phases here ?

Meiosis where 2n=2


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! Genetic variation in gametes results from

–  independent orientation at metaphase I resulting in different possibilities of gametes

–  If the number of homologous chromosomes = n, the different possible gametes can be 2n

–  So for humans , n= 23.. Thus possible different combinations of chromosomes in gametes is 223

Genetic Variation


Possibility A

Two equally probable arrangements of chromosomes at

metaphase I

Possibility B

Metaphase II

Gametes

Combination 3 Combination 4 Combination 2 Combination 1

!  This diploid cell has 2 pair of homologous chromosomes… thus n = 2

!  Possible combination in gametes = 2n =22 = 4

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8.16 Homologous chromosomes may carry different versions of genes


Coat-color genes

Eye-color genes

Brown Black

White Pink

Tetrad in parent cell (homologous pair of

duplicated chromosomes)

E C

e c e

E

E

e c

c

C

C Locus for eye color

Different versions for eye color

If we use this example above, and assume that the diploid cell only has 1 pair of chromosomes, the possible gametes = 2 The next slide shows the meiosis end result of possible gametes.

C c

e E

C c

e E

C

E

C

E

c

e

c

e

Chromosomes of the four gametes. Note that there are actually only 2 different kinds of gametes.

Meiosis

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!  An additional mechanisms, called genetic recombination, increases genetic variability among the gametes.

!  Genetic recombination is the production of new combinations of genes due to crossing over.

!  Crossing over is an exchange of corresponding segments between separate, non-sister chromatids on homologous chromosomes.

8.17 Crossing over : further increases genetic variability


!  A reminder of the definitions



Sister chromatids

Sister chromatids

Pair of homologous chromosomes

NON- Sister chromatids

!  Crossing over exchanges DNA segments between separate, non-sister chromatids on homologous chromosomes.

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!  In late prophase I, homologous chromosomes pair laterally, or side-by-side. At this time they are said to be in synapsis, forming tetrads.

!  During synapsis, cross-connections are formed from breakage and rejoining between sister chromatids.



!  First nonsister chromatids will join at a point called chiasma (plural, chiasmata), the site of attachment and crossing over.

!  Corresponding amounts of genetic material are exchanged between maternal and paternal (nonsister) chromatids.



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Piece of DNA that has ‘crossed’ over


!  In figure below, following crossing over, the blue and red chromosomes, which originally carried AA and aa alleles, respectively, now carry Aa alleles in both chromosomes at the end of prophase I.



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C c

e E

C c

e E

C

E

c

e

Chromosomes of the four gametes: note that now we have 4 different kind of gametes instead of 2 !

C c

e E

c

E

C

e

!  Using our previous example, crossing over now creates different kind of gametes

Crossing over during synapsis

Tetrad (pair of homologous chromosomes in synapsis)

Breakage of homologous chromatids

Joining of homologous chromatids

1

2

C

c e

E

C

c e

E

C

c e

E

Chiasma

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Figure 8.17B_2

Separation of homologous chromosomes at anaphase I

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C E

C e

Chiasma

c

c

E

e

C E

c e

Figure 8.17B_3

Separation of chromatids at anaphase II and completion of meiosis

Parental type of chromosome

Recombinant chromosome

Recombinant chromosome

Parental type of chromosome

End result : Gametes of 4 genetic types

C E

e C

4

E c

c e

E C

e C

c E

c e

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ALTERATIONS OF CHROMOSOME NUMBER

AND STRUCTURE


!  A karyotype is an ordered display of magnified images of an individual’s chromosomes arranged in pairs.

!  Karyotypes

–  are often produced from dividing cells arrested at metaphase of mitosis and

–  allow for the observation of

–  homologous chromosome pairs,

–  chromosome number, and

–  chromosome structure.

8.18 A karyotype is a photographic inventory of an individual’s chromosomes


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Blood culture

Packed red and white blood cells

Centrifuge

Fluid

Hypotonic solution Fixative

White blood cells

Stain

3

2

1

Centromere

Sister chromatids

Pair of homologous chromosomes

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Sex chromosomes

Human karyotype

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!  Nondisjunction is the failure of chromosomes or chromatids to separate normally during meiosis. This can happen during

–  meiosis I, if both members of a homologous pair go to one pole or

–  meiosis II if both sister chromatids go to one pole.

!  Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes.

8.20 Accidents during meiosis can alter normal chromosome number


Nondisjunction

MEIOSIS I

MEIOSIS II

Normal meiosis II

Gametes

Number of chromosomes

Abnormal gametes

n + 1 n + 1 n - 1 n - 1

The diploid cell has 2n =4

Gametes should have n =2

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Normal meiosis I

MEIOSIS I

MEIOSIS II

Nondisjunction

Abnormal gametes Normal gametes

n + 1 n - 1 n n

!  Trisomy 21, called Down syndrome, –  involves the inheritance of three copies of chromosome

21 and –  is the most common human chromosome abnormality.

!  Trisomy 21, produces a characteristic set of symptoms, which include:

–  characteristic facial features and short stature, –  mental retardation,heart defects, –  susceptibility to respiratory infections, leukemia, and Alzheimer’s

disease, and shortened life span.

!  The incidence increases with the age of the mother.

Abnormalities in karyotypes


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Figure 8.19A

Trisomy 21

Figure 8.19B

Age of mother 50 45 40 35 30 25 20

0

10

20

30

40

50

60

70

80

90

Infa

nts

with

Dow

n sy

ndro

me

(per

1,0

00 b

irths

)

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!  Other trisomies include !  Trisomy 18, called Edwards syndrome,

–  Very low survival rate due to multiple organ disorders.

!  Trisomy 13, Patau syndrome

Abnormalities in karyotypes


!  While an extra chromosome in the autosomal chromosomes can be quite dramatic, sex chromosome abnormalities tend to be less severe, perhaps because of

–  the small size of the Y chromosome and/or

–  X-chromosome inactivation.

8.21 CONNECTION: Abnormal numbers of sex chromosomes do not usually affect survival


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!  The following table lists the most common human sex chromosome abnormalities. In general,

–  a single Y chromosome is enough to produce “maleness,” even in combination with several X chromosomes, and

–  the absence of a Y chromosome yields “femaleness.”

–  a single X chromosome is a viable outcome (Turner syndrome) ; a single Y chromosome is non-viable.

8.21 CONNECTION: Abnormal numbers of sex chromosomes do not usually affect survival


Table 8.21

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!  Errors in mitosis or meiosis may produce polyploid species, with more than two chromosome sets.

!  The formation of polyploid species is –  widely observed in many plant species but

–  less frequently found in animals.

8.22 EVOLUTION CONNECTION: New species can arise from errors in cell division


Figure 8.22

The gray tree frog (Hyla versicolor), a tetraploid organism

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!  Chromosome breakage can lead to rearrangements that can produce –  genetic disorders or,

–  if changes occur in somatic cells, cancer.

–  All cancers are due to genetic chromosomal changes

8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer


!  These rearrangements may include –  a deletion, the loss of a chromosome segment, –  a duplication, the repeat of a chromosome segment,

–  an inversion, the reversal of a chromosome segment, or

–  a translocation, the attachment of a segment to a nonhomologous chromosome that can be reciprocal.



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!  Chronic myelogenous leukemia (CML) –  is one of the most common leukemias, –  affects cells that give rise to white blood cells

(leukocytes), and –  results from part of chromosome 22 switching places

with a small fragment from a tip of chromosome 9.



Chromosome 9

Chromosome 22 Reciprocal translocation

“Philadelphia chromosome”

Activated cancer-causing gene

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Figure 8.UN03

Number of chromosomal duplications

Number of cell divisions

Number of daughter cells produced

Number of chromosomes in the daughter cells

How the chromosomes line up during metaphase

Genetic relationship of the daughter cells to the parent cell

Functions performed in the human body

Mitosis Meiosis

Documents

The Cellular Basis of Reproduction and Inheritance : part Cfaculty.collin.edu/cdoumen/1408/1408_Ch8_9/1408_Ch8C.pdf · 1 © 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko