basic genetic

Human Genetics Dr. Belal Azab

First Semester 2013-14

Overview: The Key Roles of Cell Division

•  The ability of organisms to produce more of their own kind best distinguishes living things from nonliving matter

•  The continuity of life is based on the reproduction of cells, or cell division

© 2011 Pearson Education, Inc.

•  Multicellular organisms depend on cell division for –  Development from a fertilized cell – Growth –  Repair

•  Cell division is an integral part of the cell cycle, the life of a cell from formation to its own division


Cellular Organization of the Genetic Material

•  All the DNA in a cell constitutes the cell’s genome •  A genome can consist of a single DNA molecule

(common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells)

•  DNA molecules in a cell are packaged into chromosomes


•  Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division

•  Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus

•  Somatic cells (nonreproductive cells) have two sets of chromosomes

•  Gametes (reproductive cells: sperm and eggs) have half as many chromosomes as somatic cells


Distribution of Chromosomes During Eukaryotic Cell Division

•  In preparation for cell division, DNA is replicated and the chromosomes condense

•  Each duplicated chromosome has two sister chromatids (joined copies of the original chromosome), which separate during cell division

•  The centromere is the narrow “waist” of the duplicated chromosome, where the two chromatids are most closely attached


Figure 12.4

0.5 µm Centromere

Sister chromatids

•  During cell division, the two sister chromatids of each duplicated chromosome separate and move into two nuclei

•  Once separate, the chromatids are called chromosomes


Figure 12.5-1

Chromosomes Chromosomal DNA molecules

Centromere

Chromosome arm

1

Figure 12.5-2


Centromere

Chromosome arm

Chromosome duplication (including DNA replication) and condensation

Sister chromatids

1

2

Figure 12.5-3


Centromere

Chromosome arm

Chromosome duplication (including DNA replication) and condensation

Sister chromatids

Separation of sister chromatids into two chromosomes

1

2

3

•  Eukaryotic cell division consists of – Mitosis, the division of the genetic material in the

nucleus –  Cytokinesis, the division of the cytoplasm

•  Gametes are produced by a variation of cell division called meiosis

•  Meiosis yields nonidentical daughter cells that have only one set of chromosomes, half as many as the parent cell


Phases of the Cell Cycle

•  The cell cycle consists of – Mitotic (M) phase (mitosis and cytokinesis) –  Interphase (cell growth and copying of

chromosomes in preparation for cell division)


Figure 12.6

INTERPHASE

G1

G2

S (DNA synthesis)

MITOTIC (M) PHASE

Human chromosomes •  DNA and associated proteins are organized into

chromosomes •  Human somatic cells are diploid and have 22 pairs of

autosomes AND 1 set of sex chromosomes (XX or XY)= total of 46 –  Females XX –  Males XY

•  Germ cells are haploid and contain 22 chromosomes plus 1 sex chromosome (X or Y)

Mitosis Meiosis

Confocal image of cells at various stages of mitosis. The markers are microtubules (in red), DNA (in blue) and the Golgi apparatus (which temporarily fragments during mitosis, in green).

Credit: © Thomas Deerinck/Visuals Unlimited

900084

Interphase •  Gap 1 (G1)– many cytoplasmic

organelles are constructed; RNA, protein and other molecules are synthesized; cell almost doubles in size

•  Synthesis (S)– DNA is replicated

and chromosomes duplicate, forming 2 sister chromatids attached at the centromere

•  Gap 2 (G2)– more cell growth; mitochondria divide; spindle precursors form

Interphase 18-24 hours

Fig. 2.8a

Figure 12.6

INTERPHASE

G1

G2

S (DNA synthesis)

MITOTIC (M) PHASE

One chromosome (unreplicated)

One chromosome (replicated) A chromatid

Its sister chromatid

Centromere

Mitosis •  Produces identical daughter cells

–  (46 chromosomes) •  It must be accurate for cells to function properly •  Continuous process but divided into distinct

steps: –  Prophase –  Metaphase –  Anaphase –  Telophase

Stepped Art

(a) Cell at interphase

Pair of centrioles

Nuclear envelope Chromosome

Mitosis

(b) Early Prophase (c) Late Prophase (d) Transition to Metaphase

Microtubule

(e) Metaphase (f) Anaphase (h) Interphase (g) Telophase

Stepped Art

Prophase

•  Chromosomes condense

•  Nuclear membrane breaks down

•  Spindle fibers (Mitotic Spindles) form

Fig. 2.8b

Prophase

Late prophase Prometaphase

Prophase into metaphase

Fig. 2.8d Fig. 2.8c

Nuclear membrane breaks up Chromosomes disperse and attach to mitotic spindle Chromsomes start to move toward the equatorial plane (process called congression)

The Mitotic Spindle: A Closer Look

•  The mitotic spindle is a structure made of microtubules that controls chromosome movement during mitosis

•  In animal cells, assembly of spindle microtubules begins in the centrosome, the microtubule organizing center

•  The centrosome replicates during interphase, forming two centrosomes that migrate to opposite ends of the cell during prophase and prometaphase


•  An aster (a radial array of short microtubules) extends from each centrosome

•  The spindle includes the centrosomes, the spindle microtubules, and the asters


•  During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes

•  Kinetochores are protein complexes associated with centromeres

•  At metaphase, the chromosomes are all lined up at the metaphase plate, an imaginary structure at the midway point between the spindle’s two poles


Figure 12.8

Sister chromatids

Aster Centrosome

Metaphase plate (imaginary)

Kineto- chores

Overlapping nonkinetochore microtubules Kinetochore

microtubules

Microtubules

Chromosomes

Centrosome

0.5 µm

1 µm

Metaphase

•  Chromosomes line up on the midline (Metaphase plate)

•  Spindle fibers attach to centromeres

Fig. 2.8e

Metaphase

Anaphase

•  Centromeres divide •  Spindle fibers shorten •  Sister chromatids separate and

move along the kinetochore microtubules to opposite poles

•  The microtubules shorten by depolymerizing at their kinetochore ends

Fig 2.8f

Anaphase

Chromosome movement

Microtubule

Motor protein

Chromosome

Kinetochore

Tubulin subunits

CONCLUSION

•  Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell

•  In telophase, •  Cytokinesis begins during anaphase or telophase

and the spindle eventually disassembles


Telophase genetically identical daughter nuclei form at opposite ends of the cell

•  Cell elongates •  Nuclear membrane

reforms •  Chromosomes

decondense •  Spindle disappears •  Mitosis is completed Fig. 2.8g

Telophase

Cytokinesis •  Division of the cytoplasm •  Cleave furrow forms at

equator of the cell •  Constriction tightens by

contraction of filaments •  Cell is divided into two

identical cells

Fig. 2.11

Figure 12.10a (a) Cleavage of an animal cell (SEM)

Cleavage furrow

Contractile ring of microfilaments

Daughter cells

100 µm

Two identical daughter cells Interphase

Fig. 2.8h

Figure 12.7a

G2 of Interphase Prophase Prometaphase

Centrosomes (with centriole pairs)

Chromatin (duplicated)

Nucleolus Nuclear envelope

Plasma membrane

Early mitotic spindle

Aster Centromere

Chromosome, consisting of two sister chromatids

Fragments of nuclear envelope

Nonkinetochore microtubules

Kinetochore Kinetochore microtubule

Figure 12.7b

Metaphase

Metaphase plate

Anaphase Telophase and Cytokinesis

Spindle Centrosome at one spindle pole

Daughter chromosomes

Cleavage furrow

Nucleolus forming

Nuclear envelope forming

Mitosis functions in growth and cell replacement •  Cells from adults can divide only ~10–30 times •  Most cells in the adult human body undergo mitosis;

however, brain and spinal cord nervous tissues do not. –  Highly differentiated cells (such as neurons) are stuck in G0 in the

cell cycle and are terminally differentiated. •  Cell division is tightly controlled. The cell cycle is

governed by a series of checkpoints—these monitor the quality of the process. –  If damage is detected, then the cell may undergo programmed cell

death (apoptosis). –  If checkpoints do not function properly, then uncontrolled cell

growth can occur (cancer)

G1 checkpoint

G1

G2

G2 checkpoint M checkpoint

M

S Control system

Figure 12.15

•  For many cells, the G1 checkpoint seems to be the most important

•  If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide

•  If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase


Figure 12.16

G1 checkpoint

G1 G1

G0

(a) Cell receives a go-ahead signal.

(b) Cell does not receive a go-ahead signal.

Disorders of accelerated aging include progeria and Werner syndrome

Cell cycle regulation DNA repair

Progeria ( LMNA ): rapid aging in children Werner syndrome (WRN): people age rapidly

Review of terms

•  Haploid cells – (1n) with one copy of each chromosome

•  Diploid cells – (2n) with two copies of each chromosome

•  Somatic – non-germline (body) cells

•  Gametes – sex cells (eggs and sperm)

Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid

•  Like mitosis, meiosis is preceded by the replication of chromosomes

•  Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II

•  The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis

•  Each daughter cell has only half as many chromosomes as the parent cell


The Stages of Meiosis

•  After chromosomes duplicate, two divisions follow

– Meiosis I (reductional division): homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomes

– Meiosis II (equational division) sister chromatids separate

•  The result is four haploid daughter cells with unreplicated chromosomes


Figure 13.7-1

Pair of homologous chromosomes in diploid parent cell

Duplicated pair of homologous chromosomes

Chromosomes duplicate

Sister chromatids Diploid cell with

duplicated chromosomes

Interphase

Figure 13.7-2






Homologous chromosomes separate

Haploid cells with duplicated chromosomes

Meiosis I

1

Interphase

Figure 13.7-3







Haploid cells with duplicated chromosomes

Sister chromatids separate

Haploid cells with unduplicated chromosomes

Interphase

Meiosis I

Meiosis II 2

1

•  Meiosis I is preceded by interphase, when the chromosomes are duplicated to form sister chromatids

•  The sister chromatids are genetically identical and joined at the centromere

•  The single centrosome replicates, forming two centrosomes


•  Division in meiosis I occurs in four phases –  Prophase I – Metaphase I –  Anaphase I –  Telophase I and cytokinesis


Meiosis I

•  Reductional division •  A diploid cell divides and forms two haploid

cells (2n n) •  In humans, the chromosome number is

reduced from 46 to 23 chromosomes •  Occurs in testis and ovary in the primary

spermatocytes and primary oocytes •  Interphase:

– Chromosomes replicate

Prophase I •  Chromosomes condense •  Nuclear membrane

breaks down •  Homologous

chromosomes pair •  Recombination occurs •  Nucleolus disappears •  Meiotic spindle begins to

form

newly forming microtubules

Prophase I

Fig. 2.13a

Stepped Art

Plasma membrane

(a) Prophase I

Meiosis I

Newly forming microtubules in the cytoplasm

The nuclear envelope is breaking apart; microtubules will be able to penetrate the nuclear region.

Interactions between motor proteins and microtubules are moving one of two pairs of centrioles toward the opposite spindle pole.

Stepped Art

Plasma membrane

(b) Metaphase I (a) Prophase I

Meiosis I




Spindle equator (midway between the two poles)

One pair of homologous chromosomes (each being two sister chromatids)

Stepped Art

Plasma membrane

(c) Anaphase I (b) Metaphase I (a) Prophase I

Meiosis I






Stepped Art

Plasma membrane

(d) Telophase I (c) Anaphase I (b) Metaphase I (a) Prophase I

Meiosis I






Stepped Art

There is no DNA replication between the two nuclear divisions.

(e) Prophase II

Meiosis II

Stepped Art


(f) Metaphase II (e) Prophase II

Meiosis II

Stepped Art


(g) Anaphase II (f) Metaphase II (e) Prophase II

Meiosis II

Stepped Art


(h) Telophase II (g) Anaphase II (f) Metaphase II (e) Prophase II

Meiosis II

Prophase I

•  Leptotene –  Replicated chromosomes align and begin to

condense •  Zygotene

–  homologous chromosomes pair along entire length (synapsis)

–  synaptonemal complex forms •  Pachytene

–  Synapsis is complete and each pair of homologues is called a tetrads (bivalent)

–  Crossing over occurs (recombination at chiasmata) •  Diplotene

–  Homologous chromosomes separate some but remain bound at chiasmata

•  usually 2 chiasmata/chromosome, more frequent in females)

•  Diakinesis –  Further chromosome condensation; tetrads viable

A tetrad showing chiasmata

Remember: •  Prophase I typically occupies more than 90% of the

time required for meiosis •  Chromosomes begin to condense •  In synapsis, homologous chromosomes loosely pair

up, aligned gene by gene •  In crossing over, nonsister chromatids exchange

DNA segments •  Each pair of chromosomes forms a tetrad, a group of

four chromatids •  Each tetrad usually has one or more chiasmata, X-

shaped regions where crossing over occurred


Metaphase I

•  Chromosome pairs line up on the midline

•  Spindles are fully formed and attached to centromeres

Metaphase I

spindle equator

one pair of homologous chromosomes

Fig. 2.13b

Anaphase I

•  Centromeres do not divide

•  Homologous chromosomes move to opposite poles and separate (disjunction)

Anaphase I

Fig. 2.13c

Telophase I •  2 haploid sets of

chromosomes are grouped at poles

•  Nuclear membrane reforms •  Replicated chromosomes may

decondense in some cells ! After cytokinesis: •  Cytokinesis separates the

cytoplasm •  Cytokinesis usually occurs

simultaneously, forming two haploid daughter cells

•  Two haploid (n) cells are produced

•  Immediate entry into interphase

Telophase I

Fig. 2.13d

Interkinesis Interphase II

•  No DNA synthesis occurs

Meiosis I--summary

•  Reduction of chromosome number (2n to n) in daughter cells

•  Random distribution of maternal and paternal chromosomes

•  Crossing-over/recombination occurs; thus, each chromosome consists of both maternal and paternal chromosomes

Meiosis II begins with haploid cells

Essentially same as mitosis except haploid cells

Prophase II •  Chromosomes condense •  Spindle forms •  Nuclear membrane breaks down •  Each chromosome is composed of

two sister chromatids attached at the centromere

Prophase II Fig. 2.13e

Metaphase II •  Chromosomes line up on

the midline and centromeres attach to spindle fibers

Metaphase II

Fig. 2.13f

Anaphase II • Centromeres divide

• Sister chromatids move to opposite poles

Anaphase II

Fig. 2.13g

Telophase II

•  Nuclear membrane forms •  Chromosomes

decondense After cytokinesis: •  Four unique haploid (n)

cells are produced with 23 chromosomes each

Telophase II

Fig. 2.13h

Members of chromosome pair

Each chromosome pairs with its homologue

Paired homologues separate

in meiosis I

Sister chromatids separate and become

individual chromosomes in meiosis II

Sister chromatids Sister chromatids

Genetic consequences of meiosis •  Reduction of chromosome number •  Diploid to haploid (essential for gametes) •  Random assortment of maternal and paternal

chromosomes –  genes on different chromosomes –  maternal/paternal chromosomes –  Number of possible chromosomal combinations = 223 or

8,388,608 –  Recombination between chromosome pairs increases the

possible combinations •  Segregation of alleles •  Recombination/crossing-over

–  Allows new combinations of genes to be produced –  Important for normal chromosome disjunction –  Ensures genetic diversity

Chromosome combinations: independent assortment

MITOSIS

Parental cell is diploid.

Doubled chromosomes appear in late prophase.

Unpaired chromosomes align at metaphase.

Sister chromatids separate at anaphase.

Two daughter cells are diploid, genetically identical to parent cell.

MEIOSIS

Four daughter cells are haploid, not genetically identical to parent cell.

Sister chromatids separate at anaphase II.

Paired homologous chromosomes align at metaphase I, then separate at anaphase I.

Parental cell is diploid.

Doubled chromosomes appear in late prophase.

No crossing-over Crossing-over Homologous chromosome

s

Crossing-over occurs

Crossing-over

complete First meiotic

division

Second meiotic division

Possible gametes

No crossing-over

Prophase I

Summary of mitosis and meiosis

Figure 13.8

MEIOSIS I: Separates homologous chromosomes

Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis

Centrosome (with centriole pair)

Sister chromatids

Chiasmata

Spindle

Homologous chromosomes

Fragments of nuclear envelope

Duplicated homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example.

Centromere (with kinetochore)

Metaphase plate

Microtubule attached to kinetochore

Chromosomes line up by homologous pairs.

Sister chromatids remain attached


Each pair of homologous chromosomes separates.

Cleavage furrow

Two haploid cells form; each chromosome still consists of two sister chromatids.

MEIOSIS I: Separates sister chromatids

Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis

Sister chromatids separate

Haploid daughter cells forming

During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing unduplicated chromosomes.

A Comparison of Mitosis and Meiosis

•  Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell

•  Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell


Figure 13.9

Prophase

Duplicated chromosome

MITOSIS

Chromosome duplication

Parent cell

2n = 6

Metaphase

Anaphase Telophase

2n 2n

Daughter cells of mitosis

MEIOSIS

MEIOSIS I

MEIOSIS II

Prophase I

Metaphase I

Anaphase I Telophase I

Haploid n = 3

Chiasma

Chromosome duplication Homologous

chromosome pair

Daughter cells of

meiosis I

Daughter cells of meiosis II n n n n

SUMMARY

Property Mitosis Meiosis

DNA replication

Number of divisions

Synapsis of homologous chromosomes

Number of daughter cells and genetic composition

Role in the animal body

Occurs during interphase before mitosis begins

One, including prophase, metaphase, anaphase, and telophase

Does not occur

Two, each diploid (2n) and genetically identical to the parent cell

Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction

Occurs during interphase before meiosis I begins

Two, each including prophase, metaphase, anaphase, and telophase

Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion

Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other

Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes

Figure 13.9a

Prophase

Duplicated chromosome

MITOSIS

Chromosome duplication

Parent cell

2n = 6

Metaphase

Anaphase Telophase

2n 2n Daughter cells

of mitosis

MEIOSIS

MEIOSIS I

MEIOSIS II

Prophase I

Metaphase I

Anaphase I Telophase I

Haploid n = 3

Chiasma

Chromosome duplication Homologous

chromosome pair

Daughter cells of

meiosis I

Daughter cells of meiosis II n n n n

Figure 13.9b

SUMMARY

Property Mitosis Meiosis DNA replication

Number of divisions

Synapsis of homologous chromosomes

Number of daughter cells and genetic composition

Role in the animal body

Occurs during interphase before mitosis begins

One, including prophase, metaphase, anaphase, and telophase

Does not occur

Two, each diploid (2n) and genetically identical to the parent cell

Enables multicellular adult to arise from zygote; produces cells for growth, repair, and, in some species, asexual reproduction

Occurs during interphase before meiosis I begins

Two, each including prophase, metaphase, anaphase, and telophase

Occurs during prophase I along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion

Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically different from the parent cell and from each other

Produces gametes; reduces number of chromosomes by half and introduces genetic variability among the gametes

•  Three events are unique to meiosis, and all three occur in meiosis l –  Synapsis and crossing over in prophase I:

Homologous chromosomes physically connect and exchange genetic information

–  At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes

–  At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate


•  Sister chromatid cohesion allows sister chromatids of a single chromosome to stay together through meiosis I

•  Protein complexes called cohesins are responsible for this cohesion

•  In mitosis, cohesins are cleaved at the end of metaphase

•  In meiosis, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II (separation of sister chromatids)


Meiosis

• Used only in the production of gametes

• Produces unique haploid (n) progeny cells (x4)

• Two divisions of the nucleus; two cell divisions

• Meiosis I

• Meiosis II

Mitosis

• Asexual reproduction

• Increase in cell numbers

• Produces identical daughter cells (2n)

• Occurs in most cells

• One division of the nucleus; one cell division

Inheritance of Genes

•  Genes are the units of heredity, and are made up of segments of DNA

•  Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs)

•  Each gene has a specific location called a locus on a certain chromosome

•  Most DNA is packaged into chromosomes


Comparison of Asexual and Sexual Reproduction

•  In asexual reproduction, a single individual passes genes to its offspring without the fusion of gametes

•  A clone is a group of genetically identical individuals from the same parent

•  In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents


Sets of Chromosomes in Human Cells

•  Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes

•  A karyotype is an ordered display of the pairs of chromosomes from a cell

•  The two chromosomes in each pair are called homologous chromosomes, or homologs

•  Chromosomes in a homologous pair are the same length and shape and carry genes controlling the same inherited characters


Figure 13.3b

Pair of homologous duplicated chromosomes

Centromere

Sister chromatids

Metaphase chromosome

5 µm

•  The sex chromosomes, which determine the sex of the individual, are called X and Y

•  Human females have a homologous pair of X chromosomes (XX)

•  Human males have one X and one Y chromosome

•  The remaining 22 pairs of chromosomes are called autosomes


•  Each pair of homologous chromosomes includes one chromosome from each parent

•  The 46 chromosomes in a human somatic cell are two sets of 23: one from the mother and one from the father

•  A diploid cell (2n) has two sets of chromosomes

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


•  In a cell in which DNA synthesis has occurred, each chromosome is replicated

•  Each replicated chromosome consists of two identical sister chromatids


Figure 13.4

Sister chromatids of one duplicated chromosome

Key Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3)

Key

2n = 6

Centromere

Two nonsister chromatids in a homologous pair

Pair of homologous chromosomes (one from each set)

•  A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n)

•  For humans, the haploid number is 23 (n = 23) •  Each set of 23 consists of 22 autosomes and a

single sex chromosome •  In an unfertilized egg (ovum), the sex

chromosome is X •  In a sperm cell, the sex chromosome may be

either X or Y

© 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.

•  Fertilization is the union of gametes (the sperm and the egg)

•  The fertilized egg is called a zygote and has one set of chromosomes from each parent

•  The zygote produces somatic cells by mitosis and develops into an adult

Behavior of Chromosome Sets in the Human Life Cycle


•  At sexual maturity, the ovaries and testes produce haploid gametes

•  Gametes are the only types of human cells produced by meiosis, rather than mitosis

•  Meiosis results in one set of chromosomes in each gamete

•  Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number


Figure 13.5 Key

Haploid (n) Diploid (2n)

Egg (n)

Haploid gametes (n = 23)

Sperm (n)

Ovary Testis

Mitosis and development

Diploid zygote (2n = 46)

Multicellular diploid adults (2n = 46)

MEIOSIS FERTILIZATION

The Variety of Sexual Life Cycles

•  The alternation of meiosis and fertilization is common to all organisms that reproduce sexually

•  The three main types of sexual life cycles differ in the timing of meiosis and fertilization


•  Gametes are the only haploid cells in animals •  They are produces by meiosis and undergo no

further cell division before fertilization •  Gametes fuse to form a diploid zygote that

divides by mitosis to develop into a multicellular organism


Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution

•  Mutations (changes in an organism’s DNA) are the original source of genetic diversity

•  Mutations create different versions of genes called alleles

•  Reshuffling of alleles during sexual reproduction produces genetic variation


Origins of Genetic Variation Among Offspring

•  The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation

•  Three mechanisms contribute to genetic variation

–  Independent assortment of chromosomes – Crossing over – Random fertilization


Independent Assortment of Chromosomes

•  Homologous pairs of chromosomes orient randomly at metaphase I of meiosis

•  In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs


•  The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number

•  For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes


Figure 13.10-1

Possibility 1 Possibility 2

Two equally probable arrangements of chromosomes at

metaphase I

Figure 13.10-2



metaphase I

Metaphase II

Figure 13.10-3



metaphase I

Metaphase II

Daughter cells

Combination 1 Combination 2 Combination 3 Combination 4

Crossing Over

•  Crossing over produces recombinant chromosomes, which combine DNA inherited from each parent

•  Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene


•  In crossing over, homologous portions of two nonsister chromatids trade places

•  Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome


Figure 13.11-1 Prophase I of meiosis

Nonsister chromatids held together during synapsis

Pair of homologs



Pair of homologs

Chiasma

Centromere

TEM



Pair of homologs

Chiasma

Centromere

TEM Anaphase I



Pair of homologs

Chiasma

Centromere

TEM Anaphase I

Anaphase II



Pair of homologs

Chiasma

Centromere

TEM Anaphase I

Anaphase II

Daughter cells

Recombinant chromosomes

Figure 13.11a

Chiasma

Centromere

TEM

Random Fertilization

•  Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)

•  The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations


•  Crossing over adds even more variation •  Each zygote has a unique genetic identity


Animation: Genetic Variation


Animation: Genetic Variation Right-click slide / select “Play”

Prophase I: Each homologous pair undergoes synapsis and crossing over between nonsister chromatids with the subsequent appearance of chiasmata.

Metaphase I: Chromosomes line up as homologous pairs on the metaphase plate.

Anaphase I: Homologs separate from each other; sister chromatids remain joined at the centromere.

Figure 13.UN01

Spermatogenesis (testis) •  Spermatogonia divide mitotically from puberty to

death, producing primary spermatocytes •  Primary spermatocytes undergo meiosis I,

producing 2 haploid secondary spermatocytes, which undergo meiosis II and produce haploid spermatids, which develop into mature spermatozoa

•  Process takes about 48-64 days –  200 million sperm per ejaculate (1012 sperm in a lifetime)

Oogenesis (ovary)

•  Different than spermatogenesis--timeline •  Oogonia divide mitotically and form primary

oocytes—all oocytes are formed during gestation, before birth.

•  Primary oocytes are arrested at meiosis I prior to birth (prophase I, dictyotene stage) until ovulation

•  Females are born with all of the oocytes they will ever have (~2.5 million, ~400 of which will actually mature)

Oogenesis (ovary)

•  At puberty, primary oocytes complete meiosis I, producing 1 polar body and 1 secondary oocyte

•  Polar bodies do not usually divide •  The secondary oocyte is halted at metaphase of meiosis

II after ovulation •  If an ova or “egg” is fertilized, then the secondary oocyte

rapidly undergoes meiosis II, producing an ootid or ovum and a second polar body—and a complete zygote

Fertilization •  Takes place in fallopian tube •  Single sperm penetrates, usually

within 1-2 days, but sperm can survive up to 5-6 days in the fallopian tubes –  Once a single sperm has

penetrated, no other sperm can get in

•  After fertilization, meiosis II is complete

•  Pronuclei are formed (egg and sperm nuclei)

•  Zygote immediately starts to undergo mitosis

Figure 2.11

The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases

•  Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks)

•  Cdks activity fluctuates during the cell cycle because it is controled by cyclins, so named because their concentrations vary with the cell cycle

•  MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase


Figure 12.17

(a) Fluctuation of MPF activity and cyclin concentration during the cell cycle

(b) Molecular mechanisms that help regulate the cell cycle

MPF activity Cyclin concentration

Time

M M M S S G1 G2 G1 G2 G1

Cdk

Degraded cyclin

Cyclin is degraded

MPF

G2 checkpoint Cdk

Cyclin

Figure 12.17a

(a) Fluctuation of MPF activity and cyclin concentration during the cell cycle

MPF activity Cyclin concentration

Time

M M M S S G1 G2 G1 G2 G1

(b) Molecular mechanisms that help regulate the cell cycle

Cdk

Degraded cyclin

Cyclin is degraded

MPF

G2 checkpoint Cdk

Cyclin

Figure 12.17b

Stop and Go Signs: Internal and External Signals at the Checkpoints

•  An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase

•  Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide

•  For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture


Figure 12.18

A sample of human connective tissue is cut up into small pieces.

Enzymes digest the extracellular matrix, resulting in a suspension of free fibroblasts.

Cells are transferred to culture vessels.

Scalpels

Petri dish

PDGF is added to half the vessels.

Without PDGF With PDGF

10 µm

1

2

3 4

Loss of Cell Cycle Controls in Cancer Cells

•  Cancer cells do not respond normally to the body’s control mechanisms

•  Cancer cells may not need growth factors to grow and divide

–  They may make their own growth factor –  They may convey a growth factor’s signal without

the presence of the growth factor –  They may have an abnormal cell cycle control

system


•  A normal cell is converted to a cancerous cell by a process called transformation

•  Cancer cells that are not eliminated by the immune system, form tumors, masses of abnormal cells within otherwise normal tissue

•  If abnormal cells remain at the original site, the lump is called a benign tumor

•  Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form additional tumors


Figure 12.20

Glandular tissue

Tumor

Lymph vessel Blood vessel

Cancer cell

Metastatic tumor

A tumor grows from a single cancer cell.

Cancer cells invade neighboring tissue.

Cancer cells spread through lymph and blood vessels to other parts of the body.

Cancer cells may survive and establish a new tumor in another part of the body.

4 3 2 1

basic genetic

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