University of Sharjah

Faculty of health Sciences

Department of Medical Laboratory Sciences

Biology 1

Chapter18 ||| Lecture 1and 2

Patterns of Chromosome Inheritance

Chapter 18

Patterns of Chromosome Inheritance

Human Biology, Sylvia S. Mader & Michael Windelspecht

Learning objective

• What is the structure of chromosomes?

• What is the cell cycle and what occurs during each of its stages?

• Explain what mitosis is used for, what cells undergo mitosis, and the 4 stages of mitosis.

• Explain the 2 divisions of meiosis.

• What is meiosis used for and what cells undergo meiosis?

• Compare and contrast mitosis and meiosis.

• Compare and contrast spermatogenesis and oogenesis.

• What are trisomy and monosomy?

• What most often causes changes in chromosome number?

• What are the syndromes associated with changes in sex chromosomes?

• Explain the 4 changes in chromosome structure.

Eukaryote chromosomes are composed or "packaged" by strands of negatively-charged DNA wrapping themselves around positively-charged globular proteins called histones. The histones form clusters called nucleosomes that clump together to form chromatin fibers in the nucleoplasm or cytoplasm inside the nuclear membrane. When the DNA in a chromatin strand is being used during early protein synthesis, it is loosely packed and called euchromatin. When it is not being used, it is coiled into a tight package called heterochromatin.

18.1 Chromosomes

Chromosomes: A review

• Humans have 46 chromosomes that are in 23 pairs within a cell’s nucleus.

–

Pairs of chromosomes are called Homologous chromosomes.

– Autosomes are the 22 pairs of chromosomes that control traits unrelated to gender.

– Sex chromosomes are the 1 pair that contains the genes that do control gender (male sex chromosomes are XY and those of the female are XX.

- Cells (somatic or body) that have 46 (2n) paired chromosomes are called diploid.

- Cells (sex cells or gametes) that have only 23 (n) unpaired chromosomes are called haploid cells.

What is a karyotype?

• A karyotype (Greek karyon = kernel, seed or nucleus) is the number and appearance of chromosomes in the nucleus of a eukaryotic (true or nucleated) cell; the term is also used to denote

the complete set of chromosomes (46 in humans).

18.1 Chromosomes

What is a karyotype (cont.)?

karyotyping

The cell cycle

• 2 parts

1. Interphase

• G₁ stage – cell doubles its organelles; cell grows in size

• S stage – DNA synthesis (replication) occurs

• G₂ stage – proteins needed for division are synthesized

2. Cell division (mitosis and cytokinesis)

• Mitosis – nuclear division

• Cytokinesis – cytoplasmic division

The cell cycle

Control of the cell cycle

• Checkpoints can delay the cell cycle until certain conditions are met.

• There are 3 major checkpoints.

– G₁, if passed, the cell is committed to divide.

– G₂ cycle pauses to verify DNA replication.

– M cycle pauses to verify spindle assembly and chromosome attachment.

Control of the cell cycle

Figure 18.3 Control of the cell cycle.

External control of the cell cycle

•

External signals like hormones and growth factors (signaling molecules) can alter the rhythm of the cell cycle

• signaling molecules bind specific receptors on target cells causing a molecular signal to be generated inside the cell

(signal transduction)

• Net result is the turning off/on certain genes involved in cell cycling

Chromosome structure in mitosis

•Chromosomes contain both DNA and proteins (collectively called chromatin).

•Chromosomes that are dividing are made up of 2 identical parts called sister chromatids.

•The sister chromatids are held together at a region called the centromere.

Chromosome structure in mitosis

The spindle in mitosis

• centrosome – the microtubule organizing center of the cell

•

aster – a radial array of microtubules extends from each centrosome at one of the poles

(ends of the cell)

• Kinetochores – short cylinders of microtubules that assist in the formation of spindle fibers

The spindle in mitosis

Overview of mitosis

• A diploid cell makes and divides an exact copy of its nucleus.

• It is used in cell growth and cell repair.

• Mitosis occurs in body cells.

• There are 4 phases.

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

Overview of mitosis

1. Mitosis: Prophase

•

Chromosomes condense and become visible.

• The nuclear envelop fragments.

• The nucleolus disappears.

• Centrosomes move to opposite poles.

• Spindle fibers appear and attach to the centromeres.

18.3 Mitosis

1. Mitosis: Prophase

chromosomes, and the nuclear Centrosomes begin moving apart,

envelope is fragmenting. and spindle is in process of forming.

(early prophase, prophase): © Ed Reschke

Figure 18.8 The phases of mitosis.

2. Mitosis: Metaphase

come from opposite spindle poles.

(metaphase): © Ed Reschke; (early metaphase): © Michael Abbey/Photo Researchers, Inc.

3. Mitosis: Anaphase

• Sister chromatids separate at the equator and move towards the poles.

poles. In this way, each pole receives the same number and kinds of chromosomes as the parental

(anaphase): © Ed Reschke;cell.

4. Mitosis: Telophase and cytokinesis

• Chromosomes arrive at the poles.

• Chromosomes become

indistinct chromatin again.

• nucleoli reappear.

• Spindle disintegrates.

• Nuclear envelope reappear.

• 2 New daughter cells are formed by a ring of actin

filaments (cleavage _{Daughter cells are forming}

furrow). nucleoli reappear. Chromosomes willbecome indistinct chromatin.as nuclear envelopes and

(telophase): © Ed Reschke;

Overview of meiosis

•

Two nuclear divisions occur to make 4 daughter cells.

• It is used to make gametes (egg and sperm).

• Meiosis occurs in sex cells

• Has 8 phases (4 in each meiosis, I & II)

Figure 18.9 The results of meiosis.

n = 2

• Prophase I

– Homologous chromosomes pair (synapsis); crossing-over occurs, in which there is exchange of genetic information.

• Metaphase I

– Homologous pairs line up at the equator.

• Anaphase I

– Homologous chromosomes separate and move toward opposite poles.

• Telophase I

– 2 daughter cells result, each with 23 pairs of homologous chromosomes (in males it is 22 pairs + chromosomes X and Y) .

diploid

Prophase I

Chromosomes have duplicated. Homologous chromosomes pair during synapsis and crossing-over occurs.

Prophase I

Chromosomes have duplicated.

Homologous

chromosomes pair during synapsis and crossing-over occurs.

Metaphase I

Homologous pairs align independently at the equator.

Prophase I

Chromosomes have duplicated. Homologous chromosomes pair during synapsis and crossing-over occurs.

Metaphase I

Homologous pairs align independently at the equator.

Anaphase I

Homologous chromosomes separate and move toward the poles.

crossing-over occurs. from each homologous pair.

Prophase I

Chromosomes have duplicated. Homologous

chromosomes pair during Interkinesis ^{synapsis and}Anaphase I Chromosomes still

Daughter cells have one chromosome

^{Metaphase I}from each homologous pair.

Homologous pairs align independently at the equator.

18.4 Meiosis

What is crossing-over?

• Crossing-over is the exchange of genetic information between homologous sister chromatids during synapsis (In synapsis, homologous chromosomes loosely pair up, aligned gene by gene).

• This occurs during metaphase1 of meiosis I and increases genetic variation.

18.4 Meiosis What is crossing-over?