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Fatchiyah, Ph.D.

Lab. Molecular Biology

Brawijaya University

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Chromosome & DNA?• Chromosomes, threadlike structures, first

observed by Nageli in 1842. • Walter Flemming was the first to follow the

process of mitosis and replication of chromosomes.

• Thomas Morgan, in his experiments with fruit flies, described genetic recombination, and demonstrated that traits were to inherited together to varying degrees.

• Alfred Sturtevant extended Morgan’s ideas, used observed recombination rates to produce the first genetic maps.

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• Surrounded by a nuclear envelope•Perinuclear space

• Communicates with cytoplasm through nuclear pores

The nucleus is the center of cellular operations

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The Nucleus

Figure 3.13

Content of nucleus:•A supportive nuclear matrix•One or more nuclei•Chromosomes•Chromatin•DNA bound to histones

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What Are Chromosomes?

• Cytogenetics is the study of chromosomes and the related disease states caused by abnormal chromosome number and/or structure.

• Chromosomes are complex structures located in the cell nucleus, they are composed of DNA, histone and non-histone proteins, RNA , and polysaccharides.

• They are basically the "packages" that contain the DNA.

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• Normally chromosomes can't be seen with a light microscope but during cell division they become condensed enough to be easily analyzed at 1000X.

• To collect cells with their chromosomes in this condensed state they are exposed to a mitotic inhibitor which blocks formation of the spindle and arrests cell division at the metaphase stage

What Are Chromosomes?

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Type of chromosome structure

MetacentricChrom. 1

AcrocentricChrom. 14

SubmetacentricChrom. 9

1. The short arm is designated as p and the long arm as q.

2. The centromere is the location of spindle attachment and is an integral part of the chromosome.

3. It is essential for the normal movement and segregation of chromosomes during cell division.

The ideogram is basically a "chromosome map" showing the relationship between the short and long arms, centromere (cen), and in the case of acrocentric chromosomes the stalks (st) and satellites (sa). The specific banding patterns are also illustrated. Each band is numbered to aid in describing rearrangements.

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Figure 3.14 Chromosome Structure

Figure 3.14

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Organization of Genes on Human Chromosome

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Structural Organization of the Nucleosome

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Structural Organization of the Nucleosome

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Nucleosome Disruption

Figure 4-34. A cyclic mechanism for nucleosome disruption and re-formation. According to this model, different chromatin remodeling complexes disrupt and re-form nucleosomes, although, in principle, the same complex might catalyze both reactions. The DNA-binding proteins could function in gene expression, DNA replication, or DNA repair, and in some cases their binding could lead to the dissociation of the histone core to form nucleosome-free regions of DNA like those illustrated in Figure 4-30. (Adapted from A. Travers, Cell 96:311 314, 1999.)

http://www.ogm-info.com/chromosome.gif

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Chromosome Facts

• number of chromosomes: 22 pairs + 1 pair sex-determining chromosomes = 46– one chromosome of each pair donated from

each parent’s egg or sperm– sex chromosomes: X,Y for males; X,X for

females– largest chromosome #1 = ~263 million base

pairs (bp)– smallest chromosome Y = ~59 million bp

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Chromosomes can be “painted” for easyidentification.

By technique of multiplexfluorescence in situ hybridization (M-FISH)

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Mitoticchromosomes

G1 chromosomes

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Gene Facts

• size of human genome: 3.4 billion base pairs (bp)

• number of human genes: ~100,000

• genes vary in length and can cover thousands of bases– avg. size: ~3,000 bp

• only about 5% of the human genome contains genes

• function of much of the genome is unknown

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Chromosomes come in pairs

• Humans have 23 pairs of chromosomes–Total of 46 chromosomes

• Fruit flies have 4 pairs of chromosomes–Total of 8 chromosomes

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Normal Chromosomes

Normal human somatic cells have 46 chromosomes: 22 pairs, or homologs, of autosomes (chromosomes 1-22) and two sex chromosomes. This is called the diploid number. Females carry two X chromosomes (46,XX) while males have an X and a Y (46,XY).

Germ cells (egg and sperm) have 23 chromosomes: one copy of each autosome plus a single sex chromosome. This is referred to as the haploid number.

One chromosome from each autosomal pair plus one sex chromosome is inherited from each parent.

Mothers can contribute only an X chromosome to their children while fathers can contribute either an X or a Y.

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Chromosomes of humanMale Female

Y-chrom

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Members of a chromosome pair

• Cells with PAIRS of chromosomes are diploid– di – means “two”, “double”, “twice”– oid – “appearance”

• The two members of any chromosome pair are called homologues, or a homologous pair

• Ex:– The two “number 1” chromosomes are homologues– The two “number 21” chromosomes are

homologues.

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In sexual reproduction

• new organisms (zygotes) are formed by the joining together of two sex cells (gametes):– a sperm cell from a male and–an egg cell from a female

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What would happen if…….

• a human egg cell had 46 chromosomesAND

• a human sperm cell had 46 chromosomes?

46 chromosomes

+ 46 chromosomes

92 chromosomes

Too many for a human!

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Diploid organisms need….

• A way to produce sex cells (eggs and sperm) that only have ONE member of each chromosome pair

• In humans, sex cells each have 23 chromosomes– ONE member of each homologous pair

– Half of amount of chromosome is named genome (haploid)

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23 chromosomes

+23 chromosomes

46 chromosomes

Just right for a human!

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The different possibilities of the way chromosomes will separate into gametes helps create genetic diversity

OR

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Effect of Crossing Over

Crossing over occurs when homologous chromosomes pair with each other before the first meiotic division

Chromatids cross over one another, and the crossed sections of the chromatids are exchanged.

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Chromosomes

Figure 4-14. Two closely related species of deer with very different chromosome numbers. In the evolution of the Indian muntjac, initially separate chromosomes fused, without having a major effect on the animal. These two species have roughly the same number of genes. (Adapted from M.W. Strickberger, Evolution, 3rd edition, 2000, Sudbury, MA: Jones & Bartlett Publishers

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The emphasis in the human genome

Human

26,588 genes• The nervous system• The immune system• The blood system• Signaling and cell-cell

communication• Programmed cell

death

Arabidopsis

25, 498 genes

Missing

Missing

Missing

Different

Missing

Structure of Chromosome - NORsStructure of Chromosome - NORs

Interphase nucleus of onion root tip through light microscope

nucleolus

Structure of Chromosome - NORsStructure of Chromosome - NORs

electron micrograph of interphase cell from bat pancreas

nucleolus

Polytene Chomosome of Drosophila as Giant chromosome

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Phase-contrast image of Drosophila melanogaster polytene cromosomes.

A. The end of the X-chromosome is marked with an arrow. Chromocentre is in the upper right corner.

B. Shows a magnification of chromomere and interchromomere bands

Polytene chromosome

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W-chromosomeYeast Artificial Chromosome

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Bacterial Artificial Chromosome Manipulationby Homologous Recombinant

(HeintZ, 2001)

Metaphase: Chromosome in middle cell

Antibodies of a person with an autoimmune disease stain

centromers