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Dr MOHAMED FAKHRY 2015 1 2 PROKARYOTES EUKARYOTES single chromosome plus plasmids many chromosomes circular chromosome linear chromosomes made only of DNA made of chromatin, a nucleoprotein (DNA coiled around histone proteins) found in cytoplasm found in a nucleus copies its chromosome and divides immediately afterwards copies chromosomes, then the cell grows, then goes through mitosis to organise chromosomes in two equal groups Dr MOHAMED FAKHRY 2015 Chromosomes They were given the name chromosome (Chromo = colour; Soma = body) due to their marked affinity for basic dyes One member of each chromosome pair is derived from each parent. Somatic cells have diploid complement (set)= chromosomes i.e. 46 Germ cells (Gametes: complement i.e. 23 sperm & ova) have مجموعةof haploid The chromosomes of dividing cells are most readily analyzed at the `metaphase' stage of mitosis . Dr MOHAMED FAKHRY 2015 3 Chromosomes Chromosomes are composed of thin chromatin threads called Chromatin fibers. These fibers undergo folding, coiling and supercoiling during prophase so that the chromosomes become progressively thicker and smaller. Therefore, chromosomes become readily observable= يمكن مالحظته بسهولةunder light microscope. At the end of cell division, on the other hand, the fibers uncoil and extend as fine chromatin threads, which are not visible at light microscope Dr MOHAMED FAKHRY 2015 4 Gene for cystic fibrosis (chromosome 7) Gene for sickle cell disease (chromosome 11) • Chromosomes are made of DNA. • Each contains genes in a linear order. • Human body cells contain 46 chromosomes in 23 pairs – one of each pair inherited from each parent • Chromosome pairs 1 – 22 are called autosomes. • The 23rd pair are called sex chromosomes: XX is female, XY is male. Dr MOHAMED FAKHRY 2015 5 Chromosomes can be identified by: Their size Their shape (the position of the centromere) NB Chromosomes are flexible Banding patterns produced by specific stains (Giemsa) Chromosomes are analysed by organising them into a KARYOTYPE 6 © 2007 Paul Billiet ODWS Dr MOHAMED FAKHRY 2015 © Biologyreference.com Female Male 7 Trysomy-21 Down’s syndrome Trysomy-18 Edward’s syndrome Images believed to be in the Public Domain Dr MOHAMED FAKHRY 2015 8 The Normal Human Chromosomes Normal human cells contain 23 pairs of homologous chromosomes: 22 pairs of autosomes numbered as 1-22 in decreasing order of size 1 pair of sex chromosomes p Autosomes are the same in males & females Sex chromosomes are: - XX in females - XY in males q Both X are homologous Y is much smaller than X and has only a few genes 9 Chromosome Structure Telomere • At the metaphase stage: Each chromosome consists of two chromatids joined at the centromere or primary constriction • The centromere divides chromosomes into short (p i.e. petit) and long (q i.e. g=grand) arms. • The tip of each chromosome is called telomere • The exact function of the centromere is not clear, but it is known to be responsible for the movement of the chromosomes at cell division. p Centromere q Dr MOHAMED FAKHRY 2015 10 Chromosomes may differ in the position of the Centromere, the place on the chromosome where spindle fibers are attached during cell division. In general, if the centromere is near the middle, the chromosome is metacentric If the centromere is toward one end, the chromosome is acrocentric or submetacentric If the centromere is very near the end, the chromosome is telocentric. The centromere divides the chromosome into two arms, so that, for example, an acrocentric chromosome has one short and one long arm, While, a metacentric chromosome has arms of equal length. All house mouse chromosomes are telocentric, while human chromosomes include both metacentric and acrocentric, but no telocentric. 11 12 Chromosomes as seen at metaphase during cell division Telomere DNA and protein cap Ensures replication to tip Tether to nuclear membrane Short arm p (petit) Long arm q Telomere Light bands Replicate early in S phase Less condensed chromatin Transcriptionally active Gene and GC rich Centromere Joins sister chromatids Essential for chromosome segregation at cell division 100s of kilobases of repetitive DNA: some non-specific, some chromosome specific Dark (G) bands Replicate late Contain condensed chromatin AT rich Dr MOHAMED FAKHRY 2015 13 A pair of homologous chromosomes (number 1) as seen at metaphase Locus (position of a gene or DNA marker) Allele (alternative form of a gene/marker) Dr MOHAMED FAKHRY 2015 14 Centromeres and telomeres are two essential features of all eukaryotic chromosomes. Each provide a specific function i.e., absolutely necessary for the stability of the chromosome. Centromeres are required for the segregation of the chromosomes during meiosis and mitosis. Teleomeres provide terminal stability to the chromosome and ensure its survival Dr MOHAMED FAKHRY 2015 15 The region where two sister chromatids of a chromosome appear to be joined or “held together” during mitotic metaphase is called Centromere When chromosomes are stained they typically show a darkstained region that is the centromere. Also termed as Primary constriction During mitosis, the centromere that is shared by the sister chromatids must divide so that the chromatids can migrate to opposite poles of the cell. On the other hand, during the first meiotic division the centromere of sister chromatids must remain intact whereas during meiosis II they must act as they do during mitosis. Therefore the centromere is an important component of chromosome structure and segregation. 16 The two ends of a chromosome are known as telomeres. It required for the replication and stability of the chromosome. When telomeres are damaged or removed due to chromosome breakage, the damaged chromosome ends can readily fuse or unite with broken ends of other chromosome. Dr MOHAMED FAKHRY 2015 17 Species Arabidopsis Human Oxytricha Slime Mold Tetrahymena Trypanosome Repeat Sequence TTTAGGG TTAGGG TTTTGGGG TAGGG TTGGGG TAGGG 18 Tetrahymena - protozoa organism. The telomeres of this organism end in the sequence 5'-TTGGGG-3'. The telomerase adds a series of 5'-TTGGGG-3' repeats to the ends of the lagging strand. A hairpin occurs when unusual base pairs between guanine residues in the repeat form. Finally, the hairpin is removed at the 5'TTGGGG-3' repeat. Thus the end of the RNA Primer - Short stretches of chromosome is faithfully ribonucleotides (RNA substrates) found replicated. on the lagging strand during DNA replication. Helps initiate lagging strand replication 19 Gene Map 20 Classification Of Chromosomes Chromosomes are classified (analyzed) according to: I. Morphology (Shape) II. Staining Morphologically (shape) According to the position of the centromere : 1- Metacentric 2- Sub-metacentric 3- Acrocentric 4- Telocentric: (with centromere at one end) occurs in other species, but not in man Dr MOHAMED FAKHRY 2015 21 Metacentric Chromosomes Sub-Metacentric Chromosomes Telomeres p Centromere q Dr MOHAMED FAKHRY 2015 22 Acrocentric chromosomes (13, 14, 15, 21 and 22) They have a small mass of chromatin known as satellite attached to their short arm by narrow stalks (secondary constrict). The stakes contain genes for 18S and 28S rRNA. Satellite Stalk 23 →Classification Of Chromosomes There are several staining methods for cytogenetic analysis of chromosomes. Each stain produces specific banding patterns known as "Chromosome Banding = " تباين – نطاقات أوأاشرطة G banding Q banding R banding C banding The pattern is specific for each chromosome and is the characteristics utilized to identify each chromosome. 1) 2) 3) 4) Dr MOHAMED FAKHRY 2015 24 Staining Methods for Cytogenetic Analysis G Banding: Treat with trypsin and then with Geimsa Stain. R Banding: Heat and then treat with Geimsa Stain. Q Banding: Treat with Quinicrine dye giving rise to fluorescent bands. C Banding: Staining of the Centromere. Dr MOHAMED FAKHRY 2015 25 The G-Banding Pattern of Chromosomes Dr MOHAMED FAKHRY 2015 26 Staining and Banding chromosome Staining procedures have been developed in the past two decades and these techniques help to study the karyotype in plants and animals. 1. Q banding: The Q bands are the fluorescent bands observed after quinacrine mustard staining and observation with UV light. The distal ends of each chromatid are not stained by this technique. The Y chromosome become brightly fluorescent both in the interphase and in metaphase. 2. R banding: The R bands (from reverse) are those located in the zones that do not fluoresce with the quinacrine mustard, that is they are between the Q bands and can be visualized as green. Staining and Banding chromosome G banding: The G bands (from Giemsa) have the same location as Q bands and do not require fluorescent microscopy. 3. C banding: The C bands correspond to constitutive heterochromatin. The heterochromatin regions in a chromosome distinctly differ in their stainability from euchromatic region. 3. Dr MOHAMED FAKHRY 2015 28