1. Lecture 2-3 Genetics Dr. Heba Al-Fares
Chromosome
Lecture 2-3
Genetics
Dr. Heba Al-Fares

2. Contents of this lecture
Chromosome structure
Classification of chromosomes
Chromosomal aberrations

3. chromosomes in prokaryotic and eukaryotic cell
Chromosomes are structures within the living cells that contain the genetic material.
In Prokaryotic
Chromosomes is circular DNA molecules contain the entire set of genetic instruction essential for life of the cell. (in a region of the cytoplasm called the nucleoid).
When circular DNA copied, the genetic information is passed on to the daughter cells (new cells created by cell division) during mitosis.
In Eukaryotes
Chromosomes are threadlike strands that are consisting of chromatin and carrying the genetic information arranged in a linear sequence.
To pass genetic traits from one generation to the next, the chromosomes must be copied, and then the copies must be divided up.

4. Chromosomes (DNA) in eukaryotics
Chromosomes are double helix spirals composed of deoxyribonucleic acid (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.

5. Chromosome Structure
DNA is long and thin and fragile: needs to be packaged to avoid breaking.
First level is the nucleosome, 200 bp of DNA wrapped twice around a core of 8 histone proteins (small and very conserved in evolution). A string of beads.
The nucleosomes coil up into a 30 nm chromatin fiber. This level of packaging exists even during interphase.
During cell division, chromatin fibers are attached in loops of variable size to a protein scaffold. The DNA probably attaches at specific AT-rich areas called scaffold attachment regions.
The loops may be functional units: active vs. inactive in transcription.
Further coiling gives the compact structures we see in metaphase.

6. Centromeres
Sometimes called the “primary constriction” on a chromosome, based on microscopic appearance.
The centromere is the attachment point for the spindle.
The centromere is a region of DNA on the chromosome. During cell division, a large protein structure, the kinetochore, that attaches to the centromere DNA sequences. The spindle proteins then get attached to the kinetochore.
The centromere is many repeats of about 170 bp element .
Centromere regions also contain large amounts of repeated sequence DNA.

7. Telomeres
Telomeres are the DNA sequences at the ends of chromosomes. Chromosomes that lose their telomeres often fuse with other chromosomes or become degraded. There are telomere-binding proteins that protect the chromosome ends.

8. Euchromatin and Heterochromatin
Euchromatin is the location of active genes (although many genes in euchromatin are not active: depends on cell type). During interphase euchromatin is extended and spread out throughout the cell.
Heterochromatin is darkly staining, condensed, and late replicating. Genes in heterochromatin are usually inactive.
Some heterochromatin is constitutive : always heterochromatin: especially around centromeres. Composed mostly of repeat sequence DNA.
Other heterochromatin is facultative: can be heterochromatin or euchromatin

9. Cytogenetics Microscopic examination of chromosomes
Karyotype : the chromosome as viewed under the microscope (nuclear type).
Cytogenetics : the microscopic study of chromosomes and analysis of their genetic property it combine genetic and cytology.
It describes the light microscopic morphology of the component chromosomes, so that their relative lengths, centromere positions, and secondary constrictions can be identified.

10. Classification of chromosomes
Main features to identify and classify chromosomes
Size
Location of the centromeres
Banding patterns

11. Banding patterns
G-banding: chromosomes are treated with trypsin enzyme that partially digest chromosomal proteins when exposed to dye call Giemsa some chromosomal region bind the dye heavily and produce dark band (tightly compacted and G bands contain high proportion of A-T pairs).
R (reverse Giemsa) bands are produced by heat-treating the chromosomes in saline solution before staining with Giemsa (the R bands rich in G-C and most active gene are located in this bands)

12. G-Banded Metaphase Chromosomes
Figure 8.1

13. Chromosome size Large chromosomes Medium chromosomes Small chromosomes
Sex chromosomes

14. Types of Chromosome
Chromosomes are placed into broad categories depending on the position of the centromere.
metacentric: centromere in the middle, with arms of equal length.
telocentric: centromere at one end, with only 1 arm.
acrocentric: centromere near one end, with arms of very different lengths
sub-metacentric: centromere near the middle, with arms of slightly different lengths.

15. Chromosome are classified based on the locations of their centromeres.

16. Counting out chromosome numbers
Each eukaryotic organism has a very specific number of chromosomes per cell — ranging from one to many. For example, humans have 46 total chromosomes.
These chromosomes come in two varieties:
Sex chromosomes: These chromosomes determine gender. Human cells contain two sex chromosomes. If you’re female, you have two X chromosomes, and if you’re male, you have an X and a Y chromosome.
Autosomal chromosomes: Autosomal simply refers to non-sex chromosomes. So, sticking with the human example, humans have 44 autosomal chromosomes.

17. Counting out chromosome numbers
In humans, chromosomes come in pairs.
That means you have 22 pairs of uniquely shaped autosomal chromosomes plus 1 pair of sex chromosomes, for a total of 23 chromosome pairs.
Your autosomal chromosomes are identified by numbers — 1 through 22.
So, you have two chromosome 1s, two 2s, and so on.

18. Counting out chromosome numbers
This figure shows you how all human chromosomes are divided into pairs and numbered.

19. Homologous chromosomes
When chromosomes are divided into pairs, the individual chromosomes in each pair are considered homologous, meaning that the paired chromosomes are identical to one another in shape and size.
These homologous chromosomes are sometimes referred to as homologs for short.

20. Chromosomes carry genes
Genes are sections of DNA that make up the building plans for physical traits.
The genes tell the body how, when, and where to make all the structures that are necessary for the processes of living.
Each pair of homologous chromosomes carries the same — but not necessarily identical — genes.
For example, both chromosomes of a particular homologous pair might contain the gene for hair color, but one can be a “brown hair” version of the gene — alternative versions of genes are called alleles — and the other can be a “blond hair” allele.

21. Chromosomes carry genes
One chromosome carries the allele A while its homolog carries the allele a (the relative size of an allele is normally very small; the alleles are large here so you can see them).
The alleles code for the different physical traits (phenotypes) you see in animals and plants like hair color or flower shape.

22. Chromosomes carry genes
Each point along the chromosome is called a locus (Latin for “place”). The plural of locus is loci (pronounced low-sigh).
Most of the phenotypes that you see are produced by multiple genes (that is, genes occurring at different loci and often on different chromosomes) acting together.
For instance, human eye color is determined by at least three different genes that reside on two different chromosomes.

23. Chromosome numbers haploid and euploid
Chromosome numbers can get a bit confusing.
Humans are diploid, meaning we have two copies of each chromosome.
Some organisms (like bees and wasps) have only one set of chromosomes (cells with one set of chromosomes are referred to as haploid); others have three, four, or as many as sixteen copies of each chromosome!

24. Chromosome numbers
The number of chromosome sets held by a particular organism is called the ploidy.
The total number of chromosomes doesn’t tell you what the ploidy of an organism is.
For that reason, the number of chromosomes of an organism is often listed as some multiple of n.

25. Chromosome numbers
Humans are 2n = 46 (indicating that humans are diploid and the total number of chromosomes is 46).
A single set of chromosomes referred to by the n is the haploid number. Human sex cells such as eggs or sperm are haploid.

26. Chromosome number and ploidy condition in commercial important crop species
Chromosome no. in X
Common name 4X 32 8
alfalfa 2X 34 17
Apple 52 13
Oats 28 7
Wheat, durum 6X 42
Wheat, bread 14
Barley 8X 56
Strawberry
X : (genome) The entire complement of genetic material in chromosome set.
N: basic number of chromosome

27. Variation In Chromosome Number
Euploidy
Normal variations of the number of complete sets of chromosomes
Chromosomes number are multiples of some basic number (n)
N: number of chromosome in one nuclear genome (haploid) that is 1xn=n, diploid 2n that is 2xn=2n, and so on
N is called the haploid chromosome number.
2n is called diploid.
3n, 4n, and so on are called polyploid.
Haploid, Diploid, Triploid, Tetraploid, etc…
Genome (X): The entire complement of genetic material in chromosome set.
Aneuploidy
Variation in the number of particular chromosomes within a set
Aneuploidies: changes in part of chromosome sets
A change from euploid number
Monosomy, trisomy, tetrasomy

28. Polyploidy v Aneuploidy

29. Euploidy Variations Plants commonly exhibit polyploidy
30-35% of ferns and flowering plants are polyploid
Many of the fruits & grain are polyploid plants
Polyploid strains often display desirable agricultural characteristics
wheat
cotton
strawberries
bananas

30. Euploid Number can Naturally Vary
Most animal species are diploid
Polyploidy in animals is generally lethal
Some naturally occurring euploidy variations
bees - females are diploid.

31. Benefit of Odd Ploidy-Induced Sterility
Seedless fruit
watermelons and bananas
asexually propagated by human via cuttings
Seedless flowers
Marigold flowering plants
Prevention of cross pollination of transgenic plants

32. Chromosomal aberrations
Substantial changes in chromosome structure
Typically affect multiple genes (loci)

33. Categories of Chromosomal Aberrations
Inversions
Normal A B C D E F
Pericentric – inversion about the centromere
A D C B E F
Paracentric – inversion not involving the centromere
A B C E D F

34. Inversions
A segment of chromosome that is flipped relative to that in the homologue
Centromere lies within inverted region
Centromere lies outside inverted region
Figure 8.11

35. Deletions
Loss of a region of a chromosome

36. Cri-du-chat Syndrome
Symptom: cat-like cry, short stature, and facial abnormality.

37. Duplication
Duplications result from doubling of chromosomal segments, and occur in a range of sizes and locations.
a. Tandem duplications are adjacent to each other.
b. Reverse tandem duplications result in genes arranged in the opposite order of the original.
c. Tandem duplication at the end of a chromosome is a terminal tandem duplication.

38. Fig. 17.5 Duplication, with a chromosome segment repeated
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.

39. Forms of chromosome duplications are tandem, reverse tandem, and terminal tandem duplications
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.

40. Duplications
Phenotypic consequences of duplications correlated to size & genes involved
Duplications tend to be less detrimental

41. Translocations
When a segment of one chromosome becomes attached to another
Exchange or joining of regions of two non-homologous chromosomes
In reciprocal translocations two non-homologous chromosomes exchange genetic material
Usually generate so-called balanced translocations
Usually without phenotypic consequences.

42. Fig. 8.13b(TE Art) Nonhomologous chromosomes 1 1 7 7 Crossover between
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. 8.13b(TE Art)
Nonhomologous chromosomes 1 1 7 7
Crossover between
nonhomologous
chromosomes 1 7
Reciprocal
translocation
Nonhomologous crossover

43. Fig. 8.13a(TE Art) 22 22 Environmental agent 2 2 causes 2 chromosomes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. 8.13a(TE Art) 22 22
Environmental agent
causes 2 chromosomes
to break. 2 2
Reactive ends
DNA repair enzymes
recognize broken ends
and connect them.
Chromosomal breakage and DNA repair

44. Chromosomes Eukaryotic Linear Prokaryotic Fairly long Circular
Several chromosomes per cell.
Histone proteins package DNA Same in all eukaryotes
Housed in a nucleus.
Nucleosome—2 loops of DNA wrapped around 8 histone proteins.
Prokaryotic
Circular
Very small
1 chromosome per cell
Some enzymes and proteins are associated with the DNA.
Not housed in a nucleus.