Chromosome Structure
What is a chromosome?
''A chromosome is a deoxyribonucleic acid (DNA) molecule with part or all of the genetic material (genome) of an organism. Most eukaryotic chromosomes include packaging proteins which, aided by chaperone proteins, bind to and condense the DNA molecule to prevent it from becoming an unmanageable tangle''
Genes are organized into discrete cellular structures called chromosomes that coordinate
DNA replication and distribution of replicated genetic copies between two daughter cells.
As vehicles of genetic transmission, chromosomes play a central role in Darwinian
evolution.
Chromosome Size
Free-living bacteria need genetic information to synthesize
proteins for executing vital functions. Most bacteria have a
single chromosome with DNA that is about 2Mbp (mega
base pairs) long (1Mbp 5 1 000 000 base pairs), but the
DNA content ofdifferent species varies from 0.58 to
greater than 9 Mbp ofDNA, and some bacteria have
multiple chromosomes. For example, Leptospira has two
chromosomes of4.4 and 4.6 Mbp and the largest bacterialbreaks down once per cell cycle to distribute the 46 diploid
chromosomes equally between two daughter cells.
Plasmids
In addition to the large chromosome, many (most?)
bacteria have additional DNA molecules called plasmids
(or episomes.) Plasmids are separate DNA molecules that
contain a replication origin which allows them to multiply
independently ofthe host chromosome. Plasmids range in
size from 1 kbp (Kilo base pair) (1000 bp) to 100 kbp, and
these DNA molecules encode genetic systems for specialized functions. Some plasmids make extracellular appendages that allow bacteria to infect and colonize sensitive
eukaryotic hosts. Plasmids often carry genes that confer on
bacteria the ability to survive in the presence ofantibiotics
such as tetracycline, kanamycin and penicillin. Many
plasmids also contain genes that promote DNA transfer so
that plasmid genes can move into other bacterial species.
Plasmid transfer has caused the emergence of bacterial
pathogens that are resistant to most ofthe useful
antibiotics in medicine, with notable examples including
multidrug-resistant strains of Staphylococcus and Mycobacterium tuberculosis.
In most eukaryotes, plasmids are rare. However, S.
cerevisiae contains a plasmid called the 2-m circle which
efficiently partitions to new daughter cells at every cell
division. This DNA serves as a convenient module for gene
cloning and performing genetic experiments in yeast.
Chromosomes and DNA
Deoxyribonucleic acid (DNA) is the genetic code that allows information to be transferred from one generation to the next. DNA molecules consist of two linear chains that are wrapped around one another forming a double helix structure. These helical structures are further wound into chromosome structures. Chromosomes are divided into two parts with a constriction point in the in the middle which is known as the centromere. The four types of chromosomes in animal cells are classified by the position of the centromere.
Structure and Function of the Centromere
Centromeres consist of a complex combination of proteins and DNA. They are essential to the division of cells and ensure the accurate segregation of chromosomes. Studies have demonstrated that chromosomes without centromeres segregate randomly and are eventually lost from cells. In contrast, chromosomes that have multiple centromeres can be subject to fragmentation.
Metacentric Chromosomes
Metacentric chromosomes have the centromere in the center, such that both sections are of equal length. Human chromosome 1 and 3 are metacentric.
Submetacentric Chromosomes
Submetacentric chromosomes have the centromere slightly offset from the center leading to a slight asymmetry in the length of the two sections. Human chromosomes 4 through 12 are submetacentric.
Acrocentric Chromosomes
Acrocentric chromosomes have a centromere which is severely offset from the center leading to one very long and one very short section. Human chromosomes 13,15, 21, and 22 are acrocentric.
Telocentric Chromosomes
Telocentric chromosomes have the centromere at the very end of the chromosome. Humans do not possess telocentric chromosomes but they are found in other species such as mice.
Numerical Chromosomal Abnormalities
The condition trisomy 21 (Down syndrome) is caused by an extra copy of chromosome 21 in the egg or sperm that results in the fertilized egg receiving three copies of chromosome 21. Mosaic trisomy 21 is a rare form of Down syndrome that happens after fertilization.
Trisomy 13 (Patau syndrome) causes severe intellectual and physical disabilities. Trisomy 18 (Edwards syndrome) is even more severe and can threaten children’s survival. Trisomy X is an extra copy of the X chromosome in female sex cells. Klinefelter syndrome happens when a male inherits an extra X chromosome from his mother; the XXY condition is sometimes associated with advanced maternal age.
Monosomy occurs when one chromosome is partially or entirely missing. For example, females with Turner syndrome only have one X chromosome instead of two X chromosomes. Cri du chat syndrome results from a deletion of the short arm of chromosome 5.
Chromosome Structure
Questions-
- What are centromeres?
- What are telomeres?
- How many chromosomes do humans have?
Answer- Commenrt Now
- QUIZ
- In plant and animal cells, DNA is tightly packaged into thread-like structures called chromosome .This is in contrast to bacteria where DNA floats freely around the cell.
- A single length of DNA is wrapped many times around lots of proteins called histones, to form structures called nucleosomes.
- These nucleosomes then coil up tightly to create chromatin loops.
- The chromatin loops are then wrapped around each other to make a full chromosome.
- Each chromosome has two short arms (p arms), two longer arms (q arms), and a centromere holding it all together at the centre.
- Humans have 23 pairs of chromosomes (46 in total): one set comes from your mother and one set comes from your father.
- Of these 23 pairs, one pair are sex chromosomes so differ depending on whether you are male or female (XX for female or XY for male).
- The other 22 pairs are AUTOSOME (non-sex chromosomes) and look the same for both males and females.
- The DNA making up each of our chromosomes contains thousands of GENS.
- At the ends of each of our chromosomes are sections of DNA called TELOMERE. Telomeres protect the ends of the chromosomes during DNA REPLICATION by forming a cap, much like the plastic tip on a shoelace.
Raghvendra Singh
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