Biochemistry,  Cell biology,  Genetics


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The DNA of an organism are packed in their cells to protect, they also regulate the accessibility of the DNA. The packaging of DNA helps conserve space in the cells. Approximately, two meters of the human DNA can fit into a cell that is only a few micrometres wide. Chromosomes are made up of DNA segments. Chromosomes carry all the information that helps a cell grows, survive and reproduce. DNA segments with specific patterns are called genes. The chromosomes are found in the nucleus of the cell. In prokaryotic organisms, the DNA is not present in the nucleus; the DNA floats in the cytoplasm in the area called the nucleoid. 

The chromosomes vary widely between different organisms. Eukaryotic cells have a large number of linear chromosomes and cells of prokaryotes have smaller and circular DNA. Cells may contain more than one type of chromosome, like in most eukaryotic cells, the mitochondria and the chloroplasts in plant cells possess their own set of chromosomes. 

In the nucleus of the eukaryotic organism, the chromosomes are packed with proteins to form a compact structure called chromatin. This condensation allows long molecules of DNA to fit into the cell nucleus. Chromosomes are more condensed than the chromatin and they are essential for cell division. The chromosomes are replicated, divided and passed on to the daughter cells, to ensure genetic diversity and survival of the progeny. 

Duplicated chromosomes contain two identical copies known as chromatids or sister chromatids, they are joined by a centromere. Compaction of the chromosomes during the cell division process results in the four-arm structure.

Recombination of chromosome plays a vital role in genetic diversity. Incorrect multiplication of the chromosomes may lead to mitotic failure or death of the cell, it may lead to apoptosis and sometimes may be cancerous. 

What are Chromosomes? 

Chromosomes are the organized structure of DNA and proteins found in cells. they are thread-like structures located inside the nucleus of animal and plant cells. Chromosomes are made up of proteins and a molecule of deoxyribonucleic acid (DNA). Chromosomes are passed on from parents to offspring. The term chromosome is derived from the Greek word ‘chroma’ which means ‘colour’ and ‘soma’ which means ‘body’. The chromosomes are named so because they are cellular structures or cellular bodies and they are strongly stained by some dyes used in research. Chromosomes play an important role that ensures DNA is copied and distributed accurately in the process of cell division. In most of the organisms, chromosomes are arranged in pairs in the nucleus of the cell. We have 23 pairs of chromosomes

Chromosome Structure

  • In eukaryotic cells, chromosomes are composed of the single molecule of DNA with many copies of five types of histones.
  • Histones are proteins molecules and are rich in lysine and arginine residues, they are positively charged. Hence they bind tightly to the negatively charged phosphates in the DNA sequence. 
  • A small number of non-histone proteins are also present, these are mostly transcription factors. Transcription factors regulate which parts of DNA to be transcribed into RNA. 
  • During most of the cell’s life cycle, chromosomes are elongated and cannot be observed under the microscope. 
  • During the S phase of the mitotic cell cycle, the chromosomes are duplicated. 
  • At the beginning of mitosis, the chromosomes are duplicated and they begin to condense into short structures which can be stained and observed easily under the light microscope.
  • These duplicated condensed chromosomes are known as dyads.
  • The duplicated chromosomes are held together at the region of centromeres. 
  • The centromeres in humans are made of about 1-10 million base pairs of DNA.  
  • The DNA of the centromere are mostly repetitive short sequences of DNA, the sequences are repeated over and over in tandem arrays. 
  • The attached, duplicated chromosomes are commonly called sister chromatids. 
  • Kinetochores are the attachment point for spindle fibers which helps to pull apart the sister chromatids as the mitosis process proceeds to anaphase stage. The kinetochores are a complex of about 80 different proteins. 
  • The shorter arm of the two arms of the chromosome extending from the centromere is called the p arm and the longer arm is known the q arm.

Bacterial Chromosome

Bacterial chromosomes contain circular DNA molecule unlike the linear DNA of vertebrates. Most of the chromosomes are circular DNA molecules and there are no free ends to the DNA.  The bacterial DNA is packaged into a single chromosome into a continuous loop. The DNA is folded or coiled to fit into the cell. The compaction of the DNA involves the binding of proteins to the DNA that helps form initial loops which are then coiled.

Prokaryotic Chromosome

Prokaryotes like the bacteria and archaea typically have a single circular chromosome. The chromosome size of most bacteria is from only 160,000 base pairs to 12,200,00 base pairs. Some bacteria in exceptions contain a single linear chromosome. The base sequences in prokaryotic chromosomes are less than in eukaryotic cells. Bacterial chromosomes have a single origin of replication from which the replication starts. In some archaea, there are multiple replication origins. The prokaryotic genes are organized into operons and it usually it does not contain introns. The nucleus is absent in prokaryotes, the DNA is organized into a structure called the nucleoid. The DNA of the archaea are more organized, they are packaged within structures similar to eukaryotic nucleosomes. The chromosomes in the prokaryotes and plasmids are generally supercoiled like that of the eukaryotes. The DNA is released into the relaxed state for the process of transcription, replication, and regulation.

Eukaryotic Chromosomes

In eukaryotes, the chromosomes are multiple large, linear and are present in the nucleus of the cell. Each chromosome typically has one centromere, one or two arms that project from the centromere, the arms are usually not visible during most of the time. Most of the eukaryotes have a small circular genome in the mitochondria. Some of the eukaryotes have small linear or circular chromosomes in the cytoplasm. To fit into the compartment in which it is contained the DNA has to be condensed and the degree to which it is condensed is expressed as its packaging ratio. Packaging ratio is the length of the DNA divided by the length into which it is packaged. The chromosomes of eukaryotic organisms consist of complexes made of DNA and protein, and it is organized in a condensed manner. This condensation permits a large amount of DNA to be stored in the nucleus of the cell.

Human Chromosomes

Humans chromosomes are two types of autosomes and sex chromosomes. Genetic traits that are linked to the sex of the person are passed on through the sex chromosomes. The rest of the genetic information is present in the autosomes. Humans have 23 pairs of chromosomes in their cells, of which 22 pairs are autosomes and one pair of sex chromosomes, making a total of 46 chromosomes in each cell. Many copies of the mitochondrial genome are present in human cells.

Sex Chromosomes

Sex chromosomes differ in form of size, behaviour from the ordinary chromosome. The sex chromosomes determine the sex of an individual during reproduction. These sex chromosomes differ between the male and the females. Females have two copies of the X chromosome, males have one X chromosome and one Y chromosome.  In the process of sexual reproduction in humans, two different gametes fuse to form a zygote. 

Homologous Chromosomes

Homologous chromosomes are also known as homologs or homologs. The homologous chromosomes are pairs of chromosomes that are approximate of the same length, position of the centromere, and pattern of staining, genes for the same characteristic are at corresponding loci. In an organism one of the homologous chromosomes is inherited from the mother and the other from the father. These chromosomes are usually not identical, but they carry the same type of genes. During the process of mitosis, the daughter chromosomes carry the same sequence of nucleotide, assuming there are no errors during the replication process.  The genome of diploid organisms is composed of homologous chromosomes One of the homologous pairs is the maternal chromosome and the other is the paternal chromosome. During the process of meiosis, the homologous chromosomes cross over. Homologous chromosomes are not identical but they are similar. The genes are carried in the same order, but the alleles for the trait may not be similar.

Function of Chromosomes

  • Genetic Code Storage: Chromosome contains the genetic material that is required by the organism to develop and grow. DNA molecules are made of the chain of units called genes. Genes are those sections of the DNA which code for specific proteins required by the cell for its proper functioning.
  • Sex Determination: Humans have 23 pairs of chromosomes out of which one pair is the sex chromosome. Females have two X chromosomes and males have one X and one Y chromosome. The sex of the child is determined by the chromosome passed down by the male. If X chromosome is passed out of XY chromosome, the child will be a female and if a Y chromosome is passed, a male child develops. 
  • Control of Cell Division: Chromosomes check successful division of cells during the process of mitosis. The chromosomes of the parent cells ensure that the correct information is passed on to the daughter cells required by the cell to grow and develop correctly. 
  • Formation of Proteins and Storage: Proteins are essential for the activity of a cell. The chromosomes direct the sequences of proteins formed in our body and also maintain the order of DNA. The proteins are also stored in the coiled structure of the chromosomes. These proteins bound to the DNA help in proper packaging of the DNA.
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