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Genes - Module 1 Reading / Watching

సైటు: Justwrite
కోర్సు: An introduction to Gene Regulation and Gene Expression
పుస్తకం: Genes - Module 1 Reading / Watching
ముద్రించినది: Guest user
తేదీ: బుధవారం, 4 జూన్ 2025, 10:41 AM

వివరణ


విషయసూచిక

1. Genes

In this book you will see an overview of the topics of Module one and reference links.

By the end of this book, you will be able to:

  • Discuss what are genes?
  • Explain the organization of genes in Prokaryotes
  • Discuss the organization of genes in Eukaryotes
  • Compare prokaryotic and eukaryotic gene Organization
  • Explain what are chromosomes, their types and function

2. What are Genes?

Genes are the basic unit of inheritance containing the information for physical and biological traits. They contain the information necessary for living cells to survive and reproduce . Genes are regions of the DNA .They are passed from parents to offspring. They code for specific proteins or segments of proteins. Humans have approximately 20,000 protein coding genes . These  20,000 protein coding genes are encoded only 1.5% of the entire human genome. Not all genes code only for proteins , some genes encode information for making an RNA molecule that functions other than directly coding for a protein.

Genes in genome of eukaryotes consist of exons and introns.  are the protein coding regions of mRNA. These are the regions that contain information for making a protein whereas other regions of the RNA are non-coding and these regions are called 

The majority of human genes have two or more possible , which are alternative forms of a gene. Differences in alleles account for the considerable genetic variation among people. In fact, most human genetic variation is the result of differences in individual DNA bases within alleles.


3. Prokaryotic Genes and their Organization

Prokaryotes do not have a well defined nucleus. The genome is present in a region of the cytoplasm called nucleoid .The genome of prokaryotes is composed of a single , double stranded DNA in the form of a circle or loop. Apart from the genome DNA some prokaryotes also have smaller loops of DNA called plasmids that are not essential for normal growth. The DNA of prokaryotes are compressed through supercoiling .

Genomes can be twisted in the same direction as the double helix or positively super coiled . Alternatively genome can be twisted in the opposite direction of the double helix or       negatively supercoiled . Most bacterial genomes are negatively supercoiled . This is achieved with the help of a protein called HU,  which is the most abundant protein in the nucleoid. The protein Hu along with an enzyme called topoisomerase I to bind DNA and introduce sharp bends in the chromosome. This  generates  the tension necessary for negative supercoiling.

Another protein called  Integration Host Factor (IHF), can bind to specific sequences within the genome and introduce additional bends. The folded DNA is then organized into a variety of conformations that are supercoiled and wound around tetramers of the HU protein. DNA topoisomerase I, DNA gyrase, and other proteins help maintain the supercoils.


4. Chromosomes

The chromosomes are thread like structures present in the nucleus of an eukaryotic cells . They are DNA molecule compacted with proteins .Chromosomes vary widely between different organisms. Each species has a characteristic number of chromosomes. Eukaryotic cells (cells with nuclei) have large linear chromosomes and prokaryotic cells (cells without defined nuclei) have smaller circular chromosomes. There are many exceptions to this rule. Mitochondria in most eukaryotes and chloroplasts in plants have their own small chromosomes.

Humans have 23 pairs of chromosomes , of which 22 pairs are  or body chromosomes  . Autosomes  look the same in both males and females . Apart from autosomes there is a  pair of  or sex chromosomes which differ between males and females.

Each pair contains two chromosomes, one acquired from each parent, which means that children inherit half of their chromosomes from their mother and half from their father. Chromosomes can be seen through a microscope when the nucleus dissolves during cell division.

The matched pairs of chromosomes in a diploid organism are called homologous chromosomes. are of  the same length and have genes in exactly the same location, or locus.A somatic cell contains two matched sets of chromosomes, a configuration known as diploid ( 2n)  where ‘ n’ represent a single set of chromosomes;

Gametes, or sex cells  (eggs and sperm )  contain only one set of 23 chromosomes ( n)  and are designated  as haploid.

The variation of individuals within a species is caused by the specific combination of the genes inherited from both parents. Genes are expressed as characteristics of the organism . Each characteristic may have different variants called traits . Traits are caused by differences in the DNA sequence for a gene. An organism’s traits are determined largely by the genes     inherited from each parent and also by the environment that they experience.


5. Chromosomes – Structure ,Types and Functions

Structure of Chromosomes 

Each chromosome has a constriction point called the centromere. The centromere divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the  The long arm of the chromosome is labeled the  The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.

Types of Chromosomes 

Apart from the two types of  chromosomes namely Autosomes and Allosomes , Chromosomes may be classified further :

I. On the basis of location of centromere

  • Telocentric– Telocentric chromosomes have the centromere at the end of the chromosome . Here centromere occupy the terminal position. Thus, the chromosome in this case  has just one arm. Such rod shaped chromosomes are found in species such as mice. 
  • Acrocentric- In this type ,centromere occupying a sub-terminal position  making one arm very long and the other very short. Human chromosomes 13, 15, 21, and 22 are acrocentric. 
  • Sub-metacentric– This type of chromosomes are with centromere slightly away from the mid-point. The two arms are unequal. Human chromosomes 4 through 12 are sub-metacentric. 
  • Metacentric- Metacentric chromosomes are V-shaped . The  centromere lies in the middle of chro­mosome or in the center.  The two arms are almost equal. Human chromosome 1 and 3 are metacentric.

II. On the basis of number of centromere

  • Monocentric- Chromosomes are with one centromere. 
  • Dicentric-Chromosomes  with two centromeres. 
  • Polycentric- Chromosomes with more than two centromeres. 
  • Acentric- Such chromosomes are without centromere. These are freshly broken segments of chromosomes. 
  • Diffused or non-located- These are chromosomes with indistinct centromere diffused throughout the length of chromosome.
Functions of Chromosomes 
  • Chromosomes are the essential unit for cellular division
  • Chromosomes must be replicated, divided, and passed successfully to their daughter cells so as to ensure the genetic diversity and survival of their progeny.
  • Chromosomes protect the DNA from damage
  • The dynamics of  chromosome structural changes regulate gene expression.
  • Chromosomal recombination plays a vital role in genetic diversity.


6. Eukaryotic gene organization – Packaging of DNA to Chromosome

Gene Organization in Eukaryotes 

The large eukaryotic genome or DNA approximately 2 m in length is packed  in to a nucleus of roughly 10 µm diameter through an orderly packaging.

Nucleosomes

Nucleosomes are the basic unit of DNA packaging in eukaryotes and represent the first level of chromatin organization. A nucleosome consist of a segment of DNA wound around a   protein core called –histone octamer. This structure is often compared to thread wrapped around a spool.

The nucleosome core particle consists of approximately 146- 147 base pairs of DNA  wrapped  around a histone octamer , consisting of 2 copies each of the core histones namely  H2A, H2B, H3, and H4.The octamer structure is a four-helix bundle comprised of two H3-H4 and two H2A-H2B dimers.

Nucleosome cores are separated by linker DNA of variable length ( 20 to 80 base pairs of linker DNA ) and are associated with the linker histone H1. Nucleosomes connected by a DNA linker of variable  forms a 10-nm beads-on-a-string array. 

Nucleosomes  pack DNA in a sequence-independent fashion. The core histones make contact with the DNA primarily through three to six hydrogen bonds between the protein main chain amides and the DNA phosphate backbone.

 Nucleosome particles can be modified in their composition, structure and location by chromatin remodeling complexes and regulates  gene expression. Nucleosomes are folded through a series of successively higher order structures to eventually form a chromosome. A nucleosome plus histone H1 is referred to as 

The 30 nm Fibre 

The chromatosome fold  up to a 30 nm fibre .Nucleosome core particles can stack on top of each other to form extended structures. These stacks can adopt a gentle helical configuration.  The 30 nm fibre  consists of a helical array of nucleosomes, each comprising a core particle wrapping ∼146 or 147 base pairs (bp) of DNA associated with a linker histone. Secondary structures to chromatin are demonstrated by the solenoid model and the zigzag model. The solenoid model consists of tightly wound nucleosomes containing 6 nucleosomes per turn in a regular, spiral configuration . The zigzag model is a bit looser form of chromatin with irregular configuration. In this model, nucleosomes have little face-to-face contact.

Nuclear Scaffold 

Higher order packaging consisting of loops of the 30-nm fibers attached to a proteinaceous nuclear scaffold by an interaction between the scaffold and specific DNA sequences called ) . These are DNA elements that serve to compartmentalize the chromatin into structural and functional domains.

Loops are formed averaging about 300nm in length.  The 300nm fibre are compressed and folded  producing a 250 nm wide fibre .

Upon tight coiling of the 250nm wide fiber the chromatid of chromosome is formed.