Genetics


Genetics 


Human knew from as early as 8000 _ 1000 B.C. than one of th causes of variation was hidden in sexual reproduction. They exploited the variations that were naturally present in the wild populations of plants and animals to selectively breed desirable characteristics. 
1) The meaning of genetic is a branch of biology that deals with the heredity and variation of organisms. 

Mendel is known as the father of genetics.

 He worked upon the pea plants and proposed the fundamental laws of inheritance. There are.

2) law of Dominance: 

States that when two different Genes controlling for a same character come together in an organism, only one is expressed and this expressed gene is known as dominant gene and the remaining one as recessive gene.


3) law of independent assortment: 

Separate gene for separate traits are passed independently of one another from parents to offspring. Genes do not influence each other about the sorting of alleles into gametes. 

4) law of segregation:

States that a diploid organism passes a randomly selected allele for a trait to its offspring such that the offspring receives one allele from each parent. 

(i) According to the law of segregation, only one of the two gene copies present in an organism is distributed to each gamete ( egg or sperm cell) that it makes and the allocation of the gene copies is random.


(ii) When an egg and a sperm join in fertilization, they form a new organism, whose genotype consists of the alleles contained in the gametes. 

Mendel published his work on inheritance of characters in 1865 but for several reasons, it remained unrecognized till 1900.

Chromosomal basis of Inheritance:



In 1900 three Scientific ( de.vries, correns and con Tschermak) independently rediscovered Mendel's results on the inheritance of characters. 

1) Chromosomes as well as genes occur in pairs.

2) The two alleles of a gene pair are located  on homologous sites on homologous Chromosomes. 

3) The pairing and separation of  a pair of Chromosomes would lead to the segregation of a pair of factors they carried. 

Molecules basis of inheritance 



DNA: 

DNA or deoxyribonucleic acid, is the central information storage system of most animals and plants, and even some viruses.

1) The name comes from its structure which has a sugar and phosphate backbone which have based sticking out from it _ so_ called based.

2) It is a polymer of four bases __ _  _ A, C, T and G.


3) DNA is organized structurally into Chromosomes and then wound around nucleosome as part of those chromosomes. 

4) The two chains have anti _ parallel polarity. It means, if one  chain has the polarity 5' to 3' then the other has 3' to 5'






Discovery of DNA

In 1953 James Watson and Francis crick, based on the X_ ray diffraction data produced by Maurice Wilkins and Rosalind Franklin,  proposed a very simple but famous  Double Helix model for the structure of DNA .One of the hall Mark's of their proposition was base pairing between the two stands of polynucleotide chains.


RNA 

Like DNA, each RNA strand has the same basis structure, composed of nitrogenous bases covalently bound to a sugar _ phosphate backbone.However, unlike DNA, RNA is usually a single _ stranded molecule. 

1) Also, the sugar in RNA is ribose instead of deoxyribose ( ribose contains one more hydroxyl group on the second carbon), which accounts for the molecules name.

2) RNA consists of four nitrogenous bases: adenine, cytosine, uracil , and gunanine.


3) Uracil is  a pyrimidine that is structurally like the thymine, another pyrimidine that is found in DNA like thymine, Uracil can base _ pair with adenine. 


Why is DNA widely acceptable as better Genetic material?

1) 2 _ OH group present at every nucleotide in RNA is a reactive group and makes RNA labial and easily degradable. 


2) DNA chemically is less reactive and structurally more stable when compared to RNA .Therefore among the two nucleic acids, the DNA is a better genetic material. 

Packaging of DNA 

1) Proteins called histone are responsible for the main level of DNA packing in chromatin. 

2) Histones are rich in the basic amino acid residues lysine and arginine. Both the amino acid residues carry positive charges in their side chains.

3) Histones are organized to form a unit of eight molecules called histone octamer.

4)  The negatively charged DNA is wrapped around the positively charged histone 

5) Nucleosome constitute the repeating unit of a structure in the nucleus called chromatin. 

6) Chromatin further folds and form the structure of the chromosome. 




Artificial Chromosomes:

1) Artificial Chromosome are artificially created Chromosomes having the properties of centromereas, telomeres and origins of replication, and specified sequences required for their stable maintenance within the cell as autonomous, self _ replicating chromosome. 

Types of Artificial Chromosome:

1) These include yeast artificial Chromosomes ( YACS) 


i) YACS hold the largest amount of DNA up to about 2000Kb.

ii) YACS have yeast centromeres and yeast telomeres for maintenance in yeast.

2) Bacter Artificial Chromosomes (BACS):


BACS can be circularised and grown in bacteria ; therefore, they have a bacterial origin of replication and antibiotic resistance genes .

3) PI bacteriophages artificial Chromosomes ( PACS) 

They are used to contain length of DNA from 150 in to 2000kb .

4 HAC ( Human Artificial Chromosome): 

HAC is a mini chromosome that is constructed artificially in human cells.
Using it's own self _ replicating and segregating system, a HAC can behave as a stable chromosome that is independent from the Chromosomes of host cells.

■ They are mitotically and cytogentically stable for up to six months.




5 MAC ( Mammalian Artificial Chromosome) 


Similar to YACs but instead of yeast sequences they contain mammalian or human ones.They are used to clone DNA fragments larger than 100kb and up to 3000kb.

Flow of genetic information: From DNA to proteins 

Central Dogma states that the genetic information flows from DNA to RNA to protein.

1) DNA Replication:

 DNA replication is the process by which DNA makes a copy of itself during  cell division. 

2) Transcription:

The process by which a cell makes an RNA copy of  a piece of DNA .The RNA copy, called messenger RNA ( mRNA), carries the genetic information needed to make proteins in a cell.

3) Translation

The process by which a cell makes proteins using the genetic information carried in messenger RNA ( mRNA).

■ The MRNA is made by copying DNA, and the information it carries tells the cell how to link amino acids together to form proteins. 



Genetic Code:

1) The process of translation requires transfer of genetic information from a polymer of nucleotides to Synthesis a polymer of amino acids.

2) This led to the proposition of a genetic code that could direct the sequence of amino acids during Synthesis of proteins
 
3) The genetic code is a set of rules defining how the four _ letter code of DNA is translated into the 20 _ letter code of amino acids, which are the building blocks of proteins. 

4) The genetic Codd is a set of three letter combinations of nucleotides called codons, each of which corresponds to a specific amino acid or stop signal.

■ For example,  The RNA Sequence UUU specifically coded for the amino acid phenylalanine. 

■ 3 codons in Human are stop codon : 

UGA, UAA, UAG,

5) There are 64 possible permutations, or combinations, of three_ letter nucleotides sequence that can be made from the four nucleotides. 


6) of these 64 codons, 61 represent amino acids, and three are stop signals.




DNA profiling or DNA Fingerprinting 


1) DNA profiling is the process where a specific DNA pattern, called a profile, is obtained from a person or sample of bodily tissue.

2) It is a forensic technique in criminal investigations, comparing criminal suspects profiles to DNA evidence to asses the likelihood of their involvement in the crime. 

3) It is also used in parentage testing to establish immigration eligibility, and in genealogical and medical research. 

Genetic Disorders:

● A genetic disorder is a disease that is caused by a change, or mutation,  in an industrial's DNA sequence. 

● These mutations can be due to an error in DNA replication or due to environmental factors,  such as Cigarette smoke & exposure to radiation,  which cause changes in the DNA sequences. 

The Three main categories are


■ Single Gene Disorders: 

Disorders caused by defects in one gene, often with simple and predictable inheritance patterns. Ex.Huntington's disease, cystic fibrosis. 









■ Chromosome Disorders:

Disorders resulting from changes in the number or structure of the chromosomes. Ex:Down's syndrome, which results from an extra Chromosome 21.

■ Multifactorial Disorders ( Complex Diseases): 

Disorders caused by changes in multiple Gene's, often in a complex interaction with environmental & lifestyle factors such as diet or cigarette smoke.Ex. cancer









































































Stomach               Pancreas

RNA                    Nucleic Acids




















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