Genes

Characters of DNA

1. G ENES AND ITS CHARACTERISTICS

2. GENES AND DNA The genes are segment of DNA(deoxyribonucleic acid). DNA has its own nitrogenous base, which are : A, C, G, T. These letters in different combinations code for different functions. Each letter represents a different base of a molecule called nucleotide. Each gene is a specific sequence of nucleotide. The A stands for adenine, the C stands for Cystosine, the G stands for Guanine and T stands for thymine. Adenine and guanine are double ring base called purines. Cystonine and thymine are single ring base called pyrimidines.

3. W ATSON AND CRICK DNA DOUBLE HELIX Watson and crick (1953) proposed a DNA exists in the form of double helix i n which the pol y nucleoti d e c h ains a r e t w o c oiled spiral abut one a noth e r like a ladder. Two strands of alternating phosphate groups and deoxyribose sugar form the upright the ladder and have opposite chemical polarity. The b a se p a irs alw a y s m atch up according to these rule: Adenine is always form hydrogen bond with thymine. Cyst o sine i s alw a ys f o r m hydrogen bond with guanine.

4. Diameter o f DN A i s 2 A , a p i tch of distance spiral i s 34 A , between adja c ent nucleotide pairs is 3.4A0 and there are 10 base pairs per turn.

5. S TRUCTURE OF A TYPICAL HUMAN GENE Most human genes are split genes, in which message of gene is interrupted by some non coding intervening sequence called introns. The coding sequence are called exons. At start of each gene is a promoter region where RNA polymerase enzyme bind during the process of transcription. At the end of gene is terminator region where transcription stops and RNA polymerase dissociates from DNA.

6. C ON T …. Genes are chemically composed of DNA and are situated on the chromosomes. They are determination of inherited characters. There are two types of genes: Structural genes: synthesis the specific protein. Control gene s : cont r ol g enes a re responsible for regulation of activity of structural genes.

7. D OMINANT AND RECESSIVE GENE The genes are present on 23 pairs of chromosomes. Each pair of chromosomes comprises of 2 allele which carry genetic material specific to that gene. Each gene in a pair has the same overall function but may have different version. Different versions of a gene are called alleles. Dominant allele : it produces a dominant phenotype in individuals who have one copy of the allele, which can come from just one parent. Recessive allele: it produces a recessive phenotype, the individual must have two copies, one from each parent.

8. C ON T …. For eg. Take stature of the man. There are two allele for stature. One allele carries genes for tallness, while the other allele carries for shortness. An allele, which get expressed even if one copy is partnered is called dominant which is partnered with allele for shortness called recessive genes. Since the single gene for tallness has expressed, therefore , he/she will be tall. A person also would have tall stature if both the alleles were identical and are coded for tallness. For this reason , a person to have short stature, both the alleles should be coded for short stature.

9. H OMOZYGOUS AND HETEROZYGOUS STATES Homozygous state: if a person has two identical allele(TT) for a particular trait(i.e tallness), that person is called homozygous for that trait. His genotype is TT. Heterozygous state: when a person has two non identical alleles for a trait for example: one is for tall stature (T) and other is for short stature (t), then that person is heterozygous for that trait. His genotype is Tt.

10. C ON T …. A person can be homozyg o us domina n t or homozygous recessive. Homozygous person having two dominant alleles (for eg. TT for tallness) is called homozygous dominant. When a person has two recessive allele (tt for short stature) then it is called homozygous recessive state for that trait.

11. C HARACTERISTIC OF GENES Genes are the functional unit of heredity. The genetic information of each gene of the organism is tranismitted faithfully from one cell to another and parents to progeny . This funtion of genes is called genotype function or replication. Genes are situated in chromosomes. Every gene occupies a fixed position in a chromosome. A unit of genetic material can undergo mutations and evolution. Genes control the functions of DNA and RNA. Genes consist of a particular set of instructions or specific functions. For example, the globin gene was instructed to produce haemoglobin. Haemoglobin is a protein that helps to carry oxygen in the blood.

12. CON T … Every person has two copies of each genes, one is heritened from each parent. Genes consists of long strand of DNA that contain promoter, which control the activity of a gene and coding and non coding sequence. The genes control the development of the phenotype of the organisms. Total set of genes in a person is called genome. Gene is basically an instruction for human body. Each gene has specific function and every single function of the human body is coded in one or more genes.

13. C ON T …. Dominant allele : it produces a dominant phenotype in individuals who have one copy of the allele, which can come from just one parent. Recessive allele: it produces a recessive phenotype, the individual must have two copies, one from each parent. Homozygous state: if a person has two identical allele(TT) for a particular trait(i.e tallness), that person is called homozygous for that trait. His genotype is TT. Heterozygous state: when a person has two non identical alleles for a trait for example: one is for tall stature (T) and other is for short stature (t), then that person is heterozygous for that trait. His genotype is Tt.