genetics

Dentistry college - first class
Medical biology- Lec. 8

Genetics is a branch of biology concerned with study of heredity and variation in vast array that form properties of organisms. The unite of heredity is a gene was located in chromosome, it is a segment of DNA that carries in its nucleotide sequence information for specific biochemical or physiologic properties and this information is passed from generation to generation by DNA replication, the gene becomes known in 20th century by Johansson 1909.
The nucleus contains genetic information in the form of chromosome, highly folded ribbon-like consisted of deoxyribonucleic acid (DNA) and a class of proteins called histones. Gnomic DNA is tightly and orderly packed in a process called DNA condensation to fit the small available volumes of cell. The German embryologist Walter Fleming was the first described the chromosome during follow-up the cell division in Salamander larvae in 1882. DNA usually occurs as linear chromosomes in eukaryotes and circular chromosomes in prokaryotic cells. The set of chromosome in a cell makes up its genome in human genome has approximately 3 billions base pairs of DNA arranged into 46 ( 23 pairs ). Eukaryotic organisms store most of their DNA inside of cell nucleus and some of their DNA in mitochondria or chloroplast, in contrast to prokaryotic cell stores their DNA only in cytoplasm.

























The number of chromosome in cell is constant for a particular species. The somatic cells contain two haploid set are known as diploid cells (2n) while reproductive cells such as sperm or ovum has one set of chromosomes and it is known as the haploid set (n), the diploid condition is arrived at by the union of the haploid male and female gametes in the sexual reproduction. The size of a chromosome can be measured during mitotic metaphase; it may range from 0.25 ?m to 30 ?m. The shape of chromosome changes from phase to phase, each chromosome has a clear zone, known as centromere or kinetocore. The centromere divides the chromosome into two parts, each part is called the chromosome arm according to the position of the centromere and nature of the chromosome arm, the chromosomes may be telocentric, acrocentric, submetacentric and metacentric.



















Fig. structure of chromosome

Chromosome: is a thread – like structure composed of chromatin that carries a large numbers of genes which determine all the individual characters.
Chromatin: is a DNA- protein (histone) complex found in eukaryotic chromosomes.

Histone: a group of water soluble proteins found in association with the DNA chromosomes, they believed to be involved in the condensation and coiling of chromosome during cell division .It is found in eukaryotic cell and deficient in bacteria.

Centromere : it is a constriction area that linked two chromatid in the chromosome this part of chromosome free of genes.

Kintichore : two parallel brush- like filaments situated in the centromere of a chromosome and attach chromatids to the spindle fibers during cell division.




The chemical composition of DNA and RNA
Cells consist of four basic types of molecules involved in life, proteins, polysaccharide, lipid, nucleic acids (DNA and RNA) are known collectively as biological macromolecules. Both DNA and RNA are called nucleic acid because they can be isolated from nuclei and they are acidic tendency; they are polymeric molecules made up of several monomeric units called nucleotides, it is a basic building block of DNA and RNA molecules consist of pentose (5- carbon) sugar, nitrogenous (N2 – containing) base and a phosphate group (the sugar and base only is called a nucleoside). Below the characters of DNA and RNA strands:

RNA strand DNA strand
RNA is the genetic material of some viruses DNA is the usual genetic material
RNA is single stranded. DNA is usually double-stranded
The pentose sugar is ribose The pentose sugar is deoxyribose
The common nitrogenous bases are adenine, guanine, cytosine and uracil The common nitrogenous bases are adenine, guanine, cytosine and thymine
There are three types of RNA messenger ribosomal and transfer DNA is only of one type
Messenger RNA, rRNA and tRNA found in the nucleolus and cytoplasm. Most of the DNA is found in the nucleus and in the cytoplasm (mitochondria and chloroplasts)
Usually RNA does not repli¬cate or transcribe. DNA on replication forms DNA, and on transcription forms RNA.
The usual function of RNA is translating messages en¬coded in DNA into proteins. Genetic messages are usually encoded in DNA




















Fig. Chemical structure of DNA strand Fig. Chemical structure of RNA strand


Watson – Crick?s double helical model of DNA
In 1953, Watson, an American biologist and Crick, an English biologist, proposed the double helix structure for DNA. This development set the stage for a new and continuing era of chemical and biological investigation.
Watson and Crick?s helical model of DNA has main features as fellow:
1- The DNA molecule consist of two polynucleotide chains wound around each other in a right – handed double helix ( clockwise fashion) .
2- The external diameter of the helix is 2 nm.
3- The two chains are antiparallel that is , the two strands are oriented in opposite directions , with one strand oriented in the 5? to 3? way , while the other strand oriented 3? to 5? , antiparallel means that the head of one chain is against the tail of the other chain .
4-The sugar – phosphate backbones are on the outsides of the double helix, while the bases oriented toward the central axis.
5-The two sugar – phosphate backbones of the double helix are not equally spaced along the helical axis , this result in grooves of unequal size between the backbones called the major ( wider ) groove and the minor ( narrower ) groove, both of these grooves are large enough to allow protein molecules to make contact with bases .

















6-The bases of opposite strands are bonded together by hydrogen bonds, which are relatively weak chemical bonds. The specific pairings observed are A with T (two hydrogen bonds) and G with C (three hydrogen bonds). The hydrogen bonds make it relatively easy to separate the two strands of the DNA. The specific A – T and G – C pairs are called complementary base pairs , so the nucleotide sequence in one stand may compatible with nucleotide sequence of the other , for instance , if one chain has the sequence 5?- TATTCCGA- 3? then the opposite antiparallel chain must bear the 3?- ATAAGGCT - 5? .


















Fig. sequences of bases in DNA strand

Central Dogma of molecular biology
Replication of DNA and transcription of genetic information into mRNA then translated into protein this process is called central dogma of molecular biology.






















Replication involves the separation of DNA strands to synthesis of a complementary copy of new DNA on each of the two parent strands. Replication begins when the hydrogen bonds that hold the purine – pyrimidine bases together break apart, the separation starts at one end and proceeds along DNA molecule each half acts as a template for reconstructing the missing half. There is information that the DNA polymerase catalyzes DNA replication and also share in the repair of DNA in vivo. This process requires the presence of all four deoxyribonucleoside triphosphates, d ATP, d GTP, d TTP, d CTP and Mg2.




















Fig. transfer RNA (tRNA)

Fig. DNA replication process

Transmission of genetic information in genes is achieved via complementary base pairing in a process called transcription through attraction between the DNA and the messenger RNA nucleotides; mRNA is then exported from the nucleus to the cytoplasm, where it is bound to ribosome. The order of amino acids is determined by 3 adjacent nucleotides (triplets) on strand mRNA is called codon and translated into its corresponding protein form with the help of transfer RNA (tRNA) is a small RNA chain of about 80 nucleotides carry 3 adjacent nucleotides (triplets) called anticodon that transfers a specific amino acid to a growing polypeptide chain at ribosomal site of protein synthesis during translation. There are 64 codons and only 20 amino acids the code is redundant.

Mendel s laws in genetic
Genetics started at 19th century without any knowledge about DNA , a monk Gregor Mendel breed peas in his garden and noticed that the inheritance sometimes follows clear rules and put the principles that govern heredity in the 1860, one of these principles now called Mendel s first law of segregation states that “ allele pairs separate or segregate during gamete formation, and randomly unite at fertilization”.
There are four main concepts related to this principle. They are as follows:
1-A gene can exist in more than one form.
2-Organisms inherit two alleles for each trait.
3-When gametes are produced by meiosis, allele pairs separate leaving each cell with a single allele for each trait.
4-When the two alleles of a pair are different, one is dominant and the other is recessive.
That is mean a pair of characters only one can be represented in a gamete. for instance if we cross roses which have purple (PP) and white colors (pp), the next generation all colors will be purple, two generations later one quarter of the roses colors will be white (pp), while three quarters will be purple (PP,Pp) (fig.).





















Fig. Example on first Mendel’s law

Mendel’s second law of independent assortment
The law of independent assortment says that “alleles of different genes are inherited independently of one another”. Gregor Mendel demonstrated this with his famous experiments on peas when he a cross between 2 pea varieties shows 2 genes (seed color and shape) each having 2 possible alleles (yellow/green, round/wrinkly), where yellow and round are the dominant phenotypes (AABB).While the green and wrinkly seeds are recessive phenotype (aabb). The F1 is completely heterozygous, showing the yellow, round phenotype (AaBb).
A= yellow seed color (dominant character)
a = green seed color (recessive character)
B= round shape (dominant character)
b = wrinkly shape (recessive character)





















Fig. Example on second Mendel’s law

Two generations F2 later, show the distribution the characters against the fallowing ratio:

9/16 yellow round seeds
3/16 yellow wrinkly seeds
3/16 green round seeds
1/16 green wrinkly seeds