Cytogenetics PDF

# Cytogenetics Asst Prof/ Maggie Amer # Genotype vs Phenotype ## GENOTYPE - The genotype is an organism's genetic information. | Genotype | Description | |---|---| | **BB** | homozygous dominant | | **Bb** | heterozygous | | **bb** | homozygous recessive | ## PHENOTYPE - The phenotype is the...

# Cytogenetics Asst Prof/ Maggie Amer # Genotype vs Phenotype ## GENOTYPE - The genotype is an organism's genetic information. | Genotype | Description | |---|---| | **BB** | homozygous dominant | | **Bb** | heterozygous | | **bb** | homozygous recessive | ## PHENOTYPE - The phenotype is the set of observable physical traits. | Genotype | Phenotype | |---|---| | **BB** | purple | | **Bb** | purple | | **bb** | white | # Phenotype: - the appearance or physical structure of an individual (red vs. pink vs. white) # Genotype: - the genetic composition of an individual # Homozygous: - a gene pair in which both the maternal and paternal genes are identical (AA or aa) # Heterozygous: - a gene pair in which the maternal and paternal genes are different (Aa or aA) # History: DNA Carries Genetic Specificity - **Oswald T. Avery:** demonstrated that DNA was the active genetic component. - **Rosalind Franklin:** took the first high-quality X-ray diffraction pictures of DNA. - **Francis Crick and James Watson:** confirmed that DNA exists in a complementary double helix. # DNA structure - DNA is composed of **two polynucleotide chains** twisted around each other to form a double helix. # The Nature of Nucleotides - The DNA nucleotide contains a 2'-deoxyribose sugar backbone (called such because of the lack of a hydroxyl group at the 2' position). - A phosphate group is attached to the sugar at the 5'-OH position. - A base is attached to the sugar at the 1'-OH position. # Nucleotide vs. Nucleoside - **Nucleoside:** Nitrogenous Base + Pentose - **Nucleotide:** Nitrogenous Base + Pentose + Phosphate # DNA chains have a free 5'-phosphate at one end and a free 3'-OH at the other end (5' to 3'): - **Ribose Sugar** - **Adenine** - **Thymine** - **Guanine** - **Thymine** - **Hydrogen Bonds** - **Phosphorus** - **Oxygen** # The DNA Bases - **Purines:** adenine and guanine - **Pyrimidines:** cytosine and thymine - **Note: KNOW THESE STRUCTURES!!!!!!** - The two polynucleotide chains are held together by hydrogen bonding between the bases of the nucleotides on complementary strands. - Adenine ALWAYS pairs with Thymine in DNA. - Guanine ALWAYS pairs with Cytosine in DNA. # RNA Structure - RNA like DNA, is comprised of a string of nucleotides joined by covalent sugar-phosphate backbone bonds. - RNA is similar to DNA except in three important ways: 1) Sugar: a ribose ring instead of a deoxyribose ring. (That is, it contains a hydroxyl group at the 2' position instead of a hydrogen.) 2) RNA contains uracil instead of thymine. 3) Single strand: RNA is usually found as a single polynucleotide chain and not as an extensive double helix. - **Note:** Unlike DNA, RNA is not the genetic material so doesn't need to replicate. It acts as the messenger between DNA and protein (mRNA), an adaptor to assist in protein synthesis (tRNA), a regulator to inhibit protein synthesis. # The sugar-phosphate backbone - DNA is a polymer made up of units called nucleotides. The nucleotides are made of three different components, a sugar group, a phosphate group, and a base. There are four different bases: adenine, thymine, guanine and cytosine. # What holds DNA strands together? - DNA strands are held together by hydrogen bonds between bases on adjacent strands. Adenine (A) always pairs with thymine (T), while guanine (G) always pairs with cytosine (C). Adenine pairs with uracil (U) in RNA. # From DNA to proteins - The bases on a single strand of DNA act as a code. The letters form three-letter codons, which code for amino acids (the building blocks of proteins). - **DNA** -> **Transcription** -> **RNA** -> **Translation** -> **Protein** - An enzyme, RNA polymerase, transcribes DNA into mRNA (messenger ribonucleic acid). It splits apart the two strands that form the double helix, then reads a strand and copies the sequence of nucleotides. The only difference between the RNA and the original DNA is that in the place of thymine (T), another base with a similar structure is used: uracil (U). - In multicellular organisms, the mRNA carries genetic code out of the cell nucleus, to the cytoplasm. Here, protein synthesis takes place. 'Translation' is the process of turning the mRNA's 'code' into proteins. Molecules called ribosomes carry out this process, building up proteins from the amino acids coded for. # Cell division (mitosis) - **Somatic cell only** - **Mitosis:** process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei. - **Cytokinesis:** divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. - **Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle.** The division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell. # Chromosomal abnormalities - **1) Numerical Disorders (Aneuploidy)** - **Monosomy:** an individual is missing either a chromosome from a pair. - **Ex:** Turner Syndrome, where the individual is born with only one sex chromosome, an X. - **Trisomy:** has more than two chromosomes of a pair. - **Ex:** Down Syndrome, also known as Trisomy 21 (an individual with three copies of chromosome 21, rather than two). - **2) Structural abnormalities** - **Deletions:** Ex: cri du chat (deletion of 5q). - **Duplications:** Ex: Klinefleters' syndrome (22 pair of autosomal chromosomes+ XXY). - **Translocations:** When a portion of one chromosome is transferred to another chromosome. - **Inversions:** the genetic material is inverted. - **Rings:** A portion of a chromosome has broken off and formed a circle or ring. # Karyotyping Types - **1) G-banding:** gives lightly and darkly stained bands - the dark regions tend to be heterochromatic. The light regions tend to be euchromatic. - **2) R-banding:** (reverse of G-banding) (the R stands for "reverse"). The dark regions are euchromatic and the bright regions are heterochromatic. - **3) C-banding:** Giemsa binds to constitutive heterochromatin. - **4) Q-banding:** is a fluorescent pattern obtained using fluorochrome for staining. (Similar to that seen in G-banding). - **5) Digital karyotyping:** quantify the DNA copy number on a genomic scale (for detection of a specific gene deletion or duplication). This method is also known as virtual karyotyping. # Classic karyotyping - **Giemsa (G-banding):** is used to stain bands on the chromosomes (giemsa is specific for the phosphate groups of DNA). - Each chromosome has a characteristic banding pattern that helps to identify them; both chromosomes in a pair will have the same banding pattern. - Karyotypes are arranged with the short arm of the chromosome on top, and the long arm on the bottom.