Genetics

Learning objectives To describe the transmission of genetic information. To define mitosis and meiosis. To differentiate mitosis and meiosis. To define dominant and recessive genes. To define alleles. To define genotype and phenotype. To differentiate between sex chromosomes. To define sex-linked disease.

Genetics Gene = Hereditary unit, a sequence of DNA Genome = Entire set of genes Gamete = Mature sex cell Chromatin  not dividing cell Chromosome  dividing cell

A chromosome

DNA Deoxyribonucleic acid The material for genetic information Confined to the nucleus Structure= sugar + phosphate group +organic nitrogenous base ( A, G, C, T )

Genetic Code The base sequence in DNA It determines the amino acid sequences in protein 3 consecutive bases (triple code) in the DNA  Codon

Genetic codes

Protein synthesis The sequence of bases in DNA  genetic codes ~ genotypes Genotypes determine the phenotype through production of enzyme. One- gene- one enzyme hypothesis

RNA Single-stranded molecule differ from DNA ( ribose instead of deoxyribose ; U instead of T) mRNA, tRNA, rRNA

Protein synthesis I. Synthesis of amino acids ~ in mitochondria & chloroplast ~ non-essential & essential amino acids 2. Transcription ~ DNA strand  base sequence of mRNA ~ mRNA directs the protein synthesis 3. Translation ~ mRNA  sequence of amino acid

Transcription

Chromosome Composed of DNA, protein & RNA Invisible in non- dividing cells  chromatin shorten & intensifies in dividing cells 23 pairs in human Member of ear pair of chromosomes = homologue Homologous pair = one from father, one from mother Diploid (2N) vs Haploid (1N)

Chromosomes in cell division DNA replicate  2 identical chains of DNA  The DNA are surrounded by protein coat, 2 identical strands lyes side by side  the strands are attached by centromere

Cell cycle The sequence of events occurring between the formation of cell and its division into daughter cell.  Interphase ~ period of synthesis & growth, replication of DNA  Nuclear division ~ separation of chromatids  Cytoplasm division ~ division of cytoplasm

Mitosis The division of nucleus into daughter nuclei containing identical sets of chromosomes to the parent cell.   cell numbers  Growth, replacement, repairs cells, asexual reproduction  Interphase + Prophase + Metaphase+ Anaphase + Telophase

Prophase

Early metaphase

Metaphase

Anaphase

Telophase

Cytokinesis

The division of cytoplasm 1. Cell organelles become evenly distributed 2. Invagination of cell membrane 3. Continuous furrow around the equator 4. Complete separation into 2 cells

Significance of mitosis I Genetic stability ~ same number of chromosomes of parent & daughter cells ~ same hereditary information in parent & daughter cells ~ no variation in genetic information Growth ~  number of cells

Significance of mitosis II Cell replacement ~ mitosis produces new cells Asexual reproduction and regeneration ~ regeneration of missing parts e.g. tail in lizard

Meiotic cell division Also call reduction division. Takes place in the reproductive tissue. Single duplication of chromosomes + 2 cycles of nuclear division & cytoplasmic division A single diploid cell gives rise to 4 haploid cells.

The necessity for chromosome reduction In normal somatic cells, chromosomes are homologous & diploid(2N). Gamete contains only one member of each homologous pair and is haploid(1N) due to meiosis. In sexual reproduction, the zygote after fertilization (sperm[1N] + ovum[1N]) is 2N. So, meiosis will prevent the nuclear materials from doubling in amount in each new generation.

Process of meiosis First meiotic division: ~ interphase ~ prophase I ~ metaphase I ~ anaphase I ~ telophase Secind meiotic division ~ interphase II ~ prophase II ~ metaphase II ~ anaphase II ~ telophase II

Crossing over

Significance of meiosis Conserve the number of chromosomes in the cells of successive generation. Random orientation of chromosomes Crossing over of genetic material

Comparison between meiosis & mitosis Continuous, regular process One duplication of genetic material Involves separation of chromosomes & other cell organelles. Similar mechanism of cell division Involves increase in cell number.

Comparison between meiosis & mitosis A single division of chromosomes & nucleus. The number of chromomses remains the same(2N). No crossing over. Daughter cells are identical to parent cells 2 daughter cells are formed. A single division of chromosomes but a double division of nucleus. The number of chromosomes is halved, 2N to 1N. Crossing over present. Daughter cells are genetically different from parental ones. 4 daughter cells are formed.

Cytokinesis Normally follows telophase and leads into interphase. The cell membrane invaginate and eventually join up  complete separation of the two cells

Inheritance Terms to know Haploid Diploid Allele Homozygous Heterozygous Genotype Phenotype Dominant Recessive

Father of Genetics: Gregor Mendel

Back cross TT (homozygous dominant) x TT (homozygous dominant) TT (all are dominant)

Back cross Tt (heterozygous dominant) x Tt (heterozygous dominant) Dominant (TT, Tt) & Recessive (tt) in ration 3:1

Test cross TT (homozygous dominant) x tt (homozygous recessive) Tt (all are dominant)

Test cross Tt (heterozygous dominant) x tt (homozygous recessive) Tt or tt (ratio is 1:1)

Sex chromosomes Each body cell: 22 pairs autosomes + 1 pair sex chromosome Sex chromosomes: X & Y Except sex chromosomes, all homologous pairs of autosomes are identical X- chromosome carries many genes while Y chromosome carries fewer genes.

Sex chromosomes Female: ~ genotype: XX (homogametic sex) Male: ~ genotype: XY (heterogametic sex)

Sex-linkage Characters controlled by the genes which situated on the sex chromosomes, especially X, are called sex-linked characters. For example ~ red-green colour blindness ~ Haemophilia

Genetic diagram: color blindness

Carrier Genetic diagram: Hemophilia

Multiple alleles Genes exists in more than 2 allelis forms in the same locus of given pair of homologous chromosomes. Each allele produces a distinctive phenotype. For example: ANO blood group system ~ the human blood groups are controlled by three alleles I A, I B, I

Crossing over During pairing up, the homologous chromosomes break and re-join with non-sister chromatid of its homologous member  exchange genetic segments.

Mutation Sudden & stable inherited change of the genetic material(DNA). Leads to differences among individuals Provides raw materials for the species of the organisms. Enhances natural selection. Takes place at any stage in the development of all organisms.