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Integrated Programme Sec 2 SBGE, LSS Biology Module Topic: Simple Genetics Genetic Materials A. Heredity B. Chromosomes-Genes-DNA C. RNA D. Difference Between RNA and DNA E..Protein Synthesis 1 A. Heredity Some of our traits are largely determined by heredity Physical traits o e.g. eye colour, ability to roll tongue, shape of earlobes mental traits e.g. intelligence and personality Traits are information determined by DNA o Deoxyribonucleic acid make up our chromosomes Chromosomes normally occur in pairs in animal and plant cells o Except in sex cells o All animals and plants of the same species have the same number of chromosomes A human cell contains 23 pairs (2n=46) of chromosomes o Except in the sex cells (n=23) B. Chromosomes-Genes-DNA Chromosomes Chromatin is a nuclear material that contains the genetic code. Heterochromatin : condensed form of chromatin Euchromatin : Thread-like form Just before cell division, chromatin condenses further into individual chromosomes Dividing chromosomes appear as two chromatids 2 Each chromosome contains 1 long DNA strand and histones (proteins) Genes A gene is the basic unit of heredity Genes occur in pairs and are located on chromosomes Each gene in a pair is inherited from a different parent Dominant trait is one that prevents another trait from expressing itself Recessive trait is expressed only if the dominant gene is not present Phenotype is the observable traits of an individual o the way in which a trait expresses itself physically o a particular set of genetic instructions inherited form one’s parents Two gene copies are called alellomorphs or alleles Homologous chromosomes o a pair of matching chromosomes Alleles occupy the same locus (location) along the length of homologous chromosomes] 3 Genotype describes the alleles each cell has for a certain feature o E.g. PP for purple flowers to represent the feature of flower colour o Usually capitalised letters are used to represent the dominant trait o Lower case letters are used to represent the recessive trait o PP then represents alleles carrying two copies of dominant gene for colour of flower Phenotype expressed as dominant trait Purple flowers o carrying two copies of recessive gene Phenotype expressed as recessive trait Purple flowers o Pp as shown in picture represents alleles carrying 1 copy of dominant gene and 1 copy of recessive gene Phenotype expressed as dominant trait Purple flowers Deoxyribonucleic Acid (DNA) The total length of DNA in human genome is 1.8 metres. The nucleus is only 6 micrometers in diameter. How is the DNA packed in the nucleus? 4 Structure of DNA DNA forms a double helix DNA is a polymer (polynucleotide) Monomer - nucleotides o Nucleotides are found in the centre and linked together by phosphodiester bond o Each nucleotide consists of 5-carbon sugar (deoxyribose) nitrogen containing base a phosphate group o The sugar-phosphate units forms the backbone. Phosphate group Adapt from:users.rcn.com/.../ N/Nucleotides.html Nitrogen bases 4 different types of nitrogen bases found in DNA adenine (A) guanine (G) cytosine (C) thymine (T) Adenine and guanine belong to a group of chemicals called purines cytosine and thymine are pyrimidines. Each purine always bond with a pyrimidine. A always pair with a T C always pair with a G. The base pairs are held together by hydrogen bond 2 H-bonds between A and T 3 H-bonds between C and G Function of DNA DNA carries the information needed to construct and operate an organism o information is stored in genes Heredity is based on the transfer of genes between generations How is the structure of DNA related to its function? Contains a sequence of bases that codes for a sequence of amino acids Cells use this genetic code to make proteins that build organisms 5 Base sequence is read in blocks of three o Known as a codon o E.g. GAU DNA strand is long so can store a lot of information. Complementary base pairing on DNA allows for replication and transcription Stable sugar-phosphate backbone to give strength Weak hydrogen bonds between the bases allows the 2 strands of to separate allowing for replication and transcription. Differences Between Chromosome, Gene and DNA Chromosome Involve a series of DNA strands coiled up into single units Occurs in pairs, have a finite set in each cell Large Gene Specific sites on the chromosome that code for a function Usually seen as a set of 3 bases, numerous Small DNA Refer to the complex molecule that makes up the chromosome Occurs as a double helix Microscopic C.Ribonucleic Acid (RNA) Found in nucleus and cytoplasm Found in the mitochondria and chloroplasts too Involved in protein synthesis Amount of RNA is proportional to the rate of protein synthesis in the cell Polymer of nucleotides Single stranded Each nucleotide consist of o 5 carbon ribose sugar o Phosphate group o Nitrogen containing base Adenine A, guanine G, cytosine C and uracil U 6 3 types of RNA o Messenger RNA (mRNA) o Transfer RNA (tRNA) o Ribosomal RNA (rRNA) Messenger RNA (mRNA) single strand of nucleotides mRNA are assembled in the nucleus using DNA as template carries genetic message from DNA to ribosome and provides a template for protein synthesis carries codons to bind to tRNA Transfer RNA (tRNA) Found in the cytoplasm Carries amino acids to ribosomes for assembly into new amino acid chain Carries anticodon on one end and amino acid binding site on the other end o tRNA binds to mRNA by means of anticodon each tRNA specific for an amino acids o 20 amino acids naturally synthesised by the body 7 Ribosomal RNA (rRNA) Makes up part of the ribosome Brings together all the chemicals and enzymes associated with protein synthesis Site of protein synthesis rRNA D. Differences Between DNA and RNA Sugar Bases Strands Size of molecule Life span Site of action DNA Deoxyribose C, G, A, T Two Enormous Long term Nucleus mRNA Ribose C, G, A, U One Small compared to size of DNA, varies with size of gene Short term Nucleus and cytoplasm tRNA Ribose C, G, A, U One Small, constant size Short term Cytoplasm 8 E. Protein Synthesis 2 stages o Transcription o Translation Transcription Genetic code in DNA is transferred, or transcribed to mRNA o mRNA then passes the genetic code to the ribosomes o which act as a template for protein synthesis. DNA helix unwind RNA polymerase attaches to DNA o mRNA molecules assembled by adding of nucleotides mRNA then detaches from DNA o complete mRNA molecule leaves the nucleus. 9 Translation Base sequence on the mRNA is translated into a polypeptide mRNA strand passes out of nucleus and attaches to ribosome tRNA binds to mRNA at the ribosome o anticodon of tRNA that is complementary to codon of mRNA tRNA delivers amino acids one by one o polypeptide chain grows Process continues until an mRNA stop codon is read A completed polypeptide is released Process of transcription Enzymes separate the two strands of DNA molecule and RNA polymerase catalyses the assembly of mRNA, adding one nucleotide at a time until the gene has been transcribed. Sites of transcription and translation in the cell. Transcription occurs in the nucleus and involves assembly of mRNA molecule. Translation occurs on the ribosomes and involves assembly of a protein. 10 11