CONTINUITY AND CHANGE

DEOXYRIBONUCLEIC ACID, (DNA). DNA is the genetic material of all cellular organisms. In eukaryotes it is found in the nucleus. However, there is some DNA in the mitochondria of animal and plants cells and the chloroplasts of plant cells. The Building Blocks. DNA is a macro-molecule, (large) which is made up of a series of chemical building blocks called nucleotides. Each nucleotide consists of 3 very different and separate components: a phosphate group (P), a five-carbon sugar, (S), (deoxyribose), and one of four nitrogen-containing bases: adenine (A), guanine (G), thymine (T) and cytosine (C).

Bases The 4 bases can be classified into 2 groups.                                                                                                                              cytosine (C) thymine (T) uracil (U) Bases The 4 bases can be classified into 2 groups. The purines, which have a double ring structure. These are adenine & guanine. Adenine Guanine The pyrimidines, which have a single ring. These are thymine and cytosine. Cytosine Thymine Uracil

DNA - Nucleotides Nucleotides join together to form a polynucleotide chain in which consecutive nucleotides are attached by phosphodiester bonds, (a type of covalent bond) between the number 3 carbon in the sugar of one nucleotide and the phosphate of the next. Nucleotides joined Nucleotide Structure

DNA – Nucleotides con’t DNA is composed of two chains of nucleotides linked together in a ladder-like arrangement with the sides composed of alternating deoxyribose sugar and phosphate groups and the rungs being the nitrogenous bases as indicated by the diagram below. The two strands are held together by hydrogen bonds (electrostatic attraction). Two hydrogen bonds hold adenine to thymine. Three bonds attach cytosine to guanine as indicated in the diagram above. Note: * The 2 ends of the strands are called the 5’ & 3’ ends. The 5’ has a free phosphate and the 3’ has a free hydroxyl group. * The DNA strand is referred to as antiparallel, meaning one strand runs 5’ to 3’ and the other 3’ to 5’. Hydrogen bonds

DNA Structure

Chromosomes Chromosomes are found in the nucleus. They consist of DNA and therefore, carry the genes. They change shape at various stages of the cell’s life. The DNA molecule is very long. In a human nucleus of 6μm, the DNA will be 1.8m long. In eukaryotes the DNA is coiled around proteins called histones. When the DNA is coiled it is called a nucleosome. Note: the coiling protects the DNA and gives it the appearance of a string of beads. When the cell is about to divide the chromosomes condense. The nucleosome folds in a regular manner producing supercoils.

See Fig 10.6 page 233 of Heinemann, Biology 2 GENE LOCUS, (pl. Loci) Genes control the way an organism develops, grows and functions. Genes are composed of long segments of DNA. They vary in size from about 100 base pairs to 2 500 000 base pairs. The DNA molecule is very long, containing approximately 1740 genes separated by spacer DNA. Each gene is made up of a specific sequence of base pairs and is found in a particular location in the DNA molecule. Each gene has particular position or LOCUS on the chromosome. See Fig 10.6 page 233 of Heinemann, Biology 2

Haploid (n) / Diploid (2n) All nucleated cells contain a fixed no. of chromosomes. The no. of chromosomes in a somatic cell (body cell) is characteristic of the species. The number of chromosome sets a species carries, is it’s ploidy level. Somatic cells are diploid (2n). They contain 2 sets of chromosomes, one from each parent. Gametes (sex cells – sperm + egg) are haploid (n), containing ½ the number of chromosomes found in the somatic cells. The sperm and the egg are haploid (n). When fertilisation occurs a diploid (2n) cell (zygote) is produced. Haploid egg Haploid sperm

DNA REPLICATION Refer to the following websites: http://207.207.4.198/pub/flash/24/menu.swf Refer to the second one. http://www.wiley.com/college/pratt/0471393878/student/animations/dna_replication/index.html

Gene Expression Gene Expression: When DNA codes for and results in a gene-product, usually a polypeptide. The overall process involves 2 major stages: 1. Transcription. 2. Translation. Transcription In nucleus Translation in Cytoplasm Information in messenger RNA Amino acid sequence of a polypeptide Information in DNA

Transcription Transcription = rewriting or copying of information from DNA to ribonucleic acid ( RNA). Occurs in the nucleus. RNA is a single stranded molecule containing the sugar ribose, phosphate and the bases Adenine, Guanine, Cytosine and Uracil. The synthesised RNA is complementary to one strand of DNA. In RNA Thymine is replaced by Uracil. The RNA molecule produced is called messenger RNA (mRNA), because it acts as a messenger, carrying the copy of the code from the nucleus into the cytoplasm.

Transcription con’t A Typical Gene. Messenger RNA synthesis is controlled by RNA polymerase. A typical gene consists of: A Promoter Region – it has a specific sequence recognised by RNA polymerase that initiates transcription. Exons – the coding segments of the gene. Introns – the non-coding segments of the gene. Promoter Exon 1 Intron 1 Exon 2 Intron 2 Exon 3 Intron 3 Exon 4

Transcription con’t Synthesis RNA polymerase moves along the template strand of DNA, adding nucleotides to the growing RNA strand. Note: RNA polymerase reads the template strand from the 5’ end to the 3’ end but builds the new mRNA molecule from it’s 3’ end to 5’ end. The RNA molecule produced is called the Primary Transcript. It is then modified by enzymes that cut out the introns and join the remaining pieces together, forming the shortened mRNA molecule. The RNA molecule is complete when the ends are chemically modified. It is chemically capped (a Methyl Cap) at the 5’ end and a tail of Adenines - A’s, (a poly - A tail) is added to the 3’ end. The mRNA the leaves the nucleus, entering the cytoplasm.

Translation Translation = Translation refers to the reading of the instructions contained in the mRNA and the assembly of the polypeptide product. mRNA travels into the cytoplasm, where a ribosome attaches to it. Once the ribosome has attached to the mRNA molecule, translation begins and the mRNA molecule is read in the 5’ to 3’ direction. The information contained in the mRNA for protein synthesis is read in a series of 3 base or triplet codes called CODONS. The sequence of codons code for the sequence amino acids that make up the polypeptide. Included in these 3 base codes are start and stop codons. The codon AUG codes for Methionine and is use to start translation. The codons UAA, UAG and UGA are 3 codons that signal stop, hence no more amino acids are added to the polypeptide.

Translation con’t There are 20 amino acids commonly found in organisms. Of the 20 there are only 2 that are specified for by only 1 codon, (Tryptophan – UGG and Methionine – AUG). The other 18 can be specified for by 2 or more codons, (eg. CCU, CCC, CCA & CCG all specify for Proline) Base sequence in GCT AAA TCC TAG DNA template strand CGA TTT AGG ATC Base sequence in mRNA GCU AAA UCC UAG Amino acid sequence in Stop Polypeptide translation Transcription (copying) Translation (change language) Ala Lys Ser

Translation con’t tRNA Molecule Transfer RNA, (tRNA), are short single-stranded molecules approx. 80 nucleotides long, which fold to form a cloverleaf shaped structure. There are many different types of tRNA. tRNA acts as transport molecules, bringing the correct amino acid to the ribosome for assembly into the polypeptide. To be able to do this there must be 2 binding sites on the tRNA molecule. 1) The binding site that binds to the codon, known as the anticodon where the nucleotides on the tRNA molecule are complementary to the mRNA, and 2) The amino acid attachment site where the amino acid binds to the tRNA molecule for transport to the ribosome. tRNA Molecule