DNA RNA Gene Expression

 DNA deoxyribonucleic acid ◦ Deoxribose sugar ◦ 2 strands held together with H bonds ◦ ATGC are the nitrogen bases ◦ Double helix ◦ Found only inside the nucleus  RNA ribonucleic acid ◦ Ribose sugar and AUGC nitrogen bases ◦ 1 strand ◦ 4 types, mRNA, tRNA, rRNA, and snRNA

 DNA replication ◦ Replication is semi-conservative ◦ Occurs during interphase when each chromosome makes an exact copy (sister chromatid)

◦ Involves several enzymes  Helicase- unwinds the helix  DNA polymerase- adds new nucleotide units to the parent strand of DNA  Ligase- connects the new strand and zips the molecule together.

 At the end of each DNA molecule is a section of noncoding nucleotides called a telomere

 Each time a cell divides and replicates its DNA a small part of the telomere is lost  This loss of telomere limits the number of times a cell can divide.  This erosion of the telomere during cell division is linked to the aging process and cancer  The enzyme telomerase is inactive in somatic cells but is active in cancer tumors. Can you identify the function of telomerase????

 In each cell DNA molecules are organized and wrapped around small proteins called histones.  The density of this wrapping is directly related to the expression of the genes found on the chromosome.  Dense wrapped – no gene expression  Lose wrapped – open to gene expression

 Genes are located on the chromosome at a particular loci  When the gene is signaled to be expressed a series of enzymes go to work  helicase, RNA polymerase, ligase  Transcription occurs when the template strand of DNA is exposed and mRNA is made

 Process of making a mRNA molecule from the DNA template.  DNA - A T A G C A T A C T A G G  mRNA- U A U C G U A U G A U C C Transcription occurs with in the nucleus, after the mRNA is made however it goes through a series of modifications call RNA processing. This process allows for one gene to express multiple proteins, and is considered to be an element of the evolution process.

 mRNA leaves the nucleus of the cell and moves into the rough ER.  A ribosome attaches to the mRNA and starts to Translate the sequence of nucleotides into a protein.  Translation involves several players: rRNA, mRNA, tRNA, amino acids, and enzymes

 Translation occurs at the ribosome, and reads the mRNA 3 nucleotides at a time  3 nucleotides make up a codon, which codes for 1 amino acid.  There are 64 different codons and only 20 amino acids. So there is repetition in the code  AUG is the universal start codon and codes for the amino acid Met.  There are 3 stop codons that terminate translation: UGA, UAA, UAG

 Once the codon enters the ribosome the complementary, anticodon, of the tRNA attaches to the mRNA.  The tRNA carries with it a specific amino acid, the amino acids are then bonded to each other as each sequential codon is read by the ribosome The end result is a very specific amino acid chain called a polypeptide or protein.