NUCLEOSOME STRUCTURE Chromosomes are made of DNA, protein, & small amounts of RNA. Prokaryote chromosomes, like bacteria, are only DNA.

DNA has negative charges along the strand & positively charged proteins are bonded by electromagnetic forces. Basic protein structure is called a histone. Complex of DNA & protein is chromatin. Histone proteins form skeleton of the helix. DNA helix combines with 8 small histone molecules & are called nucleosomes.

In eukaryotes, less than 10% of DNA codes for proteins. Order of nucleotides is the “genetic code”. One gene contains info to produce one polypeptide. Some genes do not code for proteins but control how other genes are expressed. Every cell contains ALL the genetic info.

NOT EVERY CELL EXPRESSES EVERY GENE. Cells differentiate by having certain genes inactivated. Staining shows euchromatin (light stained, condensed, transcribed); heterochromatin (dark stained,compact, rarely transcribed).

Most genes have very few copies. Some genes, like those for RNA & histones, are present many times. DNA is degenerate. More than 1 base triplet codes for the same amino acid. This is universal in all living organisms. Part of non-coding DNA is in the form of repetitive sequences & these are used for DNA profiling.

PRIMARY ENZYMES DNA polymerase III creates covalent bonds between the new nucleotides of the complementary strand. Helicase unwinds DNA. RNA primer bonds a few nucleotides to the old DNA strand & RNA primase binds the RNA nucleotides together. DNA polymerase removes the RNA primer & replaces the RNA bases with DNA bases.

The leading strand forms & the lagging strand fills in with the Okazaki fragments. DNA ligase then connects the fragments. This is done in a 5’ to 3’ direction. An enzyme that checks for possible base errors works next. In eukaryotes, DNA replication is initiated at many points. Eventually, the replication “bubbles” join together as the whole DNA replicates.

DNA & RNA COMPARED DNA Double stranded A,T,C,G Deoxyribose Never leaves nucleus RNA Single stranded A,U,C,G Ribose Three types-mRNA, tRNA, rRNA mRNA made in nucleas & moves to ribosome

MAKING THE PROTEINS Sense-nontranscribed DNA strand Antisense-transcribed DNA strand mRNA is complement to antisense DNA strand Exons- coding sequences (expressed) Introns- noncoding intervening sequences 20 amino acids are coded by 4³=64 possibilities

TRANSCRIPTION Promoter region is site for binding RNA polymerase which in turn attaches the nucleotides together to form a single strand of mRNA. At the 5’ end if the structural gene is a terminator site which stops transcription. RNA polymerase is like DNA polymerase & works in almost the same way.

RNA nucleotides in the form of ribonucleoside triphospahtes form H- bonds with the complementary nucleotide of the DNA strand. One hydroxyl group on carbon-2 in the pentose is the only difference between ribonucleoside triphosphate and deoxynucleoside triphosphate.

As in DNA replication, the ribonucleoside triphosphate attaches covalently to the 3’ hydroxyl group of the growing strand. 2nd & 3 rd phosphate are removed and provide energy to drive the reaction. Transcription involves initiation, elongation, & termination.

TRANSLATION Involves all 3 kinds of RNA. Desired sequence of amino acids is found in the sequence of mRNA. tRNA molecule is single stranded & fcolds back on itself because of base pairing between nucleotides of the strand. It forms a clover-leaf shape.

Ribosomal structure Cells have thousands of chromosomes, eukaryotes even tens of thousands. Nucleolus has ifo for making rRNA. Polysomes are a cluster of ribosomes so that several copies of the same polypeptide can be made at once. Ribosomes are made of proteins and rRNA.

Ribosome is made of small & a large subunit. “S” measures size & density of ribosomes. Larger subunit is 50S & smaller is 30S. Larger subunit is 2 molecules of rRNA & some protein, smaller subunit is 1 molecule of rRNA & some protein. Peptidyl transferase is the enzyme that links the amino acids together.

Proteins made that are needed for use within the cell are made on free ribosomes & are found throughout the cytoplasm. Proteins made for secretion or use by lysosomes are made on ribosomes attached to the ER. It is easier to reach the lumen of the ER for production, processing, & movement to the Golgi apparatus.

EVERY POLYPEPTIDE IS CODED FOR BY ONE GENE. i.e. hemoglobin contains 4 polypeptide chains (2β & 2 α )- 2 genes are required.