Cell Biology Unit Six

The Nucleus A prominent, spherical organelle found only in eukaryotes It is the repository for the cell’s genetic information (DNA)

The Nucleus It compartmentalizes the activities of the genome (DNA replication & RNA transcription) from the rest of the cell It is enclosed by a double layered nuclear envelope made up of an inner & an outer membrane, separated by a perinuclear space

The Nucleus The nuclear envelope is embedded with numerous nuclear pores which allow for exchange between the nucleus & cytoplasm Each nuclear pore has a protein diaphragm stretched across it, regulating what goes through the pore

The Nucleus

The Nucleus The nucleolus is a spherical organelle without a membrane found inside the nucleus that functions as the ribosome factory for the cell Each cell has one or two Proteins & rRNA are combined here to form ribsomes

The Nucleus The nucleolus consists of fibrils and granules The fibrils are DNA transcribing rRNA The granules are rRNA being combined with proteins to form the two ribosomal subunits

The Nucleus

The Nucleus

Chromosomes Chromosomes are found in the nucleus and are comprised of approximately 50% DNA & 50% protein They occur in pairs, one coming from each parent (sexual) or both coming from one parent (asexual)

Chromosomes Most of the proteins are histones, which determine the molecular anatomy of the chromosome The non-histone proteins also determine the molecular anatomy and also determine gene activity

Chromosomes Nucleosomes

Chromosome Packing

Chromosomes During mitosis the DNA & proteins condense and are referred to as chromosomes During non-division periods the structures relax and are referred to as chromatin

Chromosomes Since chromosomes contain the cell DNA their functions are in heredity, RNA template and protein synthesis

DNA & RNA Functions

DNA & RNA Structure

DNA & RNA Structure

DNA & RNA Base Pairing DNA replication | | | | | | | | | | | | | | | | | | A T G C A T T G A A G C T G G T A G T A C G T A A C T T C G A C C A T C | | | | | | | | | | | | | | | | | | RNA transcription | | | | | | | | | | | | | | | | | | A U G C A U U G A A G C U G G U A G T A C G T A A C T T C G A C C A T C | | | | | | | | | | | | | | | | | |

RNA Synthesis RNA synthesis (transcription) occurs at all times with all three types being produced by the same process In general, DNA is separated and one strand is used as a template, with the sequence of bases determining the sequence of RNA bases

RNA Synthesis RNA polymerase will initiate the process ~ one in prokaryotes ~ three in eukaryotes Occurs in two stages ~ synthesis of RNA nucleotides ~ polymerization of RNA nucleotides on the DNA template

RNA Synthesis The four RNA nucleotides are synthesized from triphosphate forms by way of pyrocleavage They are synthesized in the cytoplasm and transported into the nucleus

RNA Synthesis Binding of RNA polymerase to DNA promoter temporarily separates DNA

RNA Synthesis 3. DNA bases attract RNA pairing partners 4. RNA polymerase binds RNA nucleotides together into a RNA polymer

RNA Synthesis 5. DNA terminator sites eject RNA polymerase 6. RNA post transcriptional processing

Ribosomes Ribosomes are non-membrane bound organelles found in all cells and function as the manufacturing sites of proteins

Ribosomes Each ribosome is composed of three types of rRNA & fifty different ribosomal proteins and are produced in the nucleolus

Ribosomes Ribosomes made up of two subunits - one small, one large

Ribosomes Small subunit binds mRNA Large subunit has an A site to bind tRNA carrying amino acids, a P site for the tRNA holding the growing protein and an E site for tRNA exiting

Ribosomes

Protein Synthesis - Translation mRNA carries a series of codons that code for specific amino acids A codon is a sequence of three nucleotide bases on mRNA Since there are only four RNA bases and there are three in each codon, there are 64 condon possibilities

Protein Synthesis - Translation The genetic code is the use of these 64 codons coding for 20 amino acids Remember - the synthesis of a specific protein requires a specific sequence of amino acids Therefore the sequence of codons on mRNA must be sequentially specific

Protein Synthesis - Translation The genetic code is commaless and non-overlapping, meaning it is read in frames of three without gaps or overlaps There is one initiation codon - AUG There are three stop codons - UAA, UAG & UGA

Protein Synthesis - Translation Translation is the process by which RNA produces proteins in ribosomes - mRNA carries the code from DNA - tRNA carries amino acids to the ribosome - rRNA is a component of the ribosome By way of the genetic code, RNA “translates” from the language of nucleic acids to that of proteins

Protein Synthesis - The Genetic Code

Protein Synthesis - Translation

Protein Synthesis - Translation There are four steps in adding amino acids to a protein + amino acid activation + transfer of amino acids to the ribosome + peptide bond formation + ribosome shift

Protein Synthesis - Translation Amino acid activation occurs with the binding of a particular amino acid to a specific tRNA This process requires a specific enzyme for each amino acid & tRNA

Protein Synthesis – Amino Acid Activation A group of 20 aminoacyl-tRNA synthetases along with pyrophosphated ATP create a two step process that activates the amino acid

Protein Synthesis – Amino Acid Activation The activation results in the formation of an ester bond between the amino acid and the tRNA

Protein Synthesis - tRNA

Protein Synthesis – Transfer of Activated Amino Acid The transfer of the activated amino acid begins with initiation

Protein Synthesis – Transfer of Activated Amino Acid

Protein Synthesis – Peptide Bond Formation

Protein Synthesis – Ribosome Shift

Protein Synthesis – Termination of Translation

Protein Synthesis – Overview

Protein Synthesis – Post Translational Processing

Genes, Genetic Code & Genetic Disease