Cell Nucleus and Gene Expression LECTURE 7 – CHAPTER 3

Most cells have one nucleus. a.Muscle cells have hundreds; b. mature RBCs have none.

CELL NUCLEUS Two membranes Outer membrane fused with rough ER Inner membrane continuous with outer membrane

DNA and Genes The nucleus contains DNA. A gene is a length of DNA that codes for a specific protein. Transcription – DNA is used as a code to make mRNA Translation – mRNA is used as a code to make protein These two steps can be called Gene expression.

Nucleoli The nucleoli contain the DNA that codes for the production of ribosomal RNA. The nucleus also has one or more darker regions not surrounded by a membrane; these are called nucleoli. Singular = Nucleolus

Genome and Proteome Genome - all the genes in a particular individual or species -humans have ~23,000 genes. Proteome - all the proteins that are produced from the genome.

Chromatin = histones bound to DNA Nucleosome – 2 turns of DNA around histones Euchromatin - active in transcription, looser. Heterochromatin - inactive regions, highly condensed.

ACETYLATION OPENS UP CHROMATIN FOR TRANSCRIPTION

1)Start and stop regions 2)Promoters, areas of DNA that are not part of the gene but tell enzymes involved where to begin Transcription factors bind to the promoter to begin transcription What is a Gene? A GENE CONTAINS:

RNA Synthesis RNA polymerase assembles the growing mRNA --RNA nucleotides pair up to the DNA template --Assembly is complementary. --only 1 strand is transcribed

4 Types of RNA a.Precursor messenger RNA (pre-mRNA) b.Messenger RNA (mRNA) – c.Transfer RNA (tRNA) – d.Ribosomal RNA (rRNA) –

Transcription and RNA Modification in the Nucleus Exons are joined together by spliceosomes and snRNPs

RNA molecules may prevent some mRNA molecules from being translated. -- siRNA (short interfering) and miRNA (micro interfering) -- The expression of at least 30% of genes is regulated in this way. RNA Interference

Protein Synthesis (Translation) mRNA attaches to a string of ribosomes to form a polyribosome. The order of codons give the sequence of amino acids

TRANSCRIPTION & TRANSLATION:

Transfer RNA (tRNA) Cloverleaf shape Further twisted into an upside down L Aminoacyl-tRNA synthetase adds an amino acid to the “accepting end” If the anticodon is complementary to the codon, the amino acid will be inserted.

FORMATION OF A POLYPEPTIDE CHAIN Chaperones help the new protein fold

How Secretory Proteins Enter the ER A leader sequence is formed, which allows the protein to go through the hydrophobic membrane into cisternae

Formation of insulin hormone Figure ) preproinsulin enters the cisterna (109aa) 2) Leader is removed, producing proinsulin (86aa) 3) Central region is removed, leaving a 21 and 30 aa hormone that is two chains. NOTICE THE DISULFIDE BRIDGES

Secretory proteins are next sent to the Golgi complex. a.Proteins may be further modified. b.Proteins are - separated according to destination. - packaged and shipped in vesicles to their destinations.

The Ubiquitin-Proteasome System -

DNA Synthesis

DNA Replication DNA REPLICATION HELICASE DNA POLYMERASE 5’  3’ SYNTHESIS SEMICONSERVATIVE REPLICATION

THE CELL CYCLE

Interphase: G1 If a cell does not divide, it remains in a modified G 1 phase its whole life. The cell is performing the functions characteristic of cells in that tissue. -Cyclin D moves the cell through G 1. - p53- a tumor suppressor gene – inhibits cancer How do they know? Read p. 75, about knockout mice. Turn to your neighbor and explain how it’s done

S-Phase and G 2 S phase: “SYNTHESIS” G 2 phase: Chromosomes start to condense ; consist of two strands called sister chromatids joined by a centromere. A CHROMOSOME AFTER DNA REPLICATION “Condensed”

Cell Death Necrosis: Cell DEATH due to deprivation of blood supply. Apoptosis: Programmed cell death is performed by enzymes called caspases. 1)Extrinsic Apoptosis: “Death ligands” and their receptors 2)Intrinsic Apoptosis: Intercellular signals trigger death

Interphase – Prophase – nuclear membrane disappears, spindle forms Metaphase – Chromosomes line up in center and attach to spindle fibers

Anaphase – Centromeres split, spindle fibers shorten & pull chromatids to opposite sides 3.Telophase- Cytoplasm is divided (cytokinesis), nuclear membranes re- form, cells split.

Role of the Centrosome -forms the spindle -helps form cleavage furrow during cytokinesis Non-dividing cells: centrosome migrates to the plasma membrane and forms the nonmotile primary cilium. -In ciliated cells, hundreds of centrosomes form and become the basal bodies of the cilia Looks like a churro!

Telomeres and Cell Division a.Telomere loss may signal end of cell division b.Damaged telomeres activate p53 which induces cell cycle arrest, senescence, and apoptosis c.Cells that can divide indefinitely, (e.g. bone marrow), have telomerase that replicates the telomere. Telomeres – ends of chromosomes

Hypertrophy and Hyperplasia Hyperplasia: growth due to an increase in the number of cells; Hypertrophy: growth due to an increase in cell size;

Karyotype – Chromosomes are arranged in homologous pairs Humans have 23 pairs 1 pair – sex 22 - autosomes

Meiosis I – “reduction division” Meiosis – how cells make gametes

Homologous chromosomes pair up. Prophase I: parts are swapped in crossing-over. Crossing over is a source of genetic variability

Metaphase I - Homologous chromosome pairs line up at the equator [chromosomes shuffle] Anaphase I - Homologous chromosomes are pulled apart. Telophase I - Homologous chromosomes are separated. This results in two daughter cells with 23 chromosomes each

Prophase II – spindles form Metaphase II – sister chromatids line up in center Anaphase II- Centromeres are broken, chromosomes pull apart Telophase II – 4 cells with 23 chromosomes each For animations, click hereclick here

Meiosis Summary

Epigenetic Inheritance When silenced genes are passed to daughter cells during mitosis and meiosis without a change in DNA base sequence. Mechanisms of epigenetic inheritance a.Post-translational modifications of histones b.Methylation of Cs before Gs c.Acetylation of histones