Learning Outcome 16: Describe the structure and function of the nucleus. Nucleus

 Spherical or oval shaped  Contains most of the cell’s DNA

Parts of the Nucleus  Nuclear Envelope  Nucleolus  Chromatin

Figure 3.24 Nucleus

Figure 3.24c Nucleus

Nuclear Envelope  A double membrane composed of lipid bilayers.  Perforated by nuclear pores.  Envelope is semipermeable.

Nucleolus  Darkly staining spherical body inside nucleus.  Composed of DNA, RNA, and protein.  Site of rRNA synthesis and ribosomal subunit assembly.

Chromatin  Is a threadlike mass of genetic material consisting of DNA and proteins.  DNA is organized into distinct segments called genes.  A gene contains the genetic instructions needed to build a protein.  Genes control cellular structure and direct cellular activities.

Figure 3.24c Nucleus

Nucleic Acids

DNA Nucleotides  The monomers of DNA  Each DNA nucleotide consists of 3 parts: 1. Nitrogenous base; one of Adenine (A) Thymine (T) Cytosine (C) Guanine (G) 2. Pentose sugar; a 5-carbon sugar called deoxyribose attached to each base 3. Phosphate group; alternate with pentose sugars to form DNA “backbone”

Figure 2.24 DNA molecule

DNA Structure  DNA is double stranded  Phosphate groups and deoxyribose form the uprights of the ladder  Paired bases, held together by hydrogen bonds, form the rungs  Bonding of the bases is very specific:  Adenine pairs with thymine via 2 hydrogen bonds  Cytosine pairs with guanine via 3 hydrogen bonds

Figure 2.24 DNA molecule

RNA Nucleotides  The monomers of RNA  Each RNA nucleotide consists of 3 parts: 1. Nitrogenous base; one of Adenine (A) Uracil (U) Cytosine (C) Guanine (G) 2. Pentose sugar; a 5-carbon sugar called ribose attached to each base 3. Phosphate group; alternate with pentose sugars to form RNA “backbone”

RNA Structure  RNA is single stranded  Cells contain 3 different kinds of RNA, each of which has a specific role to perform during protein synthesis:  Ribosomal RNA (rRNA)  Messenger RNA (mRNA)  Transfer RNA

Learning Outcome 17: Explain how genes are organized in non-dividing and dividing cells. Nucleus

DNA Packing  Chromatin has a “beads on a string” structure.  Each “bead” is called a nucleosome.  DNA wrapped twice around a core of histone proteins.  Just before cell division, the chromatin strand replicates its DNA and condenses into a pair of identical chromatids.  The chromatid pair is held together by a centromere and is called a chromosome.

Figure 3.25 DNA Packing in a dividing cell

Learning Outcome 18: Discuss how DNA and RNA stores genetic information. Protein Synthesis

The Genetic Code  Genetic information is stored as sets of three nucleotides.  In DNA - base triplets.  In RNA - codons.  A given base triplet codes for a specific codon.  A given codon codes for a specific amino acid.  E.g. AAA  UUU  Phenylalanine

Learning Outcome 19: Define gene expression. Protein Synthesis

Gene Expression  Genes contain instructions for protein synthesis  Gene expression - a gene’s DNA is used as a template for protein synthesis  Two distinct steps: 1. Transcription – DNA triplets are “read” and “written” as mRNA codons 2. Translation – mRNA codons are “read” and “written” as amino acids

Figure 3.26 Overview of gene expression

Learning Outcome 20: Describe the sequence of events in gene transcription. Protein Synthesis

Transcription  Takes place in the nucleus.  DNA triplets are transcribed into mRNA codons.

Transcription Recipe

Figure 3.27 Transcription

Post-Transcriptional Modification of Pre-mRNA

Learning Outcome 21: Describe the sequence of events in mRNA translation. Protein Synthesis

Translation  Messenger RNA translation requires two other types of RNA: 1. Ribosomal RNA 2. Transfer RNA

Ribosomal RNA  Ribosomal RNA (rRNA) joins with ribosomal proteins to make ribosomes.  A ribosome has two subunits:  Large subunit P site and A site  Small subunit mRNA binding site

Figure 3.28 Translation

Transfer RNA  Transfer RNA (tRNA) delivers amino acids to the ribosome.  tRNA molecules carry specific types of amino acids.  tRNA molecules have anticodons.

Translation “Recipe” #1

Translation “Recipe” #2

Translation “Recipe” #3

Figure 3.29 Translation