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

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

Huge organic molecules

• Forms the inherited genetic material (genes) inside each human cell

Deoxyribonucleic acid (DNA)

• Relays instructions from the genes to guide protein synthesis

Ribonucleic acid (RNA)

Contain C, H, O, N, and P

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

RNA polymerase

binds to DNA promoter

RNA polymerase “reads” DNA triplets and

“writes” RNA codons

Transcription ends when

RNA polymerase reaches DNA terminator

Figure 3.27 Transcription

Post-Transcriptional Modification of Pre-mRNA

SnRNPs remove introns

from pre-mRNA.

SnRNPs splice

together remaining

exons

mRNA passes through nuclear

pore

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 RNA2. 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

Small ribosomal subunit binds to mRNA

Initiator tRNA binds

to start codon

Large ribosomal subunit binds to

small subunit

Translation “Recipe” #2

Initiator tRNA is put into P site

Next tRNA binds to

codon in A site

Peptide bond forms

between P site’s amino acid and A

site’s amino acid

Translation “Recipe” #3

Empty tRNA

leaves P site

Ribosome moves to

next codon

tRNA with growing

peptide is put into P

site

Translation

continues until

ribosome reaches

stop codon

Figure 3.29 Translation