learning outcome 16: describe the structure and function of the nucleus. nucleus
DESCRIPTION
Parts of the Nucleus Nuclear Envelope Nucleolus ChromatinTRANSCRIPT
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