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 AG C T G G T A G
TACGTAACTTCGACCAT C
| | | | | | | | | | | | | | | | | |
RNA transcription
| | | | | | | | | | | | | | | | | |
AU G C A U U G A AG C U G G U A G
TACGTAACTTCGACCATC
| | | | | | | | | | | | | | | | | |
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
1. Binding of RNA
polymerase to DNA
promoter
2. RNA polymerase
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
Ribosome
s
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 nonoverlapping, 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