16/09/2021

Lesson 1.3.
Gene function: Protein and
Enzymes

Learning outcomes:

• Explain the genetic control of proteins

• Describe the process of transcription
• Describe the different steps in RNA processing
• Describe the process of translation

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The Link Between DNA and

Protein
❑ DNA contains the molecular blueprint of
every cell
❑ Proteins are the “molecular workers” of the
cell
❑ Proteins control cell shape, function,
reproduction, and synthesis of biomolecules
❑ The information in DNA genes must
therefore be linked to the proteins that run
the cell

What are proteins and what do they do?

• Proteins are large macromolecules that perform a

diverse range of critical functions in both unicellular
and multicellular organisms.
• Proteins are composed of amino acids –20
different amino acids
• Different proteins are made by combining these
20 amino acids in different combinations
• Phosphodiester bond

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Proteins are manufactured by the

ribosomes – the protein machinery
of the cell

Function of proteins:
• Antibody - help protect (defense) the body from
diseases
(Ex: Immunoglobulin G (IgG)
• Enzymes – facilitates/ speeds up chemical/ biochemical
reactions (amylases)
• Messenger - Messenger proteins ex. hormones,
transmit signals to coordinate biological processes
between different cells, tissues, and organs (Ex: Insulin)
• Structural component –Responsible for cell structure
and movement.
• Transport proteins- Carry substances through
body fluids.
• Storage proteins – make essential substances readily
available

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How do genes direct the production of proteins?

The concept of the Central dogma of molecular
biology

The central dogma of molecular biology describes the two-

step process on how the genetic information flows from DNA
sequence to a protein product

• Before a protein can be made, it all started with DNA

• DNA ‘s code must be copied.
• In the cytoplasm, this code must be read so amino
acids can be assembled to make polypeptides
(proteins)
• The process by which DNA directs protein synthesis is
called gene expression
• The whole process consists of two major steps:
transcription and translation

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2 Steps to Make a Protein:

1. Transcription
◼DNA → RNA
2. Translation
◼RNA → Protein (Chain of
amino acids)

1st step: Transcription

• Copying of genetic information from DNA to
RNA
Why? DNA has the genetic code for the protein
that needs to be made
◼enzymes help:
1. RNA polymerase – brings RNA nucleotides
over to be synthesized into mRNA
◼Template strand: the side of DNA that will be
used to create an mRNA strand

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Three Types of RNA Involved

during protein Synthesis

• Messenger RNA (mRNA) copies DNA’s

code & carries the genetic information to
the ribosomes
• Ribosomal RNA (rRNA), along with
protein, makes up the ribosomes
• Transfer RNA (tRNA) transfers amino
acids to the ribosomes where proteins are
synthesized.

Overview of Transcription

• Transcription of a
DNA gene into RNA
has three stages

– Initiation
– Elongation
– Termination

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Initiation
• Start” sequence called
promoter region of
DNA
• Promoters exist
upstream of the genes
they regulate.
• Promoters are regions
on DNA that show
where RNA
Polymerase enzyme
must bind to begin the
Transcription of RNA
• Called the TATA box
• Enzyme needed: RNA
polymerase 13

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Elongation
• RNA polymerase - functions in adding
new nucleotides to create a new strand of
RNA.
• RNA polymerase synthesizes a sequence of
RNA nucleotides along one strand of the DNA
called template strand the template
strand/coding strand

RNA polymerase
synthesizes the mRNA
strand using
complimentary base-
pairing

• Remember…there aren’t “T” bases in RNA

• “C” binds with “G”
• DNA “A” binds with RNA “U”
16

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• The elongation process

synthesizes mRNA in the 5' to 3'
direction

Termination
Whenever the RNA polymerase has reached
one of three specific codons (UAA, UAG, or
UGA) codes a "stop" signal, which triggers the
enzymes to terminate transcription.

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Termination

• when RNA polymerase reaches the

end or "STOP" part of the genetic code for that
protein, it releases the newly-made mRNA
transcript
• DNA re-zips
• finished mRNA leaves the nucleus and goes
to a ribosome in the cytoplasm

Summary of Transcription
• Step #1 (of 2) of protein synthesis
• Transcribe: to make a copy from DNA
to mRNA
• Occurs in the nucleus (eukaryotes) or
in the cytoplasm (prokaryotes)
• RNA Polymerase enzyme –makes a
section of mRNA to be used for
protein synthesis.

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Processing of RNA
transcript in eukaryotic cells
• In prokaryotes, the mRNA produced is
translated directly to for protein
synthesis.

• In eukaryotic cells the RNA is processed.

•The initial product of transcription of an
mRNA is called the pre-mRNA or
primary transcript.

Processing of RNA transcript in

eukaryotes

1.Capping at the 5' end -a 7-

methyl guanosine = the
5'cap protects the RNA from
degradation
2.Addition of poly-a tail at the
3’ end – addition of a series
of repeating adenine
nucleotides at 3’
3. Splicing- – removal of
introns and rejoining of coding
sections or introns

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Removal of introns
❑ Eukaryotic genes have introns –a noncoding regions that interrupt
the gene.
❑Introns are removed from the pre-mRNA by the activity of a
complex called the spliceosome.
❑Spliceosomes are composed of smaller particles called snRNP
(made of proteins and snRNA).
❑ splicing – removal of introns and rejoining of coding
sections or introns

Ready for Translation

▪ This mRNA has been processed and is

called mature mRNA.
▪ It is ready to go to the cytoplasm for
translation.
▪ mRNA then goes through the pores of the
nucleus and attaches to the ribosome.

24

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2. Translation (Polypeptide
synthesis)

Translation:
➢ The process by
which the nucleotide
sequence of mRNA
is converted into the
sequence of amino
acids of the
corresponding
protein.

TRANSLATIONAL COMPONENTS

1. Ribosomes (large and small subunits)

2. Messenger RNA (mRNA)

3. Transfer RNAs (tRNAs)

4. Amino Acids (aa’s)

5. Enzymes (“factors”)

6. Energy (ATP, GTP)

26

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1. Ribosome Structure

27

The ribosome has 3 binding sites

• Peptidyl site or “P” site
• Aminoacyl site or “A site”
• Exit site or “E site” A site (Aminoacyl
tRNA binding site
E site
Amino end Growing polypeptide (Exit site)
Large
subunit
Next amino acid E P A
to be added to
polypeptide chain

mRNA
binding site
tRNA Small
subunit

mRNA 3
Schematic model showing binding sites. A
ribosome has an mRNA binding site and three tRNA
binding sites, known as the A, P, and E sites. This
Codons
5 schematic ribosome will appear in later diagrams.

Schematic model with mRNA and tRNA. A tRNA fits into a binding
site when its anticodon base-pairs with an mRNA codon. The P site
holds the tRNA attached to the growing polypeptide. The A site holds
the tRNA carrying the next amino acid to be added to the polypeptide
chain. Discharged tRNA leaves via the E site.

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• Building a Molecule of tRNA:

- A specific enzyme called an aminoacyl-
tRNA synthetase joins each amino acid to
the correct tRNA.

• Building a Polypeptide
I. Initiation
II. Elongation
III. Termination

Initiation of Translation:
The initiation stage of translation brings together mRNA, tRNA
bearing the first amino acid of the polypeptide, and two
Large
subunits of the ribosome ribosomal
subunit
P site
C 5
5A U G 3

Initiator tRNA GTP GDP

3 5 E P A 3
5 Start codon
mRNA
mRNA binding site

Small ribosomal Translation initiation

subunit
complex

1. A small ribosomal subunit binds to a molecule 2. The arrival of a large ribosomal subunit
of mRNA.. An initiator tRNA, with the anticodon completes the initiation complex. The
UAC, base-pairs with the start codon, AUG. initiator tRNA is in the P site; the A site is
This tRNA carries the amino acid methionine available to the tRNA bearing the next
(Met). amino acid.

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▪ In prokaryotes the selection of the start codon is

facilitated by the binding of the ribosome to certain
sequences in the mRNA called the Shine Dalgarno
sequence, a purine-rich region located upstream of the
start codon
▪ The Shine-Dalgarno sequence marks the start of each
coding sequence, letting the ribosome find the right start
codon for each gene.

Assembly of the prokaryotic Initiation Complex

• A series of three adjacent

bases in an mRNA molecule
codes for a specific amino
acid—called a codon.

• Each tRNA has 3

Amino acid
nucleotides that are
complementary to the
codon in mRNA.

• Each tRNA codes for a

different amino acid.
Anticodon

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Elongation
1. Codon recognition. The anticodon of an incoming tRNA base-pairs with
the complementary mRNA codon in the A site
2. Peptide bond formation. An rRNA molecule of the large subunit catalyzes
the formation of a peptide bond between the new amino acid in the A site and
the carboxyl end of the growing polypeptide in the P site.
3. Translocation. The ribosome translocate the tRNA in the A site to the P
site. The empty tRNA in the P site is moved to the E site, where it is released. -
The mRNA moves along with its bound tRNAs, bringing the next codon to be
translated into the A site.

Translation elongation process

Termination of Translation:
release factor

Free polypeptide
3 3
5’ 3
5
5
Stop codon
(UAG, UAA, or UGA)

1.) When a ribosome reaches 2.) The release factor hydrolyzes

3. The two ribosomal
a stop codon on mRNA, the the bond between the tRNA in subunits and the other
A site of the ribosome the P site and the last amino
components of the
accepts a protein called a acid of the polypeptide chain.
assembly dissociate.
release factor instead of The polypeptide is thus freed
from the ribosome.
tRNA.
• Prokaryotes release factors: RF1 (UAA & UAG) and RF2 (UAA
&UGA) and RF3 (facilitates binding of RF1 and RF2)
• Eukaryotes release factor : eRF1 recognizes all three stop
codons

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Eukaryotic Protein Synthesis vs. Prokaryotic Protein

Synthesis

• Transcription and translation

are spatially and temporally
separated in Eukaryotic cells

• Transcription occurs in the

nucleus, and translation
occurs in the cytoplasm
• In prokaryotic cells,
transcription and translation
occurs in the cytoplasm

Eukaryotic Protein Synthesis vs.

Prokaryotic Protein Synthesis
Prokaryotes Eukaryotes
• mRNA molecules are monocistronic • mRNA molecules are
• Protein synthesis occurs in the cytoplasm polycistronic
• eukaryotic protein synthesis uses 80 S • Prokaryotic protein synthesis
uses 70 S ribosomes
• Most of the genes have introns or non-
coding sequences along with exons • There no introns
(coding region) • mRNA is not processed,
• Pre-mRNA molecules are directly translated
modified/processed
• Met is formylated – n-formyl
• The first amino acid methionine (AUG) is met
not formylated
• one release factor, eRF1
• Three known release factors are recognizes all three stop
involved, RF1. RF2, RF3 codons

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Lesson 1.3 Part 2

The Genetic Code and

Regulation of Gene Action

Genetic code
❑ The letter A, U, G & C corresponds
to the nucleotides found in the
mRNA.
❑ The nucleotides are organized into a
three letter code called codons
❑ 1 codon =3 nucleotide bases
(triplet)
❑ The collection of codon is called the
genetic code
❑ 61 codons code for 20 amino acids
found in protein
❑ 3 codons do not code for an amino
acids (Stop codons - UAG, UAA,
UGA)

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Genetic code

❑ The Genetic code will able

you to translate sequences of
RNA into a specific sequence
of amino acids.
❑ The base sequence in a DNA
gene dictates the sequence
and type of amino acids in
translation

For example, you wanted to determine what

amino acids would result from the following mRNA
sequence:
mRNA sequence:
AUGGUCAAGGUUCUCGAUGCAGUCGU

1. The first step is to divide the mRNA sequence

into codons (triplets of N-bases).
AUG GUC AAG GUU CUC GAU GCA GUC CGU

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2. Next, find the start codon, AUG (AUG signals to

ribosomes where to start the translation).
3. The next step is to look up each codon in a
Genetic code table to determine the specific amino
acid it corresponds to.
AUG GUC AAG GUU CUC GAU GCA GUC CGU

AUG GUC AAG GUU CUC GAU GCA GUC CGU

Ex: What amino acid codes for the codon GUC?

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Properties of the Genetic code

1. The genetic code is universal.
➢All known living organisms irrespective of
their taxonomic position uses the same
genetic code

2. The genetic code is a triplet in nature.

➢ RNA sequences is read into a sequence of
amino acids by grouping the nucleotide bases
into 3 (triplet) successive nucleotides called
codons.

Properties of the Genetic code

3. The Genetic Code is Unambiguous.
There is no ambiguity in the genetic code. This means
that one particular codon shall not or never codes for two or
more different amino acids. One codon specifies only one
particular amino acid.

4. The Genetic code is Degenerate

• Most of the amino acids — except methionine and
tryptophan — are coded by several codon
• One amino acid can have many codons
• Differences is found mainly at the third base in the
triplet-nucleotide

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• The degeneracy of the genetic code comes in two types;

❑ Partial
- The first two bases in a degenerate codon are identical
but the third nucleotide is different.
Example: CAA and CAG that codes for glycine
❑ Complete
- any of the 4 nucleotide bases can take the third
position and still code for the same amino acid.
Example: CGG, CGA, CGC, CGU all specified for arginine.

❖ The degenerative characteristics of the genetic code

is an important cellular mechanism to reduce the
deleterious impact of random mutations

Degeneracy

Properties of the Genetic code

❖ The genetic code is continuous
❖ Non- overlapping
❖ There is no punctuation that between
codons
❖ The message is read up to a stop codon

5. The Genetic code is Non-

overlapping

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Properties of the Genetic code

6. Some codons serve as punctuation.

- AUG," (methionine) signals the start

of a polypeptide chain.
- Three other codons UAA, UAG, and
UGA tells the cell when to stop polypeptide
synthesis.

Gene Regulation and

the Control of
Prokaryotic & Eukaryotic
regulation
How are genes turned
on & off ?

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Gene Regulation
- the process of turning genes on and off
❑ Gene regulation function
❑ flexibility & reversibility
❑ ensures that the appropriate genes are
expressed at the proper times.
❑ adjust levels of enzymes for synthesis &
digestion
❑ help an organism respond to its environment
❑ Important for organism’s normal development
❑ explain some of the differences in form and
function between different species with
relatively similar gene sequences.

Prokaryotic gene regulation

• The primary mechanism to regulate what

protein and how much protein to expressed
is through regulation primarily at the
transcription level.

• Prokaryotic genes with related function are

often grouped together called an operon.

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Parts of an Operon
▪ Promoter
▪ Operator
▪ Repressor protein
▪ Regulatory gene
▪ Structural genes

Prokaryotic gene regulation

Two Types of Negative Gene

Regulation in prokaryotes

•Repressible operon
•Inducible Operons

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Repressible operon
➢A repressible operon is one
that is usually on; binding of
a repressor to the operator
shuts off transcription.
➢Example is the trp operon
• By default the trp operon is on and the
genes for tryptophan synthesis are
transcribed
• tryptophan - present, it binds to the trp
repressor protein, which turns the
operon off
• The repressor is active only in the
presence of its corepressor
tryptophan; thus the trp operon is turned
off (repressed) if tryptophan levels are
high

Inducible operon

• An operon that is usually off but can be

stimulated or activated
• A molecule called an inducer inactivates the
repressor.
• Inactivating the repressor will allow for the
transcription of the operon.
• Example - lac operon
• Genes codes for enzymes that breakdown lactose

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Inducible operon

• The lac operon is an inducible

operon and contains genes that
code for enzymes used in the
hydrolysis and metabolism of
lactose
• By itself, the lac repressor is
active and switches the lac
operon off
• A molecule called an inducer
(lactose) inactivates the
repressor to turn the lac
operon on

The lac operon in prokaryotes: regulated

synthesis of inducible enzymes

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Eukaryotic Gene Regulation

Eukaryotic gene expression is

regulated at a number of points
along the pathway of information.

Points of control
➢ The control of gene
expression can occur at any
step in the pathway from
gene to functional protein
1. packing/unpacking DNA
2. transcription
3. mRNA processing
4. mRNA transport
5. translation
6. protein processing
iology
7. protein degradation

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1. DNA packing as gene control

Degree of packing of DNA regulates transcription
◆ tightly wrapped around histones

✓ no transcription
✓ genes turned off

▪Heterochromatin -
tightly packed
▪Euchromatin - loosely
packed

1.a. DNA methylation

➢ Methylation of DNA blocks

transcription factors
◆ no transcription

◆ genes turned off

➢ attachment of methyl groups

(–CH3) to cytosine

➢ nearly permanent inactivation

of genes
ex. inactivated
mammalian X
chromosome = Barr body

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Histone acetylation and deacetylation

Acetylation of histones unwinds DNA
◆ loosely wrapped around histones

enables transcription
genes turned on
◆ attachment of acetyl groups (–COCH3) to histones

conformational change in histone proteins

transcription factors have easier access to genes
▪ Deacetylation
-Tightly packed nucleosomes
- genes turn off

2. Transcription initiation

◆ Promoter - binding of RNA

polymerase & transcription
factors
• Enhancer DNA sequences
distant control sequences
• Activator proteins - bind to
enhancer sequence &
stimulates transcription
• Silencer proteins - bind to
enhancer sequence
& block gene transcription

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3. Post-transcriptional control

Alternative RNA
splicing
◆variable processing

of exons creates a
family of proteins
◆different mRNA

molecules from the

same primary
transcript.

4. mRNA stability
➢ Life span of mRNA determines amount of
protein synthesis
➢ microRNAs (miRNAs) or small
interfering RNA (siRNA) could control the
lifespan of mRNA in the cytosol
- bind to mRNA
- create sections of double-stranded
mRNA
- triggers degradation of mRNA
◆ cause gene-silencing

- post-transcriptional control
- Functionally turns genes off= no protein
produced
AP Biology

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5. Control of translation

Block initiation of translation stage

◆ Phosphorylation of eukaryotic

initiation factor-2 (eIF-2)

- prevent attachment of ribosomal
subunits & initiator tRNA
- block translation of mRNA to
protein

6. Post-translational activity or
protein processing
Protein processing
◆ folding, cleaving, adding sugar groups,
targeting for transport
Protein degradation
◆ ubiquitin tagging

◆ proteasome degradation

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6. Post-translational activity or
protein processing

Attachments of certain
chemical groups that
modifies the function of
proteins

Eukaryotic gene regulation

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END….

17
 Biol 22p
Principles of Genetics
Module 1: The nature, expression, and regulation
of genes in individual

Lesson 1.1: Introduction to Genetics

Lesson 1.2: Chemical and Molecular Basis of Heredity
Lesson 1.3: Gene function: Protein and Enzymes
Lesson 1.4: The chromosomal basis of heredity –Part 1
Lesson 1.4: The Chromosomal Basis of
Heredity-Part 1

1. The Organization of Chromosome Structure

2. The Cell Cycle with Mitosis
Learning Outcomes Part 1:
You can…
✓Explain the organizational structure of chromosomes
✓Differentiate the phases/stages of cell cycle with
mitosis as the mode of cell division
Chromosome
… a threadlike structure of nucleic acids and protein found in
the nucleus of most living cells, carrying genetic information in
the form of genes.
 The Organization of Chromosome Structure

Prokaryotic Chromosome
….is formed by compressing their DNA into
smaller spaces through supercoiling with the aid
of HU protein and integration host factor
(IHF).
 The Organization of Chromosome Structure

Prokaryotic Chromosome
Positive supercoiling refers to the twisting
of the DNA in the same direction as the
double helix (right handed).

Negative supercoiling refers to the twisting

of the DNA toward opposite direction of the
double helix winding pattern (right handed).
 The Organization of Chromosome Structure
Prokaryotic Chromosome
HU or Heat unstable protein
…is the most abundant protein in the
nucleoid which bind DNA and introduce
sharp bends in the chromosome,
generating the tension necessary for
negative supercoiling.

Integration host factor (IHF)

… is also a protein that binds to specific
sequences of the DNA which introduce
additional bends to the chromosome.
 The Organization of Chromosome Structure

Eukaryotic Chromosome
… is consist of DNA tightly coiled around clusters of histone
proteins forming a structure called nucleosome.

Histone proteins
...are categorized into
two groups:
core histones (H2A,
H2B, H3, and H4)
linker histones (H1 Chromatosome
and H5)
The Organization of Chromosome Structure

Eukaryotic Chromosome
• DNA coiled around a histone
protein core forming nucleosome
• Group of nucleosomes forms the
“beads on string “ structure
• The beads on string forms into
chromatin fiber.
• Chromatin fibers organized
together forming higher order
of chromatin fiber.
The Organization of Chromosome Structure

Eukaryotic Chromosome
• The higher order of chromatin
fiber is condense into section of
the chromosome.
• Final form - Chromosome with:
• Telomere ( both end potions)
• Gene desert, poor and rich regions
• Centromere ( primary constriction)
Parts of Eukaryotic Chromosome
Unduplicated Chromosome Duplicated Chromosome
Satellite
Secondary Constriction

p– arm (short )

Centromere

q– arm (long )

Telomere(s)
Types of Eukaryotic Chromosome
(based on centromere placement)
Cell Cycle
Cell cycle
…is a series of events that takes place in a cell as it grows
and divides.
Cell growth
…refers to an increase in the total mass of a cell, including
both cytoplasmic, nuclear and organelle volume.
Cell division

…refers to the process by which a parent cell divides into two

or more daughter cells.
Phases of Cell Cycle
•Interphase/ Cell growth
G1
S-phase
G2
•M-phase/ Cell division
Mitosis (PMAT-C)
Meiosis (PMAT)2-C

o G0 Cell at rest
o Restriction Points
 Sub-phases of Interphase
G1 or Gap 1
... all cell organelles except the chromosome
or DNA are duplicated
… 11 hours in most eukaryotes

Restriction point (R1)

• The cell is irreversibly committed to the
cell cycle process.
• The cell is at its appropriate size and
adequate energy reserves.
 Sub-phases of Interphase

Restriction point (R1)

• The cell check for damage DNA before
proceeding to the next stage.

If these conditions are not met, the cells

will:
➢ Undergo inactive phase or G0
➢ Wait for further signal when the
conditions are improved.
Sub-phases of Interphase
S-Phase
… “S” means synthesis of DNA
or DNA replication
… 8 hours in most eukaryotes
Steps in DNA Replication (L1.2)
➢ Initiation
➢ Unwinding
➢ Elongation
➢ Termination
 Sub-phases of Interphase
G2 or Gap 2
…serves as the preparatory stage prior to
cell division.
…preparation involves producing more
enzymes through protein synthesis (L1.3)
…4 hours in most eukaryotes
Restriction point (R2)
➢ The cell size and protein reserves are at
optimal volume.
 Sub-phases of Interphase
Restriction point (R2)
➢ The chromosomes are accurately
replicated without any mistake or
damage.
If these conditions are not met, the cells will:
➢ The cell attempts to repair the damage
DNA.
➢ If the damage is beyond repair, the cell
will undergo the apoptosis or program
cell death.
M-phase: Mitosis
The is a type of cell division which
takes place in a somatic (body) cell
and produces two identical
daughter cells.
PMAT-C
Importance of Mitosis:
➢ Growth and development
➢ Replacement of damage
tissues
Sub-phases of M-phase:
Mitosis
Prophase
• Chromatin starts to condense
into duplicated chromosome
• Nuclear envelop disappears
• Mitotic spindle of microtubules
develops from the centrosome
appears
Sub-phases of M-phase:
Mitosis
Metaphase
• Microtubules extending from
opposite centrosome attached to
the kinetochore – protein attached
to the centromere of each sister
• Chromosomes align at the
metaphase plate
Sub-phases of M-phase:
Mitosis
Anaphase
• Microtubules attached to the
kinetochore of the centromeres of each
sister chromatids contracts until the
connection breaks.
• Each member of the sister chromatids
moves towards the opposite poles.
 Sub-phases of M-phase:
Mitosis
Telophase
• Each member of the sister chromatids
reaches the opposite poles.
• chromosome uncoil and untangle into
chromatin
• Spindle fiber disappear
• nuclear envelope starts to re-form.
 Cytokinesis
…refers to the division of cytoplasm half
way along the length of the elongated
mother cell after karyokinesis or nuclear
division.
❑ Cell Plate Formation
❑ Cleavage or contractile ring formation
 Genetic Implication of Mitosis
2n = 2 • Mitosis produces daughter
2n = 2 cells which are identical to
each other and to its
2n = 2 original/ mother cell.
2n = 2 • Mother cell has 2n = 2,
then two daughter cell has
2n = 2 2n =2 also
2n = 2 Where:
2n = 2 2n = 2 • n = haploid number which
represents total number
of chromosomes found in
a gamete (♂ and ♀)
2n = 2
2n = 2
Reference

https://www.nature.com/scitable/topicpage/genome-
packaging-in-prokaryotes-the-circular-chromosome-
9113/
Rice, P. A., et al. Crystal structure of an IHF-DNA
complex: A protein-induced DNA U-turn. Cell 87, 1295–
1306 (1996)
 Biol 22p
Principles of Genetics
Module 1: The nature, expression, and regulation
of genes in individual

Lesson 1.1: Introduction to Genetics

Lesson 1.2: Chemical and Molecular Basis of Heredity
Lesson 1.3: Gene function: Protein and Enzymes
Lesson 1.4: The chromosomal basis of heredity –Part 2
Lesson 1.4: The Chromosomal Basis of
Heredity-Part 2

1. The Cell Cycle with Meiosis

2. Types of Eukaryotic Life Cycle
Learning Outcomes Part 2:
You can…
✓Explain the mechanism of chromosome in each
phases/stages of cell cycle with meiosis
✓Differentiate the different types of eukaryotic life
cycle
Cell Cycle with Meiosis
 M-phase: Meiosis
The is a type of cell division which takes place in a reproductive
cell (diploid) and produces four unique haploid daughter cells.
It involves two karyokinesis and two cytokinesis.
• Reduction Meiosis – MI (separation of homologous chromosome)
• Equatorial Meiosis –MII (separation of sister chromatids)
Importance of Meiosis
• Production of gametes
• Maintain the ploidy level of each generation
*Ploidy level is a term referring to the number of chromosome sets in somatic cells of
the diplophase (2n) or gametophytic cells of the haplophase (1n). It is indicated by a
number followed by the x letter. Diploid cells have two sets of chromosomes and are
indicated by 2x.
Sub-phases of M-phase:
Meiosis I- Reduction Meiosis
Prophase I –Fives Sub-phases: (LZPDD)
1. Leptonema
2. Zygonema
3. Pachynema
4. Deplonema
5. Diakinesis
 Sub-phases of M-phase:
Meiosis I
Prophase I
I. Leptotene/Leptonema
• The chromosomes thicken and become
visible, but the chromatids remain
invisible.
• The centromeres begin to move
towards the opposite poles.
 Sub-phases of M-phase:
Meiosis I
Prophase I
II. Zygotene/Zygonema
• Homologous chromosomes enter synapsis (the
fusion of homologous chromosome) forming the
synaptonemal complex.

B b BB bb

♂ ♀
 Sub-phases of M-phase:
Meiosis I
Prophase I
III. Pachytene/Pachynema
• The synapsis is complete all throughout each
pair of homologous chromosomes.
• Crossing over (genetic exchange between non-
sister chromatids of a homologous pair) occurs.
.
 Sub-phases of M-phase:
Meiosis I
Prophase I
IV. Diplotene/Diplonema
• The synaptonemal complex dissolves.
• The tetrad or four chromatids of the
homologous pair is visible.
• Crossing over points appear as
chiasmata (holding of non-sister
chromatids together).
• Meiotic arrest may occur at this time
in many species.
 Sub-phases of M-phase:
Meiosis I
Prophase I
V. Diakinesis
• The chromatids thicken and
shorten.
• At the end of prophase I, the
nuclear envelop breaks down
and the spindle fibers begins to
form.
 Sub-phases of M-phase:
Meiosis I
Metaphase I
• Tetrads line up along the metaphase
plate.
• The centromere of each homologous
chromosome is attaches to centrosome in
each opposite pole by microtubule.
Sub-phases of M-phase:
Meiosis I
Anaphase I
• The homologous chromosomes
move to the opposite poles.

n=2

2n = 4
n=2
 Sub-phases of M-phase:
Meiosis I
Telophase I
• Each complement of the
homologous pair reaches the
opposite poles.
• The nuclear envelop reforms.
n=2

Cytokinesis separates the daughter cells.

n=2
 Product of MI or Reduction Meiosis

Intermediate daughter cells: 2n = 4

• Haploid (n) – separation homologous
chromosome
• Unique
➢Crossing over during pachynema
➢The orientation of each homologous pair is n=2
independent from each other resulting to
independent assortment of alleles different
genes.
* Allele is alternative form of the gene.
n=2
 Sub-phases of M-phase:
Meiosis I
Possible Orientation of
Homologous Pair
Gene: A & B
Allele: A/a & B/b A a A a
b B
B b

A a A a
B Unique b b Unique B
intermediate intermediate
daughter cell daughter cell
 Sub-phases of M-phase:
Meiosis II – Equatorial Meiosis

Prophase II
• The chromosomes condense.
• The centrosomes appears in
each the opposite poles of
the cell.
• The nuclear envelope breaks
down at the end of this
phase.
 Sub-phases of M-phase:
Meiosis II – Equatorial Meiosis

Metaphase II
• The centromeres of each
sister chromatids are attach
to the microtubules
connected to the centrosome
located on the opposite poles
of the cell.
• Each chromosome aligns at
the metaphase plate.
Sub-phases of M-phase:
Meiosis II – Equatorial Meiosis
Anaphase II
• The sister centromeres detach
from each other.
• Sister chromatids to move to
opposite poles.
Sub-phases of M-phase:
Meiosis II – Equatorial Meiosis
Telophase II
• Each individual sister
chromatids begins to uncoil.
• The nuclear envelopes re-
form.

Cytokinesis proceeds producing

four unique daughter cells.
 Cell cycle with Meiosis
2n = 4
Interphase (G1, S-phase, G2)
Meiosis I
1. Prophase I (LZPDD)
n=2
2. Metaphase I
3. Anaphase I
4. Telophase I
n=2
5. Cytokinesis
Meiosis II
1. Prophase II
2. Metaphase II
3. Anaphase II
4. Telophase II
5. Cytokinesis
 Cell cycle with Meiosis
Genetic Implication
➢Meiosis maintains the ploidy level of an organism

♂ ♀

Gen 1 2n = 4 2n = 4

n=2 + n=2

Gen 2 2n = 4
 Cell cycle with Meiosis
Genetic Implication
➢Essential in creating variation (since meiosis produces the
gametes/sex cells)
1. Crossing over in the Pachytene stage
 Cell cycle with Meiosis
Genetic Implication
A a 2. Independent
A a orientations of each
b B
B b homologous pairs
during metaphase
I
Law of Independent
A a A a Assortment
B b b B The orientation of
each homologous pair
is independent from
each other resulting
A A a a A A a a to independent
assortment of alleles
B B b b b b B B different genes.
Cell cycle with Meiosis
Genetic Implication
3. Segregation of sister
chromatids during
anaphase II
Law of Segregation
• The separation and
movement of sister
chromatids toward each
opposite pole ensures that
the alleles of a particular
gene separates from each
and segregated on different
gametes.
 Cell cycle with Meiosis

Sources of Genetic Variation

During Meiosis
Eukaryotic Life Cycle
Eukaryotic Life Cycle
… is a series of developmental stages
which start from the inception/first
stage of initial generation to the
inception of the succeeding
generation

The significant difference between

eukaryotic life cycles includes:
➢ duration between haploid and
diploid phases
➢ meiotic product such as spores
and gametes
 Gametic life cycle
• Common in animal and some Protista
• Meiosis occurs in specialized tissue of
diploid individual thereby produces
gametes – gametic meiosis
1. Growth and development of diploid
individuals
2. Production of reproductive cell of
diploid individuals
3. Formation of gametes (haploid cell;
egg /female gametes and sperm/
male gametes) through meiosis
4. Fertilization of gametes
(egg+sperm) forming diploid
individuals
 Sporic life Cycle
• Common in algae and some plants
• Alternate generation of haploid and
diploid organism
• Meiosis occurs specialized diploid tissue
of an individual producing spore and
spores under go mitosis to produce
gametes – Sporic meiosis
1. Zygote under goes mitosis and develops
into multicellular diploid individual called
sporophyte.
2. The spore forming cells of sporophyte
undergo meiosis producing spore.
3. Spore divides (mitosis) and develops
into haploid multicellular individual
called gametophyte
4. Gametophyte produce eggs and sperm
through mitosis.
 Zygotic Life Cycle
• simplest sexual life cycle
• common among fungi and protists
• organisms are haploid during most
of their life cycle.
• Meiosis occurs immediately after
fertilization, also known as Zygotic
Meiosis
1. Zygote is the only diploid stage.
2. After fertilization, zygote
under goes meiosis to produce
haploid cells.
3. Haploid cells undergo mitosis
forming a haploid multicellular
organism.
4. Specific haploid cells under goes
mitosis to produce haploid cells.