Cytoplasmic inheritance and Maternal Effect

Chapter 7
Cytoplasmic inheritance and maternal effect
12/6/2019 By:Asmamaw Menelih 1

Outlines
 Cytoplasmic inheritance
Features of cytoplasmic inheritance
Genes in the cytoplasm
Differences b/n nuclear and cytoplasmic
inheritance
Maternal effect
Endosymbiont theory

Cytoplasmic inheritance
The inheritance of most of the characters of an individual is
governed by nuclear genes.
But in some cases, the inheritance is governed by cytoplasmic
factors or genes.
The transmission of characters from parents to offspring is
governed by cytoplasmic genes is known as cytoplasmic
inheritance
Cytoplasmic inheritance also named as:
 Extra chromosomal inheritance
 Non-mendelian inheritance
Organellar inheritance.
Extra nuclear inheritance

Cytoplasmic inheritance
 The first case of cytoplasmic inheritance was reported by
Conens in 1909 in four „o‟ clock (Mirabilis jalapa) for leaf
colour.
 Later on, cytoplasmic inheritance was reported by various
workers in various organisms.
Features of Cytoplasmic Inheritance
 The important characteristic features of cytoplasmic
inheritance are briefly described below:

Con’t
 Reciprocal Differences
 Characters which are governed by cytoplasmic inheritance invariably
exhibit marked differences in reciprocal crosses in F1,
 whereas in case of nuclear inheritance such differences are not observed
except in case of sex linked genes.
 Maternal Effects:
 In case of cytoplasmic inheritance, distinct maternal effects are
observed.
 This is mainly due to more contribution of cytoplasm to the zygote by
female parent than male parent.
 Generally ovum contributes more cytoplasm to the zygote than sperm.

Con’t
 Mappability:
 Nuclear genes can be easily mapped on chromosomes, but it is
very difficult to map cytoplasmic genes or prepare linkage map
for such genes.
 Now chloroplast genes in Chlamydomonas and maize, and
mitochondrial genes in human and yeast have been mapped
 Non-Mendelian Segregation:
 The mendelian inheritance exhibits typical segregation pattern.
 Such typical segregation is not observed in case of cytoplasmic
inheritance.
 The segregation when occurs, is different from mendelian
segregation.

Cytoplasmic Inheritance vs Nuclear Inheritance

Genes in the cytoplasm
 Eukaryote cells contain mitochondria
(plants, fungi, and animals) and chloroplasts
(plants only).
 Both have genomes organized as a single
circular chromosome.
 (This is very similar to bacterial genomes.)

Mitochondria
 Animal mitochondrial genomes are 13-18 kb in
size.
 Fungal mitochondrial genomes are ~75 kb.
 Higher plant mitochondrial genomes are 300-
500 kb.
 Each mitochondrion has 5-20 copies of the
mitochondrial chromosomes.

Chloroplasts
 Chloroplast genomes are 130-150 kb in size.
 Chloroplasts have more genes than mitochondria.
 Most genes are involved in photosynthesis.
 Corn has 20-40 chloroplasts per cell, with each
chloroplast having 20-40 chromosomes (can make
up 15% of DNA)

Con’t
 Mitochondria are organelles which function to transform energy
as a result of cellular respiration.
 Chloroplasts are organelles which function to produce sugars
via photosynthesis in plants and algae.
 The genes located in mitochondria and chloroplasts are very
important for proper cellular function.
 The mitochondrial DNA and other extra nuclear types of DNA
replicate independently of the DNA located in the nucleus
 The nuclear DNA replicate only one time during cellular
division.

Con’t
 The extranuclear genomes of mitochondria and
chloroplasts however replicate independently of cell
division.
 They replicate in response to a cell's increasing energy
needs which adjust during that cell's lifespan.
 Since they replicate independently, genomic recombination
of these cytoplasimic genome is rarely found in
offspring,
 However, nuclear genomes recombination is common.

Maternal effect
 Nuclear inheritance occurs due to the genes present on the
chromosomes.
 Therefore, the mother nucleus and father nucleus equally
contribute to nuclear inheritance.
 Moreover, the offspring inherits millions of genes from parents
via nuclear inheritance.
 during fertilization, haploid sperm and haploid egg cell unite and
form a diploid zygote.
 Sperm cell transfers its nucleus to the egg cell for the fusion.
The resultant zygote has the cytoplasm of the egg cell.

Con’t
 In simple words, the cytoplasm of the egg cell becomes
the cytoplasm of the zygote.
 Moreover, the offspring receives genes from both nuclei
and genes of the maternal cytoplasmic organelles..
 The gene products stored in cytoplasm of the egg are
directly affect the phenotype of individuals.
 This phenomena is known as maternal effect
 Mitochondria with their mitochondrial DNA are already
present in the egg cell before it gets fertilized by a
sperm.

Con’t
• In many cases of fertilization the head of the sperm
enters the egg cell leaving its middle part with its
mitochondria behind.
• The mitochondrial DNA of the sperm often remains
outside the zygote and gets excluded form inheritance.

Inheritance of leaf variegation in Mirabilis
1

Endosymbiont theory:
 Mitochondria and chloroplasts originated as symbionts
◦ Both organelles reproduce by fission and have
genomes remarkably like bacteria and algae.
◦ Organelle DNA synthesis is not regulated like
nuclear DNA (occurs at all stages of cell cycle).
◦ The endosymbiotic theory states that some of the
organelles in today's eukaryotic cells were once
prokaryotic microbes.

Con’t
◦ In this theory, the first eukaryotic cell was probably an amoeba-
like cell that got nutrients by phagocytosis and contained a
nucleus that formed
◦ when a piece of the cytoplasmic membrane pinched off around
the chromosomes.
◦ Some of these amoeba-like organisms ingested prokaryotic cells
that then survived within the organism and developed a
symbiotic relationship.
◦ Some of these amoeba-like organisms ingested prokaryotic cells
that then survived within the organism and developed a
symbiotic relationship.

Con’t
◦ Mitochondria formed when bacteria capable of aerobic
respiration were ingested;
◦ chloroplasts were also formed when photosynthetic
bacteria were ingested.
◦ They eventually lost their cell wall and much of their DNA
because they were not of benefit within the host cell.
◦ Mitochondria and chloroplasts cannot grow outside their
host cell.

Evidences
 Chloroplasts
have the same size as prokaryotic cells, I,e 1-10 microns
divide by binary fission, and, like bacteria.
 Mitochondria
 have the same size as prokaryotic cells,
 divide by binary fission,
 Mitochondria and chloroplasts have their own DNA that is
circular, not linear.
 Mitochondria and chloroplasts have their own ribosomes
which have 30S and 50S subunits, not 40S and 60S.

Chapter Eight
The genetic material

Outlines
 The genetic material
 Important experiments that led to the identification of
DNA as the genetic material
 DNA definition
 DNA structure and replication mechanism
 RNA ,structure, transcription process and classes of
RNA

The genetic material
 The materials used to store genetic information
in the nuclei or cytoplasm of an organism's cells.
 These are either DNA or RNA.

Identification of DNA as the genetic material
 DNA was identified as the genetic material through a series
of experiments.
 Some these experiments are:
 Mendel‟s hybridization experiment
Chromosome theory of heredity (Walter Sutton)
Frederick Griffith‟s Experiment
Oswald Avery
Alfred Hershey and Martha Chase
• Historical timeline of discovering DNA 1875 - 1953

Mendel’s hybridization experiment
 Although Gregor Mendel‟s experiments
with pea plants in the 1870‟s led to the new
science of genetics, he was never able to
identify the “factors of heredity”
 It would be almost 100 years until the
findings of different researchers proved
DNA is the molecule responsible for genetic
inheritance and discovered its structure.

ChromosomeTheory of Heredity –Walter Sutton
 After studying cell division, Walter Sutton proposes that
chromosomes are the location of Mendel’s “factors”
 Sutton showed that the behavior of chromosomes in
meiosis and fertilization provided physical evidence of
Mendel’s Law of Segregation.

ChromosomeTheory of Heredity – Walter Sutton
 Chromosomes are composed of:
 DNA molecule
 Thousands of protein molecules, called
histones

Con’t
 Quantitative analysis of chromosomes shows a composition of about
forty percent DNA and sixty percent protein.
 At first, it seemed that protein must be responsible for carrying
hereditary information,
 Because, protein present in larger quantities than DNA,
 protein molecules are also composed of twenty different subunits while
DNA molecules are composed of only four.
 As a result, It seemed clear that a protein molecule could encode not
only more information, but a greater variety of information, because it
possessed a substantially larger collection of ingredients with which to
work.

Frederick Griffith’s Experiment (1928)
 Epidemiologist and bacteriologist
 He did research to develop vaccines against bacteria that
caused a global epidemic of the deadly influenza or
pneumonia flu.
 Griffith experimented with bacteria that cause pneumonia.
 Used two types of bacteria:
Smooth shape (Deadly) and Rough shape (not deadly).
 He found that some substance in the dead S bacteria was
taken up by the living R bacteria that made them deadly too

Con’t

Con’t
• Griffith named the substance that changed the harmless
bacteria into deadly bacteria the “transforming factor”
• “Transformation” means to change from one form to a
different form
• What was this “Transforming Factor”?

Oswald Avery’s Team Experiment
 Isolated and purified Griffith’s transforming principle.
 Performed three quantitative chemical analyses on the
transforming principle to determine what was in it.
1. Qualitative tests
2. Chemical tests
3. Enzyme tests

Con’t
 Results
– Qualitative tests showed DNA was present.
– Chemical tests showed the chemical makeup matched that of
DNA.
– Enzyme tests showed only DNA destroying enzymes
stopped transformation.

Enzyme tests

Con’t

Avery’sTeam’s Experiments
Results identified DNA as the transforming
principle
 However, still these conclusions were questioned :
 May be there was some protein in sample
 May be DNA is the genetic matter only in
bacteria
 Much doubt was due to many assumptions that
proteins had to be the genetic material.

Alfred Hershey and Martha Chase (1952)
 American biologists who studied viruses that infect bacteria
 Additional evidence that DNA was the genetic material came from
studies of viruses that infect bacteria .
 These viruses are called bacteriophages (meaning “bacteria-eaters”),or
phages for short.
 Viruses are much simpler than cells.
 A virus is little more than DNA (or sometimes RNA) enclosed by a
protective coat, which is often simply protein.
 To produce more viruses, a virus must infect a cell and take over the
cell‟s metabolic machinery.

Con’t
Hershey and Chase wanted to determine whether the virus‟
protein or DNA was injected into the bacteria
DNA tagged with radioactive phosphorus
Proteins tagged with radioactive sulfur
Once the bacteriophage virus infected the bacteria they
would be able to detect which radioactive element was
injected

Con’t

Con’t
 Only the radioactive phosphorus was found
inside the bacteria – concluded DNA was the
hereditary information!!
 Hershey and Chase’s experiment provided
conclusive evidence that confirm that DNA is
the genetic material.

Summery
1928 Griffith – Discovers a “transforming factor”
1944 Avery and his Team – Research indicates DNA is the
“transforming factor”
but much doupt from the scientific community who believe
protein is the genetic material.
1952 Hershey and Chase – Finally convinced scientists that
DNA, not protein, is the genetic material

DNA
 What is DNA?
 Is the genetic molecules carrying all
the genetic information within
chromosome
 It is complex molecule that contains
all of the information necessary to
build and maintain an organism.
 DNA is a molecule that contains the
instructions an organism needs to
develop, live and reproduce.

The structure of DNA
 Genetic material of living organisms is either DNA or RNA.
DNA – Deoxyribonucleic acid,
RNA – Ribonucleic acid
 DNA and RNA are linear polymers made of subunits known as
nucleotides.
 Each nucleotide contains a phosphate group, a sugar group and a
nitrogen base.
 The four types of nitrogen bases are adenine (A), thymine (T),
guanine (G) and cytosine (C).
 The information in each gene is determined by the order of the
different nucleotides.

Con’t
Nucleotides are joined by linking the phosphate on the 5‟-
carbon of the (deoxy) ribose of one nucleotide to the 3‟-
position of the next.
The phosphate group is joined to the sugar on either side by
ester linkages, and the overall structure is therefore a
phosphodiester linkage
Genes are lengths of DNA that code for particular proteins

Con’t

DNA structure : Double helix
X-ray diffraction photograph of the DNA double
helix by Rosalind Franklin & Maurice H. F.Wilkins
James Watson (L) and Francis Crick (R), and
the model they built of the structure of DNA
48

DNA Structure: Double Helix
 DNA backbone forms right-
handed helix
 Each DNA strand has polarity =
directionality
 The paired strands are oriented
in opposite directions =
antiparallel
49

Conclusion-DNA is a helical structure with distinctive
regularities, 0.34 nm & 3.4 nm.
50

Structure of a nucleotide
A nucleotide is made of 3 components:
 A Pentose sugar This is a 5 carbon
sugar
 The sugar in DNA is deoxyribose.
 The sugar in RNA is ribose.
 A Phosphate group
 A Nitogenous base
51

Con’t

Nitrogenous bases – Two types
Pyramidines
Thymine - T
Cytosine - C
Uracil - U
Purines
Adenine - A
Guanine - G
53

Sugar phosphate bonds (backbone of DNA)
 Nucleotides are
connected to each
other via the
phosphate on one
nucleotide and the
sugar on the next
nucleotide
56

Base pairing
 The Nitrogenous Bases pair up with
other bases.
 For example the bases of one strand of
DNA base pair with the bases on the
opposite strand of the DNA.
 Erwin Chargaff (1947) – noted that
the amount of A=T and G=C and an
overall regularity in the amounts of
A,T,C and G within species.
57

Complementary base pairing
Purines Pyramidines
Adenine Thymine
Adenine Uracil
Guanine Cytosine
60
There is exactly enough room for one purine
and one pyramide base between the two
polynucleotide strands of DNA.

NucleotidesVs Nucleosides
 A base plus a
sugar is known as
a nucleoside.A
base plus a sugar
plus phosphate is
known as a
nucleotide
61

Nature of the Genetic Material
 Property 1 - it must contain, in a stable form, information
encoding the organism’s structure, function, development
and reproduction
 Property 2 - it must replicate accurately so progeny cells
have the same genetic makeup
 Property 3 - it must be capable of some variation
(mutation) to permit evolution
62

DNA Replication
Replication
 Is a process in which DNA copies it self to produce identical daughter
molecules of DNA
 A reaction in which daughter DNAs are synthesized using the parental
DNAs as the template.
 Transferring the genetic information to the descendant generation with a
high fidelity
 Takes place during the interphase between the two mitotic cycles
 No replication takes place during interkinesis
DNA synthesis
 Doesn‟t take place during the entire interphase
 It is confined to S-phase

Four components are required
1. dNTPs: dATP, dTTP, dGTP, dCTP (deoxyribonucleoside
5’-triphosphates) (sugar-base + 3 phosphates)
2. Ds DNA template
3. Primer- short RNA fragment with a free 3´-OH end
4. Enzyme: DNA-dependent DNA polymerase (DDDP),
other enzymes, protein factor
5. Mg 2+ (optimizes DNA polymerase activity)

DNA replication
 DNA Replication is the process by which the DNA of the
ancestral cell is duplicated, prior to cell division.
 Upon cell division, each of the descendants will get one
complete copy of the DNA that is identical to its predecessor
 Synthesis of both new strands of DNA occurs at the
replication fork that moves along the parental molecule
 The replication fork consists of the zone of DNA where the
strands are separated, plus an assemblage of proteins that are
responsible for synthesis
 sometimes referred to as the replisome
65

66
• The replication fork is the site of DNA replication and, by
definition,includes both the DNA and associated proteins

DNA Replication
 When a eukaryotic cell divides, the process is called
mitosis
 DNA replication involves several processes:
 First, the DNA must be unwound, separating the two
strands
 The single strands then act as templates for synthesis of the
new strands, which are complimentary in sequence
 Bases are added one at a time until two new DNA strands
that exactly duplicate the original DNA are produced
67

Con’t
 The process is called semi-conservative
replication because one strand of each daughter
DNA comes from the parent DNA and one strand
is new
 The energy for the synthesis comes from hydrolysis
of phosphate groups as the phosphodiester bonds
form between the bases

Direction of Replication
 The enzyme helicase unwinds several sections of parent DNA
 At each open DNA section, called a replication fork, DNA polymerase
catalyzes the formation of 5’-3’ester bonds of the leading strand
 The lagging strand, which grows in the 3’-5’ direction, is synthesized in
short sections called Okazaki fragments
 The Okazaki fragments are joined by DNA ligase to give a single 3’-5’
DNA strand
69

Enzymes and Proteins Involved in DNA Replication
70

Mechanism of DNA Replication
 DNA replication is catalyzed by DNA
polymerase III which needs an RNA
primer
 DNA polymerase III cannot initiate the
synthesis of a polynucleotide, they can
only add nucleotides to the 3 end
 The initial nucleotide strand is an RNA
primer
 RNA primase synthesizes primer on
DNA strand
 DNA polymerase adds nucleotides
to the 3’ end of the growing strand
DNA polymerase I
degrades the RNA
primer and replaces it
with DNA
DNA polymerase III adds
nucleotides to primer
71

DNA Replication
72
DNA polymerase I degrades the RNA primer and
replaces it with DNA
DNA polymerase III adds nucleotides to primer

 Nucleotides are added by complementary base pairing with
the template strand
 DNA always reads from 5’ end to 3’ end for transcription
replication
 During replication, new nucleotides are added to the free 3’
hydroxyl on the growing strand
 The nucleotides (deoxyribonucleoside triphosphates) are
hydrolyzed as added, releasing energy for DNA synthesis.
 The rate of elongation is about 500 nucleotides per second in
bacteria and 50 per second in human cells 73

 Many proteins assist in DNA replication
◦ DNA helicases unwind the double helix, the template strands are
stabilized by other proteins
◦ Single-stranded DNA binding proteins make the template available
◦ RNA primase catalyzes the synthesis of short RNA primers, to
which nucleotides are added.
◦ DNA polymerase III extends the strand in the 5’-to-3’ direction
◦ DNA polymerase I degrades the RNA primer and replaces it
with DNA
◦ DNA ligase joins the DNA fragments into a continuous daughter
strand
74

Enzymes involved in DNA replication
76

DNA replication &Terminologies
 lagging strand: The new strand of DNA which is synthesized in
short pieces during replication and then joined later
 leading strand :The new strand of DNA that is synthesized
continuously during replication
 Primase: Enzyme that starts a new strand of DNA by making an
RNA primer
 RNA polymerase: Enzyme that synthesizes RNA
 RNA primer: Short segment of RNA used to initiate synthesis of a
new strand of DNA during replication
77

Enzymes in DNA replication
Helicase unwinds
parental double helix
Binding proteins
stabilize separate
strands
DNA polymerase III
binds nucleotides
to form new strands
Ligase joins Okazaki
fragments and seals
other nicks in sugar-
phosphate backbone
Primase adds
short primer
to template strand
DNA polymerase I
(Exonuclease) removes
RNA primer and inserts
the correct bases
78

The process of DNA replication
 The process of DNA replication (initiation, elongation,
termination)
 Complex endeavor involving a conglomerate of enzyme
activities.
 Different activities are involved in the stages of
initiation, elongation, and termination.
79

Initiation
 Involves recognition of an origin by a complex of proteins.
 Before DNA synthesis begins, the parental strands must be
separated and (transiently) stabilized in the single-stranded
state.
 Then synthesis of daughter strands can be initiated at the
replication fork.
80

Elongation
 Is undertaken by another complex of proteins.
 The replisome exists only as a protein complex associated with the
particular structure that DNA takes at the replication fork.
 It does not exist as an independent unit (for example, analogous to the
ribosome).
 As the replisome moves along DNA, the parental strands unwind and
daughter strands are synthesized.
81

Termination
 At the end of the replicon, joining and/or
termination reactions are necessary.
 Following termination, the duplicate
chromosomes must be separated from one
another, which requires manipulation of higher-
order DNA structure.
82

Model of DNA replication
Three models of replication:
1/ Conservative model : new/new + old/old
Two old strands in one daughters and
Two newly synthesized in the second daughters
The parental double helix remains intact and a new copy is made

DNA replication…
2/ Semi conservative : new/old + old/new
 Each daughter DNA molecule consist one old and one newly
synthesized segment of DNA
 Accepted one
 The two strands of the parental molecule separate and each functions as
a template for synthesis of a new complementary strand

DNA replication…
3/ Dispersive
 The parent DNA is broken into pieces
 The resultant DNA molecule will have one of the old and one new
synthesized segment at one strand
 Each strand of both daughter molecules contains a mixture of old and
newly synthesized parts

DNA replication model: Meselson-Stahl Experiments
 Labeled the nucleotides of old
strands with a heavy isotope of
nitrogen (15N), new nucleotides
were indicated by a lighter
isotope (14N).
 The first replication in the 14N
medium produced a band of
hybrid (15N-14N) DNA, eliminating
the conservative model.
 A second replication produced
both light and hybrid DNA,
eliminating the dispersive model
and supporting the
semiconservative model.
Bacteria
cultured in
medium
containing
15N
DNA sample
centrifuged
after 20 min
(after first
replication)
DNA sample
centrifuged
after 40 min
(after second
replication)
Bacteria
transferred to
medium
containing
14N
Less
dense
More
dense
Conservative
model
First replication
Semiconservative
model
Second replication
Dispersive
model
91

Semi-Conservative DNA Replication
92

DNA Replication Process
DNA replication- three steps
 Initiation
Recognize the starting point, Ori
 Separation of dsDNA
 Primer synthesis
 Elongation
 Add dNTPs to the existing strand
 Formation of phosphodiester bonds
 Correct the mismatch bases
 Extending the DNA strand
 Termination
 Stop the replication

Cytoplasmic inheritance and Maternal Effect

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