Extranuclear inheritance

Question 1

What is extranuclear inheritance determined by?

Answer 1

Determined in mitochondria & chloroplast
 self-replicating, double-membraned organelles inside cells
 contain their own DNA, called mitochondrial mtDNA and chloroplast cpDNA

Question 2

Structure of chloroplast

Answer 2

Outer membrane
Inner membrane
Stroma
Thylakoid

Question 3

Structure of mitochondrion

Answer 3

Outer membrane
Inner membrane
Cristae
Matrix

Question 4

How are extra-nuclear genomes structured?

Answer 4

-cp & mtDNA is circular, supercoiled, double-stranded
-stored in condensed nucleoprotein complexes called
nucleoids
-100s to 1000s copies per cell

Question 5

mtDNA is in matrix of the mitochondria

Answer 5

• Each nucleoid contains 4-6 mtDNA molecules
• There are 10-30 nucleoids in each mitochondria
• There are ~40 mitochondria/cell
   6,750 copies of mtDNA molecules per cel

Question 6

Mammalian mtDNA genome:

Answer 6

Genome is circular & supercoiled
Genome size is ~ 17 kbp
Genes coded on + and – strand
37 genes with NO introns

Question 7

Evidence for Endosymbiotic Theory

Answer 7

Mitochondria & chloroplasts are double-membraned organelles that replicate independently by binary fission like bacteria

Question 8

DNA Structure like Bacteria:

Answer 8

Chloroplasts & mitochondria have their circular DNA
* not organised into nucleosomes by histones
* functionally related proteins close together & often expressed as a single unit
(polycistronic transcripts)

Question 9

Translation Machinery like Bacteria

Answer 9

Ribosome particles have subunits the same size as prokaryotes
* Chloroplast ribosomes have similar sequences to cyanobacteria & E. coli
* MT use N-formyl methionine and tRNAfmet in translation
* Animal MT = different codon usage for translation
* Inhibitors of bacterial translation have same effect on mitochondrial translation, but not eukaryotic cytoplasmic protein synthesis

Question 10

DNA code

Answer 10

Plant mitochondria and chloroplasts have the universal genetic code
 BUT animal mitochondria have different codon usage for translation:
 UGA is NOT a STOP codon
 AGA & AGG code for STOP codons NOT Arginine
 AUA code for Methionine NOT Isoleucine

Question 11

Cyto-nuclear Coevolution

Answer 11

Most eukaryotic cells cannot survive without mitochondria & vice versa
 Size of the mtDNA & cpDNA is much smaller than bacteria & cyanobacteria genome
* Ie cpDNA is 3–5% as large as cyanobacterial genome
 Organelle evolution was accompanied by
rearrangement & transfer of genes to nucleus = cytonuclear coevolution

Question 12

Cyto-nuclear Cooperation

Answer 12

mt/cpDNA encodes only a fraction of genes they need for photosynthesis & respiration
Genes needed for mt/cp replication, transcription, translation & function come from BOTH nuclear & mt/cpDNA

Question 13

How are extra-nuclear genomes replicated?

Answer 13

Nuclear DNA replication is carried out by several enzymes encoded by nDNA specifically during S-phase of the cell cycle
- Helicase, SSBPs, Primase, DNA Pol, Ligase
Extra-nuclear genomes replicate randomly by rolling circle replication using different enzymes encoded by nDNA at any stage of cell cycle
- POLG, Twinkle & mtSSB

Question 14

How are extra-nuclear genomes replicated?

Answer 14

More replication errors
 high replication rate
 less efficient repair mechanisms
 lack of protective histones
 proximity to ROS
 High copy number
 High degree of wobble base pairing
 Compensatory mechanisms (fission/fusion)
 Controlled degradation (mitophagy)
= means that not all mutations will impact function
 NB genes for mtDNA replication & transcription are in the nucleus where the mutation rate is lower

Question 15

How are extra-nuclear genomes inherited?

Answer 15

-mtDNA is MATERNALLY inherited in higher animals because the egg contributes the cytoplasm to the zygote
-Degradation of organelles in mal gametes (zygote destroys paternal organelle after fertilization)
-Paternal organelles are distributed to cells that are NOT destined to become part of the embryo in some
plants

Question 16

Some mtDNA mutations result in disease

Answer 16

-Decreased ATP production affects the function of muscle & nerve cells;
leads to blindness, deafness, stroke etc
-These mtDNA diseases can be lethal, or can be variable in their severity
-Because tissues can have different proportions of normal vs. abnormal mtDNA = heteroplasmy

Question 17

Heteroplasmy

Answer 17

When there are two different types of mtDN present in the same cell

Question 18

Inherited Mitochondrial Diseases

Answer 18

When you have heteroplasmy, the ratio of
normal: mutant mtdna in each
Mitochondria are determined by mitochondrial replication, fission and fusion
 When mitochondria replicate, they first
double in size, then divide in half
 Mitochondria can also fuse or divide to
dilute mutant mtDNA

Question 19

With heteroplasmy, the severity of the disease phenotype reflects:

Answer 19

1. ratio of disease: normal mtDNA in same cell
2. which cell type has the highest proportion of mutated mtDNA
   Inherited Mitochondrial Diseases
3. clinical threshold for each mutation

Question 20

Identical twins don’t always have
the same mtDNA or disease phenotype

Answer 20

If the mother is heteroplasmic ,then the severity of the disease phenotype in each twin depends on:
1. How the mutant mtDNA is partitioned after fertilisation
2. Which tissue receives the mutation during development