Mitochondrial DNA

1. L H Mitochondrial DNA YILDIRIM BEYAZIT UNIVERSITY FACULTY OF MEDICINE THE DEPARTMENT OF MEDICAL BIOLOGY ASST. PROF. DR. ENDER ŞİMŞEK

2. Mitochondria • Each cell contains hundreds to thousands of mitochondria.

3. Mitochondria • membrane-bound organelle • two membranes: outer and inner. • Folds of the inner membrane, where most of oxidative phosphorylation occurs, are called as cristae. • Inside of the inner membrane = matrix. • The space of outer and inner membranes = intermembranespace. • Mitochondrial DNA is located in the matrix.

4. The Endosymbiotic Theory • Mitochondria originated as separate prokaryotic organisms which were taken inside the cell as endosymbionts (any organism that lives within the body or cells of another organism).

5. The Endosymbiotic Theory • Mitochondria originated as separate prokaryotic organisms which were taken inside the cell as endosymbionts (any organism that lives within the body or cells of another organism). • It is believed that mitochondria developed from the order of proteobacteria, Rickettsiales.

6. The Endosymbiotic Theory • Mitochondria originated as separate prokaryotic organisms which were taken inside the cell as endosymbionts (any organism that lives within the body or cells of another organism). • It is believed that mitochondria developed from the order of proteobacteria, Rickettsiales. • This has mostly likely occurred as a result of phagocytosis, the process of a cell engulfing it's food and creating a food vacuole (phagocyte); in this case, the food vacuole was undigested.

7. Endosymbiosis

8. Evidence for Endosymbiosis • mtDNA is circular; it resembles a nucleoid (bacterial DNA in the form of a single circular chromosome). • Mitochondria and bacteria both reproduce via binary fission (division in half). • Mitochondria are semi-autonomous (the nucleus contains genes which originated from mtDNA).

9. Evidence for Endosymbiosis • mtDNA is circular; it resembles a nucleoid (bacterial DNA in the form of a single circular chromosome) • mitochondria and bacteria both reproduce via binary fission (division in half). • mitochondria are semi-autonomous (the nucleus contains genes which originated from mtDNA) • Mitochondrial ribosomes resemble bacterial ribosomes. • Mitochondria have a double membrane, possibly created through phagocytosis. • The cristae of mitochondria resemble the infoldings of bacteria, where cellular respiration occurs.

10. Mitochondrial DNA • Mitochondrial DNA (mtDNA) codes for proteins and enzymes used by the mitochondria.

11. Nuclear DNA found in nucleus of the cell 2 sets of 23 chromosomes maternal and paternal double helix bounded by a nuclear envelope DNA packed into chromatin Mitochondrial DNA found in mitochondria of the cell each mitochondria may have several copies of the single mtDNAmolecule maternal only circular free of a nuclear envelope DNA is not packed into chromatin Nuclear DNA vs. Mitochondrial DNA

12. Nuclear DNA vs. Mitochondrial DNA

13. The Mitochondrial DNA • 16,569 base pairs (bp) in length • encodes 37 genes(13 proteins, 22 tRNAs, and 2 rRNAs)

14. The Mitochondrial Genome Consists of 2 different regions: • coding region: responsible for "the production of various biological molecules" involved in cellular respiration • control region: responsible for "the regulation of the mt DNA molecule" • “contains little non-coding DNA (“junk” DNA, or introns)” 610 bp with a lot of variability

15. Control Region: This region is highly variable within the human population. Consists of 2 sub-regions: HV1 and HV2. HV1 = 342 bp HV2 = 268 bp 610 bp with a lot of variability

16. mtDNAis inherited from MOTHER! • Every sibling will get their mt DNA from their mother.

17. Mt DNA is inherited from MOTHER! • Every sibling will get their mt DNA from their mother. • WHY???

18. Why Mom? • Egg contains 23 chromosomes and cell cytoplasm which contains thousands of maternal mt • Sperm contains 23 chromosomes with very little cytoplasm

19. Maternal Inheritance of mtDNA • When egg and sperm join, only female mt survive and are passed onto to new baby.

20. Maternal Inheritance Pattern with Mt DNA (Mutations occur in the control region of mt DNA at a regular rate and are passed onto children by the mom.)

21. Heteroplasmy: Heteroplasmy is the presence of a mixture of more than one type of an organellargenome (mtDNA) within a cell or individual. It is a factor for the severity of mitochondrial diseases.

22. mtDNA • mtDNA is maternally inherited in animals and plants. • Most of the mitochondrial complete sequences are belonged to animals and algae. • Plant mtDNA is far more complex than animal mtDNA – larger size (300-600kb) and variable in size (up to 2000kb).

23. Plant mtDNA • In addition to larger size, plant mtDNA are characterized by molecular heterogeneity. • Plant mtDNAs contain at least one large (1-14kb) repeated sequence.

24. Plant mtDNA Tricircular structure of the Brassica campesteris(cabbage) mitochondrial genome

25. Plant mtDNA • Plant mtDNAs change very slowly in nucleotide sequences. • Plant mtDNAsre-arrange very rapidly.

26. Animal mtDNA • Animal mtDNAs are relatively smaller than the plant mtDNAs. • The genome size is more conserved.

27. Animal mtDNA • No-recombination. • Contains less non-coding sequences. • Higher base substitution rate (even higher than the nuclear DNA regions). • Gene order/structure is more conserved (stable).

28. Human Mitochondrial Genome Human mtDNAis composed of a control region (CR), genes encoding 2 rRNAs(12S and 16S), 22 tRNAs (open circles), 13 polypeptides. Human mitochondrial genes contain no introns, although introns are found in the mitochondria of other groups (plants, for instance).

29. Mitochondrial Genes • Genes: total of 37. 22 tRNAs, 2 rRNAs, 13 polypeptides. • tRNA: only 60 of the 64 codons code for amino acids. • rRNA: 16S and 23S which are standard sizes for bacterial rRNAs. • polypeptides: all are components of the electron transport chain.

30. Genome Structure L H • The mitochondrial genome is a circle, 16.5 kb of DNA.

31. Genome Structure L H • The two (H and L) strands are notably different in base compositionleading to one strand being “heavy”(rich in G) and the other one is “light” strand (rich in C).

32. Both strands encode genes, although more are on the H strand. • A short region, the D loop (D: “displacement”), is a DNA triple helix: there are 2 overlapping copies of the H strand there.

33. The D loop is also the site where most of replication and transcription is controlled. • Genes are tightly packed, with almost no non-coding DNA outside of the D loop.

34. Mitochondria have independent replication, transcription, and translation systems.

35. Replication • Replication occurs unidirectionally, unlike replication of nuclear DNA, which proceeds in both directions simultaneously.

36. Replication of mtDNA: The mitochondrial genome is replicated in two phases. L H

37. Replication of mtDNA: The replication starts at the heavy-strand replication origin and continues clockwise around the mtDNA. L H

38. Replication of mtDNA: When the light-strand replication origin is exposed as a single strand, the second strand is replicated in the opposite direction, starting from the light-strand replication origin. L H

39. Replication

40. Transcription of mtDNA: L H • Transcription of H and L strands is from promoters in the triple stranded, non-coding D-loop region of the genome. • Transcription from Heavy strand is CLOCKWISE. • Transcription from Light strand is ANTICLOCKWISE

41. Transcription • Both strands are transcribed. • The D loop contains one promoter for each strand, and the entire strand is transcribed. L H

42. Transcription • The RNA is then cut into individual RNAs for each gene. • Protein-coding genes are given poly-A tails, and rRNA and tRNA molecules are modified as necessary. L H

43. Genetic Code • The mitochondrial genetic code has drifted from the universal code. • There are so few polypeptides that changes in the code are tolerated. • Human mitochondrial code is different from other groups such as plants or fungi.

44. Genetic Code • Uses 2 of the 3 universal stop codons, but also uses 2 other codons as stop codons. • Also, UGA codes for tryptophan in the mitochondrial, while it is a stop codon in the universal code. • AUA gives methionine in the mitochondria instead of isoleucine.

45. Mitochondrial DNA mutations in human genetic disease