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Mitochondrial DNA Replication and Regulation

Mitochondrial DNA (mtDNA) is crucial for cellular energy production, replicated by DNA polymerase gamma. Its replication is tightly controlled during embryogenesis to reduce mutation transmission. Human mtDNA contains 37 genes, with transcription varying by tissue, reflecting energy needs. The heart has the highest mitochondrial gene expression, influenced by factors like ACTH.

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1

The catalytic subunit of the enzyme complex is encoded by the ______ gene and has a molecular weight of ______.

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POLG 140 kDa

2

The accessory subunits of the enzyme complex, each weighing ______, are encoded by the ______ gene.

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55 kDa POLG2

3

______ helicase assists in DNA unwinding during replication by moving in a ______ direction.

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TWINKLE 5' to 3'

4

Despite their role in mitochondrial function, the genes for these proteins are located in ______ DNA, not the mitochondrial ______.

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nuclear genome

5

Mitochondrial bottleneck purpose

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Minimizes transmission of deleterious mtDNA mutations to offspring.

6

mtDNA replication resumption location

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Initiated in trophectoderm cells of the blastocyst.

7

Inner cell mass mtDNA replication timing

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Postponed until differentiation into specific cell lineages.

8

The ______ strand of mtDNA is guanine-rich and codes for most proteins in the ______ system.

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heavy (H) oxidative phosphorylation

9

The heavy (H) strand of human mtDNA encodes ______ rRNAs and ______ tRNAs.

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two 14

10

The light (L) strand of mtDNA encodes ______ protein subunit and ______ tRNAs.

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a single 8

11

In total, mtDNA codes for ______ rRNAs, ______ tRNAs, and ______ protein subunits vital for oxidative phosphorylation.

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two 22 13

12

The amount of mtDNA varies greatly among cell types, with over a ______ in oocytes and none in ______.

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million red blood cells

13

Mitochondrial transcription initiation site

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Initiates at mitochondrial displacement loop (D-loop).

14

Mitochondrial heavy-strand promoters

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Two types: HSP1 for rRNA transcription, HSP2 for polycistronic transcripts.

15

Mitochondrial transcript processing

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Involves cleavage at tRNA punctuation, forming individual mRNA, rRNA, tRNA.

16

The highest expression of mitochondrial genes is found in the ______, followed by the ______ and ______ tissues.

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heart brain steroidogenic

17

______ can affect the expression of genes within mitochondria, as seen with ______ influencing adrenal cortex cells.

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External factors ACTH

18

Mitochondrial gene expression in tissues is complexly controlled and can be altered by ______ like ______.

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hormones adrenocorticotropic hormone

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Mitochondrial DNA Replication and Its Enzymatic Machinery

Mitochondrial DNA (mtDNA) plays a pivotal role in cellular energy production, being replicated by a dedicated enzyme complex known as DNA polymerase gamma. This complex is composed of a catalytic subunit of 140 kDa, encoded by the POLG gene, and a pair of 55 kDa accessory subunits, encoded by the POLG2 gene. These components assemble into the replisome, which also includes TWINKLE helicase and mitochondrial single-stranded DNA-binding proteins (SSB). TWINKLE helicase unwinds the DNA in a 5' to 3' direction, facilitating replication. It is important to note that the genes encoding these proteins are found in the nuclear DNA, not within the mitochondrial genome itself.
Detailed model of mitochondrion with salmon pink outer membrane, dark pink inner cristae, blue DNA and gray ribosomes on a neutral background.

Regulation of Mitochondrial DNA Replication During Embryogenesis

The replication of mtDNA is stringently regulated during embryogenesis. There is a marked suppression of mtDNA replication from the stage of the fertilized oocyte to the preimplantation embryo. This decrease in mtDNA copy number is part of a phenomenon known as the mitochondrial bottleneck, which serves to minimize the transmission of deleterious mutations to offspring. The resumption of mtDNA replication is initiated in the trophectoderm cells of the blastocyst, whereas the inner cell mass cells postpone mtDNA replication until they differentiate into specific cell lineages, in response to developmental cues.

Structure and Gene Content of Human Mitochondrial DNA

Human mitochondrial DNA is a small, circular molecule comprising 37 genes, which are transcribed from both the heavy (H) and light (L) strands. The H strand, which is rich in guanine, encodes the majority of the proteins involved in the oxidative phosphorylation system, two ribosomal RNAs (12S and 16S rRNA), and 14 transfer RNAs (tRNAs). Conversely, the L strand encodes a single protein subunit and 8 tRNAs. Altogether, mtDNA provides the genetic code for two rRNAs, 22 tRNAs, and 13 protein subunits that are essential for the oxidative phosphorylation pathway. The copy number of mtDNA varies widely among different cell types, from none in red blood cells to over a million in oocytes.

Transcription and Processing of Mitochondrial Genes

Mitochondrial genes are transcribed as polycistronic transcripts, which are subsequently processed into individual mRNA, rRNA, and tRNA molecules. This processing involves the precise cleavage of transcripts at the tRNA punctuation marks, which are recognized by their unique L-shaped structures. The transcription process initiates at the displacement loop (D-loop) of the mitochondrial genome. Two distinct promoters, heavy-strand promoter 1 (HSP1) and heavy-strand promoter 2 (HSP2), drive the transcription of rRNAs and polycistronic transcripts, respectively.

Differential Expression of Mitochondrial Genes in Tissues

The expression of mtDNA-encoded genes varies significantly across tissues, correlating with the metabolic demands of each tissue type. The heart exhibits the highest level of mitochondrial gene expression, followed by the brain and steroidogenic tissues. This tissue-specific expression pattern indicates that mitochondrial gene expression is intricately regulated to meet the energy requirements of each tissue. Furthermore, external factors such as hormones can influence mitochondrial gene expression. For instance, adrenocorticotropic hormone (ACTH) stimulates the expression of mitochondrial protein-encoding genes in the cells of the adrenal cortex, demonstrating the responsive nature of mitochondrial gene regulation to physiological needs.