What two organelles are believed to be remnants of unicellular bacteria?

Mitochondria and chloroplast.

Explanation:

Endosymbiotic theory says that mitochondria and chloroplast are the two organelles which earlier existed as independently living unicellular bacteria. Mitochondria is believed to be remnant of a gram negative bacteria known as Rickettsia prowazekii while chloroplast is believed to be remnant of a cyanobacteria.

According to this theory, in past a eukaryotic cell engulfed these organelles and since then they become an integral part of these eukaryotic cells.

The reason behind this is that both these organelles have prokaryotic characteristics like "presence of 70S ribosome & circular DNA" and they also duplicate themselves by binary fission just like unicellular bacteria.

Apart from this, Rickettsia prowazekii is able to produce ATPs just like mitochondria does in eukaryotic cells. Also, Cyanobacteria and chloroplast are able to perform photosynthesis in a similar way.

In addition to this, the genome sequence similarity between mitochondria & Rickettsia prowazekii and Cyanobacteria & chloroplast further fortifies endosymbiotic theory.

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step 1: initiation

initiation is the beginning of transcription. it occurs when the enzyme rna polymerase binds to a region of a gene called the promoter. this signals the dna to unwind so the enzyme can ‘‘read’’ the bases in one of the dna strands. the enzyme is now ready to make a strand of mrna with a complementary sequence of bases.

step 2: elongation

elongation is the addition of nucleotides to the mrna strand. rna polymerase reads the unwound dna strand and builds the mrna molecule, using complementary base pairs. there is a brief time during this process when the newly formed rna is bound to the unwound dna. during this process, an adenine (a) in the dna binds to an uracil (u) in the rna.

step 3: termination

termination is the ending of transcription, and occurs when rna polymerase crosses a stop (termination) sequence in the gene. the mrna strand is complete, and it detaches from dna.