Unicellular functioning molecules inside multicellular cells

[icakeov]

When cells within multicellular organisms started differentiating/specializing, was this handled by a new set of chromosomes developing these functionalities, leaving the DNA that is in charge of unicellular functionality more or less consistent and similar? Or did the original chromosome develop multicellular features in a major way?

Or in other words, I am wondering whether the "unicellular functionality" (or "unicellular DNA") was retained within multicellular cells and did the differentiation functionality mainly get added as a "new feature", saved in the newly added chromosomes. That one chromosome that all unicellular organisms originally had - is that chromosome still "similar" between all cells, no matter if they are unicellular or multicellular, or is there no rule of thumb to this?

Basically, how different are "housekeeping genes" in eukaryotes (either unicellular or multicellular) from prokaryotes? For example, I am guessing that a stem cell needs to become a “proper cell” with all its regular functions before it gets differentiated. Or: how similar is a pluripotent cell to a unicellular cell?

This was a bit of a hard question to formulate, hope it made sense.

 

[jim mcnamara]

Mitochondria have DNA in a plasmid, a sort of ring, like bacteria. These are the only organelles that have their own DNA. Their DNA is handed down asexually (somatic is the term) - no meiosis, no crossing over. You got all of your Mom's mitochondrial DNA just as it exists right now in Mom. So if Mom has defective DNA in there, so do you - 100% (or as close to that as it gets in Biology) guarantee that your DNA matches Mom's. You got your mitochondria from her oocyte.

So, organelle may be the term you want. Otherwise I am not clear what you are trying to ask. Every cell in your body has virtually identical copies. Any cells or tissue with alternate DNA arose from what is called a chimera, which is uncommon in humans, very common in plants.

https://en.wikipedia.org/wiki/Chimera_(genetics)

 

[BillTre]

Mostly, genes, genomes and chromosomes are not modified during development from the single cell fertilized zygote (for animals). Plants and fungi may have different names for this.

However, here are some exceptions I can think of:
1) human (mammalian) red blood cells lose their nucleus and so have extremely modified genetics. However they don't need them anymore. They are terminally differentiated, have a limited lifespan and don't have to make anymore proteins.
2) antibody producing cells (B-cells) in the vertebrate immune system rearrange their antibody genes to produce transcripts that make a vast variety of different antibody molecules so they can defend the body from a huge variety of different invaders. T-cells do something similar with the genes for their antibody like cell surface receptors.
3) Drosophila salivary gland cells duplicate their chromosomes thousands of times (presumably to increase the copy number of genes used by the salivary gland).
4) There are other examples like this, but they are not the rule.

Another kind of change affecting genetic expression is methylation, of either nucleotides directly or of histones associated with specific sequences. Methylation (as I recall) suppresses gene expression. Other covalent modifications of histones can occur as well as many proteins that bind to histones.

Mostly however, genes are regulated by proteins produced by other genes that bind to specific sequences an activate on inactivate gene transcription. There are also controls affecting mRNA splicing, cutting of proteins into smaller parts, controlling protein production at ribosomes, and probably other mechanisms I am not remembering right now. All these can affect the overall function of cells.

These various regulatory mechanisms become more elaborated as development proceeds and an increasing number of cell types develop. As more kinds of cells develop, the possible interactions among the cells increase which can result in a large variety of different signals between cells that can trigger different developmental processes. This results in the development of a huge variety of kinds of cells.

 

[Ygggdrasil]

Not sure I have a complete answer for you, but here are some thoughts on the topic:
1) Multicellularity has evolved multiple times in independent lineages, so the exact molecular and genetic mechanisms enabling multicellularity are going to be different in different types of organisms (e.g. plants vs animals).

2) Eukaryotes evolved from a fusion between a bacterium (which became the mitochondria) and an archaeon (a domain of unicellular prokaryotes distinct from bacteria). In general, many of our metabolic genes are from the bacteria and many of our "informational" genes (e.g. those involved in DNA, RNA and protein synthesis) are from the archaeon.

3) Thus, many of the protein encoding basic cellular functions (DNA, RNA, and protein synthesis, metabolism, etc) are evolutionarily conserved between prokaryotes and eukaryotes. Further, many of the genes encoding basic cellular functions specific to eukaryotes (e.g. cell division, organelle function, membrane trafficking) are evolutionarily conserved between single-celled eukaryotes (like baker's yeast) and multicellular eukaryotes (like plants and animals).

4) It's not entirely clear which genes are responsible for the evolution of multicellularity. Obvious choices include genes involved in cell adhesion and intercellular signaling, some of which pre-dated the evolution of multicellularity (see below).

5) Many of the changes involved in the evolution of multicellularity involves the evolution of non-coding regulatory DNA sequences. In a multicellular organism, cells are specialized so one cell in the organism will only express a specific subset of the genes encoded in its genome. Thus, multicellular organisms had to develop highly sophisticated mechanisms for controlling gene expression to allow for cells to differentiate into different cell types. Single-celled organisms also face similar challenges when changing their gene expression programs to adapt to different environments, so some of the regulatory machinery likely evolved from this starting point. (For example, many of the signaling pathways in single-celled yeasts that allow the yeast to adapt to various environmental stresses are the same pathways involved in cell differentiation and development in humans).

 

[icakeov]

Thanks for all the responses! Yes, I was going more in the direction of how much the multicellular organisms percentually contain ancient unicellular chromosomes, and in which chromosomal areas, rather than whether they express themselves as unicellular at any point (just the fact that red blood cells turns out don't even have a nucleus shows that they can express in so diversely, crazy!).
Basically, the inquiry is whether the ancestral chromosomes of the archaeon that multicellular cells come from are still somewhat "intact", producing similar functionalities, or whether they would by now be unrecognizable, mixed in with the rest of the chromosomes, producing the functionalities they produce.
This is a bit of a hard one, apologies :)