When we eat raw meat, e.g. chicken or fish, we are actually consuming the DNA, proteins etc. which are present in their cells.
Wouldn't this affect our cell functions as this DNA might enter our cells?
It is known that we get energy by eating them. Is it glucose or ATP or some other form of energy that is produced from them?
Yes, we are eating the DNA and proteins of an organism when we consume and digest its cells.
However, consuming the "raw flesh of hen or fish" in your example would not cause harm strictly due to the consumption of their DNA.
DNA has two basic components - a structural 'backbone' of alternating phosphate molecules and sugar molecules (called deoxyribose) and a nitrogenous base (a group of atoms that includes nitrogen) on each backbone unit. These basic components of DNA are common to the all organisms that contain DNA, like the hen and fish in your example.
When you digest the DNA in your food, specialized molecules (in this case nuclease, which digests DNA) produced by your digestive system break down DNA into its components. These components can then be rearranged to create 'human' molecules, such as human DNA.
In other words, you don't integrate fish or hen DNA directly into your human DNA; you break it down and rearrange it into human DNA, then integrate it into your body.
To answer your second question, we get energy by breaking down the proteins, carbohydrates, and fats (known as macromolecules) of other organisms. Much like DNA, these three macromolecules are broken down into their singular components. For carbohydrates, this would be glucose.
These components are then further broken down into energy (as a force) which powers cellular machinery to 'recharge' ADP (the 'discharged' version of ATP) back into ATP, a specialized molecule which can 'bring energy' to other parts of the body to power your body's processes.
ATP is not energy per se - energy is defined as the "capacity to cause change" (Campbell Biology, pg. 160) and do work. Energy is therefore a property rather than a distinct molecule.
Summary:
The second part of your question has been answered already, but you were right about your first point. I believe the previous answers are not completely correct concerning the total break down of nucleotide sequences.
It has been shown that RNA from ingested food (specifically rice miRNA) actually does enter mammalian cells and tissues and can influence gene expression [1]. A number of exogenous miRNAs from common foods where observed in Human blood plasma in this study [2].
I believe we still have to learn a lot about this kind of interaction with our food. It might be that miRNA is not the only component of our food that can survive digestion and influence human cells directly.
To add to adam's answer:
Protein and DNA molecules are simply too big to pass through cell membranes or through the wall of the gut.
In the process of digestion:
Proteins are broken down into amino acids.
Complex carbohydrates like starch are broken down into sugars.
Fats are broken down into fatty acids and glycerol.
These molecules are small enough to pass through the wall of the gut. These are essential food. Once through the wall of the gut, the body synthesizes "human" proteins, carbohydrates and fats from them.
DNA and RNA are broken down into nucleotides, which are small enough to pass through the wall of the gut. The mass of DNA is tiny compared to the mass of the above types of food. As far as I know nucleotides are not essential nutrients (that is to say, I believe our bodies can synthesize them from the above types of food.)
Incidentally one of the nucleotides of RNA, adenosine monophosphate (AMP) is the "discharged" form of ATP. According to https://en.wikipedia.org/wiki/Nucleotide all four of the nucleotides of RNA are used in monophosphate - triphosphate energy cycles, but the others are more specific.
It's important to remember that proteins, fats, sugars (and ATP) are not "energy" but simply molecules that are high in energy.
I would guess that there is far more of these RNA nucleotides free in the cell (involved in energetic pathways) than there is in the actual RNA used for protein transcription. And there is definitely more RNA than DNA.
