Ok so I've read a lot of threads on Sage Mode and someone has yet to explain it properly.
Sage Mode is a form achieved through taking in Natural Chakra and being able to balance it within the body, allow with ones own chakra (spirit and body). A perfect balance will result in a perfect Sage Mode, which as you know is achieved by Naruto. An imperfect balance achieved can still result in a Sage Mode, however the user takes on animal features as seen with Jiraiya. Inability to control Natural Chakra result in the user turning into an animal (in Naruto's Case), then stone.
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Once Sage Mode is achieved the user can use the Natural Chakra to perform Senjutsu (Sage Techniques).
MISCONCEPTIONS
Where a lot of you "READERS" are getting confused is with the information mention today.
It was previously known that Juugo is the original source of the Curse Seal and he can transform into this creature. Orochimaru took his DNA and created an enzyme (protein catalyst) that allows bearers to take on the same transformation as Juugo.
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We learn today that the transformation is a result of the bodies ability to take in Natural Chakra. Juugo and members of his clan can take in Natural Chakra, causing them to transform. However once they transform they go berserk.
What Orochimaru tried to discover was why they went berserk and how they acquired this ability. Orochimaru later discovered the source is the Ryuchidou (dragon cave).
What this means is that the transformation and the berserk nature is caused by taking in Natural Chakra, without fully balancing their own Chakra with the Natural Chakra.Their transformation is the same as turning into a frog, like nature did when he could not control the Natural Chakra. The source of their transformation is Ryuchidou, meaning that the members of Juugo's Clan are Dragon Sages, who could not achieve perfect Sage Mode. This would explain their berserk nature and animal characteristics. Normally users would absorb Natural Chakra through meditation however, Juugo's Clan's ability allows them to take in Natural Chakra without doing so.
Orochimaru attempted to acquire perfect Sage Mode, however he did not have ability to absorb Natural Chakra like Juugo could. Therefore he developed the Curse Seal to allow the bearer to mimic Juugo's causing the bearer to mimic Juugo's transformation. Since Kabuto said the Curse Seal was fake, I would assume that means the Curse Seal allows the transformation to occur, however it is a result of Orochimaru's Chakra, not Natural Chakra. I came up with this conclusion because the Curse Seal is merely an Enzyme (speeding up the transformation) ,made from Juugo's DNA and it contains Orochimaru's Chakra (Kabuto stated that, when he Kidnapped Anko) meant to mimic Juugo's transformation.
The Curse Seal was Orochimaru's way of searching for a body compatible with Juugo's ability to transform, so he could implant them with Juugo's ability and take over the body, which would allow him to absorb Natural Chakra so he could acquire Sage Mode later in Ryuchidou. Kabuto basically took on Orochimaru's plans and did everything Orochimaru couldn't do. Pretty impressive, but Orochimaru's research skills and experimental hypothesis are pretty amazing. Props to hims, screw Kabuto for mooching off him lawl
Just like Naruto, Kabuto acquires features from a dragon, which I would assume are the horns, since he implanted Orochimaru's cells in him, resulting in the same eyes and Orochimaru wasn't able to acquire Sage Mode.
Hope you losers have a better understanding, and be sure to actually read properly before you put up stupid threads