My English teacher always says that an analytical paragraph is like an umbrella. The topic sentence is the bit right at the top, grouping everything together.
The Topic Sentence is essentially just a sentence that sketches out your overall ideas before you add line-art, colours and highlights, shadows and tiny details.
Topic sentences are STRICTLY not allowed to include devices, quotations or anything that can be used later. You don't start highlighting a sketch, do you?
A good example of a topic sentence, on the topic: How Has Scrooge Changed In the Course of The Novella?
[I'm using this book because it's very well known and most high schoolers have done it or will do it before the end of Year 9]
This paragraph is focused on Stave 1, and so we need a topic sentence to umbrella out all our ideas:
In Stave 1, Dickens has portrayed Scrooge as a miserly, stingy and isolated character who is reluctant to spend both his time and his money on others.
And we would back this up with the quotations,
'hard and sharp as flint' and 'solitary as an oyster'