[susan53]

The use of "the" indicates that the writer sees the concept as shared knowledge - ie the reader and writer both already know that the lectures and tutorials exist. So in this case there would probably have been a preceding sentence like : The course consists of lectures and tutorials. Thus, when you get to the second sentence, the concept of "lectures and tutorials" is already present and therefore "shared :
I]English 101 : The course consists of lectures and tutorials. Attendance at both the lectures and the tutorials is required[/i] (Obviously, the sentences need not be consecutive).
Alternatively, the concept may not be in the discourse at all, but by using the definite article the writer presupposes that we already know the course includes lectures and tutorials. That's what happens when we say eg There was an eclipse of the sun today. I presuppose you know a) that the sun exists and b) that you will assume that I am talking about the sun in our solar system rather than in some other. I don't have to include the knowledge in the discourse because it's already assumed to be shared.

If, on the other hand, the writer omits the article, then s/he is probably regarding "lectures and tutorials" as new (unshared) information - eg if the text ran :English 101 : The course will run from October 25th - June 10th. Attendance at both lectures and tutorials is required.

Notice that this can only be decided by seeing the full context of the sentences. it is never possible to comment on grammar from sentences in isolation.

Alternatively, there is the specific/generic distinction. If the writer uses "the", then s/he has in mind Attendance at the lectures and tutorials which are included in the course is required - ie s/he's talking about a specific series of lectures/tutorials, which may be defined afterwards in the relative clause, or (more likely) may just be something that s/he assumes you recognise to be the case - ie the writer is thinking "I know my readers know that I'm talking specifically about the lectures and tutorials included in this course. It's not necessary to say so explicitly." If the article is omitted, s/he sees the concept as generic - lectures and tutorials in general

Always remember that grammar provides the means of expressing different concepts. Which concept is expressed is entirely up to the writer - you may regard something as specific while I regard it as generic; you may assume information is shared while I want to reactivate it and therefore present it as new. The grammar chosen reflects only the "mindset" of the speaker/writer - not objective fact. Nine times out of ten there will be different ways of viewing an event/concept etc, and therefore a choice of the grammatical forms used to express it.