When Margaret Atwood wrote Oryx and Crake in 2003, most of the inventions in the book were, to the non-scientist's eye, relatively outlandish: cross-species gene-splicing; growing meat in a petri dish; man-made pandemics. Ten years later, with the publication of MaddAddam, the last book in the trilogy, they are simply part of the news cycle. Atwood has an uncanny ability to second-guess developments in biotechnology, although frankly, she thinks, if they had any sense, scientists would look to her books and do more.

"Mohair sheep, they haven't done that yet," she says in the Atwoodian tone, a kind of steely levity. "I think it would be quite a good commercial venture. You can imagine a lot of people wanting to get their own DNA hair." The 73-year-old smiles, thinly. "I'm offering it as a free gift to the world."

We're in Toronto, where Atwood has lived for much of her adult life. Much of what she says comes out lightly crusted in sarcasm. A pleasure of her books is the drive-by assassination, and in MaddAddam no one is safe: the religious maniacs, the corporate monsters, the leftwing do-gooders. Of a well-meaning but fundamentally misguided environmentalist group, she writes: "It lived off the good intentions of city types with disposable emotions who liked to think they were saving something." When the charity launches an ad campaign to save drowning polar bears it features "a smiling green furfucker telling everyone what a sterling lot of good Bearlift was doing, and please send more cash or you'll be guilty of bearicide." The evocation of the landscape is, as one would expect, secondary to that of the language.

Now everyone is doing it, but when Atwood first wrote science fiction it seemed an eccentric departure, a position with which she has always been comfortable. Ever since the early 1970s when she was asked how she coped with being a novelist and doing the housework ("I would say, look under the sofa"), she has been aggressively indifferent to criticism. "So I'm told," she says coolly on the question of whether greater visibility increases the impact of failure. She will write what she wants, and that's all there is to it.

In any case, she believes, these three novels – the middle one is The Year of the Flood, which Atwood wanted to call God's Gardeners but couldn't risk it being mistaken for "a rightwing nut-bar book" – barely qualify as science fiction. "If I were writing about Planet Xenor, that would be different. It is our world, except with a few twists." In brief: in the near future, a "bioterrorist group" attacks the corporations that have replaced government to keep most of the population in drug-assisted servitude, and unleashes a pandemic that erases humanity. (With a few exceptions.) In this world, pig hybrids outsmart humans and an off-shoot species of mankind wanders about purring, but it is, at the same time, a realistic novel full of tight domestic dramas in the midst of the apocalypse.

It is also a novel about self-definition. "There's the story, then there's the real story, then there's the story of how the story came to be told," realises Toby, the protagonist, and asks herself, as a proxy for the author: "What else to write, besides the bare-facts daily chronicle she's begun?"

Thus every story begins, unfolding on the understanding that all accounts are partial, all impressions subject to change. As a child, for example, Atwood and her family would spend their summers out in the wilds and their winters in the city, so that "my idea of a city was that it was always cold and covered with snow, because that was the only time that we went." Her father, Carl Atwood, was a zoologist conducting entomological research (she used the details in her novel Cat's Eye). Her mother was a nutritionist. Atwood's interest in science isn't coincidental and she didn't need consultants for the novel. "I grew up with the biologists. I know how they think."

The biologists, in turn, are rather grateful for her interest. "They're my readers. I have a big following among the biogeeks of this world. Nobody ever puts them in books. 'Finally! Someone understands us!'"

In fact, the only experts she consulted for MaddAddam were hackers, who she turned to for advice on how characters might pull off secret communication in the age of spying. What's interesting, Atwood says, is that with snooping potential as it currently is – and in light of the NSA eavesdropping revelations – you can "watch people revert to older technologies, as a way around some of this." Like what?

She grabs my notebook, scribbles something down and theatrically tears it up. "It's the only safe way. Then you rip it up and burn it. Don't even flush it down the toilet, it's too risky. We have ways of opening up those toilets." As a character in Year of the Flood says of digital technology: "If you can see it, it can see you."

Atwood is a polymath. She has ideas about how to fix almost everything and takes pride in her rugged resourcefulness – unlike so many namby-pamby authors who wouldn't have a clue what to do if the lights went out. When she walks down a street, for example, she likes to point out to whomever she's with what, in the natural world, they could eat, should the need arise. "I just want them to be prepared."

Until the apocalypse comes, technology fascinates her. It's not surprising that, along with Salman Rushdie, she is among the most prolific Twitter users in her peer group. She thinks of it as akin to "a radio broadcast" rather than a method of self-promotion. "So, like the host of a radio show, I can promote other people's work." But not your own?

"I wouldn't. It becomes boring. I tweet things people have shared in that area – if they've shown me a review, it's a courtesy to acknowledge it by retweeting it. But to say, 'Buy my book'? I don't think that's what it's for any more than at a party you would say, 'I want you to buy my book, that's £12 right now.' But you might say, 'See that woman over there in the purple gown? She just wrote a sensational novel, which I have read.' It's useful for that. And also for, 'Sign this petition, look at this cause. Save More Bees.' I'm very keen on Save More Bees right now."

She enthuses for 10 minutes about bees: how they feature in folklore going back to the Greeks; how for a long time botanists laboured under the misapprehension that the queen bee was actually a king, hence, in chess, the king is stationary and the queen moves around. How they buzz to "ventilate the hive", and "will sense if you're afraid or have bad emotions towards them". How they manage to be cute, even though they can sting you. Atwood looks wise for a moment. "It's because they're fuzzy."

Her capacity to absorb and retain esoteric information is a function of a ravenous intellect. (In the case of bee knowledge, it was put to good use in MaddAddam, when a character's reading of bee behaviour saves the day.) She needs a lot of intellectual movement to avoid boredom and has been known to get 150 pages into writing a novel and then abandon it because, she realised, it was a refuge from the more difficult book she should have been writing. So it went with The Handmaid's Tale – "I had a corpse immediately before writing that. Yes. And I had one immediately before writing Surfacing. And then a couple of others. I got into it a reasonable distance and realised that although it was going to work, it wasn't going to work that way. I had to go back and start at the beginning and take a different approach."

And then, presumably, you're so relieved the book's working, you don't find the material scary any more. "That's right. That's exactly right."

The Handmaid's Tale was in some ways her first foray into science fiction, and generated an interesting response. Beyond its literary merits, women lauded it for its feminism, and it appealed to a lot of male readers for the "constructed world" aspect. "They like blueprints of made-up places. It's not that women don't, but that engineering thing, 'How would I arrange the space? OK, what about the weapons, how would I do that?' If there's an underground in it and some warfare, men get into that." (It also triggered an amount of hate mail from religious groups, who, she says, you'd think would have more sense than to align themselves with the fundamentalists in the book).

Natasha Richardson in the 1990 film adaptation of The Handmaid's Tale. Photograph: Ronald Grant Archive

Atwood has always written about monsters, of one kind or another. In Cat's Eye, it's the school bully. On a forum called Wattpad, she has co-written a zombie serial with Naomi Alderman called The Happy Zombie Sunrise Home. "We each had a character. I ran the grandmother, fending off the zombies with the garden tools. She ran the granddaughter. It starts out with the granddaughter saying to the grandmother, 'Mom just ate dad in the kitchen. What do I do?' And the grandmother says, 'I never liked that woman ... self, self, self that's all she ever thought about."

This interest is something she has, at times, suspected her agents think "is beneath my dignity". Nonetheless, as with so many things, she has given the undead a great deal of thought. "There's a difference between werewolves and vampires on the one hand, and zombies on the other. Zombies are always in a group; it's never just one. Whereas Dracula, although he's made a few others, they're fairly singular. Vampires are always rich, because they've lived a long time and accumulated stuff. They tend to be aristocratic. Vampires get the joy of flying around and living forever, werewolves get the joy of animal spirits. But zombies, they're not rich, or aristocratic, they shuffle around. They're a group phenomenon, they're not very fast, they're quite sickly. So what's the pleasure of being one?"

She regards me expectantly. Is there a pleasure?

A triumphant look. "You have no responsibility! It's not your fault, you can do nothing. Don't expect anything from me, I'm just trying to shuffle. They've had to speed them up a bit."

In World War Z they swarm.

"Right. They've had to speed them up because it was getting too boring."

❦

These days, for Atwood, the most pressing question is one of time. The major downside of her success is that she is asked to lend her voice to a lot of laudable causes that take her away from her writing. She was sufficiently up against deadline with MaddAddam to have finished it on a train. "It's a consciousness of the clock ticking. So am I going to do anything that big again? Probably not. Whereas if I were 40 I would say, 'Of course!'"

When Atwood started writing, she was subject to public rebuke for trying to run a family at the same time, and elements of that debate have failed to move on. For goodness' sake, she says, why do people constantly fuss around these issues. "Who's making up the obligations, number one? Who said you have to do it, number two? Who says you have to be a writer, number three? No one's holding a gun to your head. Who says who says who says?"

The recent Atlantic essay by Lauren Sandler advising women writers to have only one child if they want to achieve anything professionally causes Atwood to make a giant eye roll. "They can do whatever they like. Leave them alone. Alice Munro has three. I would personally have liked to have had more." (She has a daughter who lives in Brooklyn).

"These are the choices you make," says Atwood. "Do what you want. If you don't want to have children, don't have them; if you do want to have them, have them. There's going to be consequences either way." She gives it a moment's thought. "I mean poor old Charlotte Brontë possibly shouldn't have had children because it killed her, but Emily didn't and she died anyway. Sooner or later, I hate to break it to you, you're gonna die, so how do you fill in the space between here and there? It's yours. Seize your space."

Hers is a toughness born of experience. "I'm of that generation that was told by all of the social historians and literary writers, that of course women writers had to dedicate themselves to their art and they couldn't have both. So I thought to hell with that. I didn't see why it had to be either/or." When her daughter was little, Atwood and her then husband lived on a farm and he was very involved. "We hired somebody to help with the sheep, cows, tractor driving and correspondence. But we ourselves split the child care. We had somebody come in a couple of mornings a week. And no, we didn't feel very guilty."

Until that point, Atwood had always fitted in writing around a day job, so she was used to squeezing it into the off hours. "If you have a job in the daytime, you write at night. It's all a question of how much you want to do it. You don't want to do it, then throw it out the window. Make your choice. Stop whining about it and filling up copy in magazines with your guilt. Sorry to sound so pragmatic. How dirty it is under your bed is your business, not anyone else's."

How was this attitude received at the time? "Mean. Hard. Aggressive. Cruel. Yes." She shrugs; tant pis.

A woman sitting several tables away interrupts to say: "Can I buy you a glass of champagne, Miss Atwood, for all the reading pleasure you have given me over the years?" ("Oh, I really appreciate it," says Atwood. "But not at this time of day.")

Can she imagine herself, as Munro did recently, announcing a retirement from writing? Atwood looks suddenly fierce. "Don't believe it. People say that, but what they're really saying is, 'Stop calling me. Don't bother me, I'm retired. Now I can write more!' It's a great temptation. But they never really follow through, because as soon as they say it, they feel free, and as soon as they feel free, they get another idea. What they're really saying is, 'I don't feel obligated.'"

The only real end is the one that is forced on you, in small or in large scale, as the characters in MaddAddam discover. What is the pleasure of anticipating the end of the world? "I think the pleasure is we like to walk it through in advance, with a consciousness that's still human. So you can't actually wipe out the human race and then tell a story about it. There has to be somebody still alive through whom you can hear that story. It's like that conundrum of where will I go after I die. You're still imagining an I."

Get it while you can, says Atwood. Move on to the next thing. Stop faffing about and making excuses. "One story ends, and you ask, 'What about later?' Well, later, the dragon will be released. You'll have the new Jerusalem. God will make another world. You just can't say, 'That's it. End of Story." A small, dry pause. "But what about the next story?"

• Margaret Atwood is in conversation with John Mullan at the Edinburgh international book festival today. edbookfest.co.uk.