Paul Gitsham - The Last Straw

“Now, you are only interested in a small piece of the DNA. To identify this piece of DNA you need to add two short pieces of artificially made DNA — maybe twenty or thirty base pairs in length — called primers that will match the DNA sequences either side of the bit you are interested in. It’s a bit like putting brackets around the word you are interested in in a sentence. To do this you cool the solution down to between forty-five degrees Celsius and sixty-five degrees Celsius for about forty-five seconds, which allows this primer DNA to join the matching sequences of the original template DNA.

“Now comes the clever bit. If you raise the temperature to about seventy-two degrees Celsius, an enzyme called a polymerase builds up the missing half of the ladder using raw chemicals that you added to the solution at the start. Depending on what type of polymerase enzyme you use, a good rule of thumb is that it takes about one minute to make a thousand base pairs of DNA. The result is that where you started off with a single very long ladder of DNA, most of which you don’t want, you now have two short DNA ladders, only containing the DNA sequence you are interested in.

“But it doesn’t stop there. If you repeat the cycle, you will use the new ladders as a template also, so those two molecules become four. Repeat again and the four become eight. Then sixteen, thirty-two, sixty-four, one hundred twenty-eight…”

Warren whistled. “The power of powers, eh? So after all that, give me a figure, Karen. How long do you think this PCR reaction would have taken, assuming that he completed it?”

Karen looked uncomfortable, nervously shuffling her notes. “There is a lot of guesswork here and I’m using figures from my own reactions, which might be completely different from anything he is using, but if we assume that he does thirty cycles, with an extension time of one minute — assuming he is amplifying one thousand base pairs — then by the time he’s added on another four minutes at the end to finish off any uncompleted reactions, I’m calculating eighty-four minutes, not including the time taken to actually set up the machine and retrieve his samples at the end of the run.”

Warren looked at her figures thoughtfully for a few long moments, before scribbling a few numbers of his own. Karen forced herself to breathe normally.

“You realise that if he did an extension time of only half a minute, he would only need about seventy minutes. Same thing if he kept it at one minute but only did twenty-five cycles. That’s getting pretty close to the sixty-eight minutes.”

“I know. There is a lot of guesswork involved. I need to get a look at the program he used ideally.”

Warren raised an eyebrow. “Is that possible?”

“I think so, sir. It should be stored in the memory of the PCR machine.”

“All of this is pretty circumstantial, you realise? It’s interesting, but not conclusive.”

Karen nodded, unable to say anything.

“Good, just so you understand.” He stood up. “Nice thinking, Karen. After my conversation with DI Sutton last night, it was decided that another look at that PCR room is on the cards — you’ve just bumped that up to our number one priority.”

Much to Karen’s surprise, Jones’ first act was to call in DI Tony Sutton. Her first impression of him that morning had been correct: he looked decidedly dishevelled. Jones filled in Sutton with an abridged version of what Karen had just told him. Sutton thought for a few moments, before nodding his head slowly. “Still doesn’t let Severino off the hook, but it’s definitely food for thought.” He turned to Karen. “Nice thinking, Detective. That insider knowledge is something that a couple of old plods like DCI Jones and me lack. You get any more good ideas, you make sure that you share them with us.”

Karen nodded, unable to speak. To her chagrin, she could feel her cheeks turning pink. Neither man seemed to notice, however, as they bantered in a way that she hadn’t witnessed before. “I don’t mind being called a plod, Tony, but a little less of the old, please.”

* * *

After a few quick phone calls to arrange for someone at the university to meet them and for a forensics team from Welwyn to provide support, the three officers clambered into Jones’ dark blue Ford Mondeo. As they pulled out of the car park Karen noticed that Sutton also appeared to be wearing rather a lot of cologne. Sitting behind him, she got a full dose blown over her by the car’s air conditioning. Karen could also smell what seemed like the faintest whiff of stale beer mingling slightly with the cologne. It didn’t take a fully trained detective to work out what had happened the previous day. Although two senior officers getting drunk whilst in the middle of an ongoing case seemed a bit unprofessional to Karen, after work or not, she couldn’t deny that it seemed to have cleared the air somewhat. The atmosphere between the two men had been almost toxic the day before, yet now seemed far more comfortable. Well, as long as it got the job done, she decided.

Pulling into the increasingly familiar car park to the Biology department, Karen noticed that there were far more cars present.

“The building reopened yesterday,” Jones explained. “The end of the corridor with Tunbridge’s lab is still taped off as a crime scene although it’ll have to reopen soon. Forensics are pretty much done so we can’t justify keeping it closed much longer.”

Sutton scowled slightly. “Let’s hope that we haven’t lost any evidence in the past twenty-four hours.”

Sitting in the back seat, Karen felt a slight stab of shame. She’d visited the PCR room Saturday morning, but it had taken her until last night to notice any potential discrepancies in Spencer’s account. Sutton was right: who knew what evidence had been destroyed in the meantime?

Exiting the car, the three police officers headed in through the entrance. Jones showed his badge to the receptionist, who had been told to expect them. By the time the three officers had signed the visitors’ log and each been given a badge on a lanyard, a soft-spoken young woman had arrived to greet them. When she introduced herself as Candice Gardner, Warren was slightly disappointed to learn that she was Professor Tompkinson’s personal assistant. For some reason, when seeing her name plate on her desk on Saturday he had pictured Mrs Gardner as a late-middle aged woman, dressed in a voluminous flowery dress, peering over half-moon spectacles disdainfully at anyone wishing to disturb the revered professor. Warren had lost count of the number of stereotypes he’d had shattered over the past few days.

As they walked through the main administration office it was obvious that the department was operating at full capacity again. When Warren commented upon this, Gardner smiled tightly. “We’ve been closed for three days. It was the last thing we needed the week that the A level results came out and clearing for university places started.” Her tone almost made Warren want to apologise on behalf of Tunbridge and his murderer for the inconvenience. He exchanged a glance with Sutton, who raised an eyebrow, clearly thinking something similar.

The three officers entered Tompkinson’s office, Warren introducing Tony Sutton, who had not yet met the professor. He remained seated, his hands shaking slightly. He looked exhausted, Warren noted.

“Forgive my manners. I’m having a bit of a flare up today.” As he said so his head bobbed backward and forward like a hen pecking for grubs. His voice was reedy and Jones noted a faint slur to the ‘S’ at the end of ‘manners’.

“Not at all, Professor. I hope not to keep you too long, I realise that this is a busy time of the year for you.”