* Dow Chemical to spin off unit, merge it with Olin
* Healthcare buoys indexes as biotechs bounce back
* Fed Chair Janet Yellen to speak shortly before the closing bell
* Indexes up: Dow 0.07 pct, S&P 0.11 pct, Nasdaq 0.28 pct (Updates to mid-afternoon)
By Chuck Mikolajczak
NEW YORK, March 27 (Reuters) - U.S. stocks were flat on Friday, with the S&P 500 on track for its worst week since late January, ahead of a speech by Federal Reserve Chair Janet Yellen that could create a volatile close on Wall Street.
Healthcare stocks helped buoy indexes with biotech stocks bounced 1.8 percent higher after suffering a 7-percent drop in the prior four sessions, while energy was the worst performing S&P sector as crude prices resumed their decline.
Fed Chair Yellen will speak on monetary policy in San Francisco just 15 minutes before the closing bell (1945 GMT) and traders will keep an ear out for clues on the timing of the start of the tightening path at the Fed.
"In the new chapter of 'no explicit forward guidance,' every speech does become incrementally more important," said Anastasia Amoroso, global market strategist with J.P. Morgan Funds in New York.
"Even though the rate hike is off the table for April, it is not off the table for June and the conditions the Fed may seek as evidence to raise the rates in June may still be satisfied between now and then."
Investors have been cautious ahead of the start of earnings season, as traders look to see how much the strong U.S. dollar will hurt corporations' bottom lines.
The Dow Jones industrial average rose 11.54 points, or 0.07 percent, to 17,689.77, the S&P 500 gained 2.29 points, or 0.11 percent, to 2,058.44 and the Nasdaq Composite
For the week, the S&P 500 is off 2.4 percent, the Dow is down 2.5 percent and the Nasdaq is 3 percent lower. The Nasdaq was on pace for its biggest weekly drop since October.
Dow Chemical shares rose 3.59 percent to $48.06 after it announced the spinoff of part of a unit that will then merge with Olin Corp. Olin shares soared 21 percent to $32.90.
BlackBerry posted a fiscal fourth-quarter profit, its turnaround efforts beginning to gain traction. U.S.-listed BlackBerry shares were up 1.8 percent at $9.47 after volatile early trading.
U.S. consumer sentiment fell month-over-month in March, a survey released on Friday showed, though the decline was smaller than forecast.
The final reading on gross domestic product for the last quarter of 2014 was unchanged from the previous forecast, at a 2.2 percent rate of expansion. After-tax corporate profits fell at a 1.6 percent rate in the fourth quarter as a strong dollar dented the earnings of multinationals.
Advancing issues outnumbered declining ones on the NYSE by 1,758 to 1,239, for a 1.42-to-1 ratio; on the Nasdaq, 1,381 issues rose and 1,236 fell, for a 1.12-to-1 ratio favoring advancers.
The S&P 500 index posted 4 new 52-week highs and 6 new lows; the Nasdaq Composite recorded 27 new highs and 36 new lows. (Editing by Chizu Nomiyama and Nick Zieminski)