US STOCKS-Wall St rises on Yellen; S&P on track for modest Q1 gain

* S&P on track for fifth straight quarterly advance

* Gains broad, materials and tech outperform

* Yellen says central bank commitment needed for some time

* Prana Biotech tumbles after Alzheimer's drug results

* Indexes up: Dow 0.8 pct, S&P 0.8 pct, Nasdaq 1.1 pct (Updates to afternoon trading)

By Ryan Vlastelica

NEW YORK (Frankfurt: HX6.F - news) , March 31 (Reuters) - U.S. stocks rose on Monday, with the S&P 500 on track to end the first quarter of 2014 with slight gains, after Federal Reserve Chair Janet Yellen calmed concerns about a rate hike coming earlier than expected.

Gains were broad, with nine of the S&P 500's 10 primary sectors rising on the day. Material stocks were the biggest gainers, jumping 1.8 percent, while utility stocks were up 1.6 percent.

Technology and financial shares, which along with materials are tied to the pace of economic growth, also outperformed on the day. Micron Tech climbed 7.2 percent to $23.48 while Oracle Corp added 3.6 percent to $40.98.

In her first public speech since becoming Fed chair two months ago, Yellen said that the U.S. central bank's "extraordinary" commitment to boosting the economy would be needed for some time to come.

Earlier this month, Yellen raised concerns by saying that the period between the end of the Fed's quantitative easing program and the first rate increase from the central bank could be six months, a faster timeline than many had anticipated.

"What Yellen did today was to alleviate that confusion and provide more clarity. When there aren't concerns about the Fed tightening, the market can breathe easier," said Joseph Tanious, global market strategist at J.P. Morgan Asset Management in New York.

Biotech stocks continued to be in focus following recent volatility when investors sharply took profits in the high-flying sector. The Nasdaq Biotechnology index jumped 2.7 percent, following a 7 percent decline last week, in what was the index's fifth consecutive weekly decline.

Among some of the more active names, Vertex Pharmaceuticals climbed 4.9 percent to $71.10 and Biogen Idec (NasdaqGS: BIIB - news) added 3.5 percent to $304.38.

The Dow Jones industrial average was up 136.83 points, or 0.84 percent, at 16,459.89. The Standard & Poor's 500 Index was up 14.44 points, or 0.78 percent, at 1,872.06. The Nasdaq Composite Index was up 45.59 points, or 1.10 percent, at 4,201.35.

Equities also received a boost from end-of-quarter "window dressing," when money managers adjust positions to improve the look of their portfolios.

With Monday's advance, the S&P 500 is up 0.7 percent for the month and up 1.3 percent for the quarter, the benchmark index's fifth straight quarterly rise, though its worst quarterly performance since the last quarter of 2012.

The Dow is up 0.8 percent for the month but down 0.7 percent for the quarter. The Nasdaq is down 2.5 percent for March but up 0.6 percent for the quarter.

The Institute for Supply Management-Chicago business barometer was at 55.9 in March, its lowest level since August and down from 59.8 in February. Economists' median forecast in a Reuters poll was 59.0.

U.S.-listed shares of Prana Biotechnology Ltd plunged 70.9 percent to $2.87 after the company said its experimental drug to treat Alzheimer's disease failed to meet the main goal of a mid-stage study in patients with a mild form of the condition.

Logistics company UTi Worldwide Inc (NasdaqGS: UTIW - news) reported a bigger-than-expected quarterly loss, hurt by weak demand for air freight as customers opted for cheaper modes of shipping. Its shares slumped 8.6 percent to $10.29.

(Editing by Bernadette Baum and Nick Zieminski)