Alexander Yakovlev whiled away part of his youth on Manhattan's Upper West Side, fashioning the life of an exchange student in New York City.

Anatoliy Dobrynin came to fancy McDonald's as a longtime Washington resident, occasionally biking off on weekends with his granddaughter for a hamburger and a Coke.

Today Yakovlev and Dobrynin are Moscow's highest-ranking specialists on the United States, occupying key positions on the ruling Soviet Politburo and the Secretariat of the Communist Party Central Committee respectively.

When Mikhail Gorbachev landed in Washington yesterday, both followed him down the ramp of his special Aeroflot jet, along with the handful of other Soviet Amerikanistii -- experts on America -- included in the Kremlin leader's delegation.

While this week's summit will give Gorbachev his first look at America, it marks a key phase in the careers of the planeload of accompanying America experts, climaxing their steady rise in power in the new Soviet leadership.

Their influence, clearly invaluable at a time of close U.S.-Soviet contacts, has already shown through in some of Gorbachev's diplomatic overtures and innovations, such as his policy of flexibility in negotiations with Americans.

Soviet watchers trace Gorbachev's frequent use of television -- and his choice of television for a presummit interview -- to Yakovlev, for instance. Yakovlev, who undoubtedly watched American television both as an exchange student at Columbia University in the late 1950s and during 10 years as Soviet ambassador to Canada, is now the Politburo member in charge of propaganda.

U.S. and Soviet arms control specialists credit Dobrynin, head of the International Department of the Central Committee, with shifts in the Soviet negotiating position that helped paved the way for the treaty to eliminate medium- and shorter-range missiles that is to be signed today by President Reagan and Gorbachev.

Dobrynin, 68, was ambassador to the United States for 24 years until last winter, when he was recalled to become one of the key figures shaping Soviet foreign policy with Gorbachev and Foreign Minister Eduard Shevardnadze.

Gorbachev does "tend to draw from American specialists right up at the top of the decision-making ladder," U.S. Ambassador to Moscow Jack F. Matlock Jr. said in a recent interview. "We have not treated our Soviet specialists the same way."

Besides Yakovlev and Dobrynin, the experts on the United States who stepped off Gorbachev's plane today include Georgi Arbatov, 68, who has headed the official U.S.A. and Canada Institute for two decades and has been a frequent guest on American television.

Shevardnadze, also accompanying Gorbachev as part of the official delegation, was a novice to American politics when he became foreign minister two years ago. But Shevardnadze, 59, has forged close ties to Secretary of State George P. Shultz during a heavy concentration of meetings the two men have held in Washington, Moscow and various West European cities.

Marshal Sergei Akhromeyev, the 64-year-old Soviet Army chief of staff and senior military specialist in Gorbachev's delegation, displayed a keen ability to negotiate with Americans during the October 1986 summit in Reykjavik, Iceland, and the crucial presummit negotiations in Geneva.

Other members of the Soviet delegation arriving with Gorbachev include Yuri Dubinin, 57, the Soviet ambassador to Washington, and Vladimir Kamentsev, a deputy prime minister. Dubinin, an expert on Western Europe, is one of the few delegates with little career experience in the United States.

Although he does not speak English, Gorbachev has spent a great deal of his 2 1/2 years in office honing an expertise in American politics.

Dozens of Americans have been invited to the Kremlin for talks with Gorbachev, including former presidents Richard M. Nixon and Jimmy Carter, former secretaries of state Henry A. Kissinger and Cyrus R. Vance, and several delegations of congressmen and academics.

The meetings have apparently left a mark. "Of course," Gorbachev said in his interview with NBC television last week, "I do now have a better understanding of American society than I did before I took up this job."

As important as the substantive impact the Soviet Americanists have left on Kremlin policymaking is the new style they have brought, developed in years of close contacts with American policymakers.

Since returning to Moscow, for example, Dobrynin has broken a taboo, long held by Soviet officials, against inviting westerners home. Dobrynin frequently plays host to visiting Americans. In August, for instance, he invited Sen. Alan Cranston (D-Calif.) to his summer dacha at the Black Sea resort of Yalta.

Some Soviet diplomats have "naturally learned how to deal with the Senate in a convincing way," Matlock said. "In that sense they have much more experience than the generation before them."

"They know how to negotiate in a businesslike way," he added. "They know how to enliven negotiations with a little humor. They know how to relax and use first names. All of this makes the mechanics of dealing certainly more pleasant for the diplomats involved."

In addition to Dobrynin and Yakovlev, the leading Soviet Americanists include Alexander Bessmertnykh and Yuli Vorontsov. Both deputy foreign ministers who have served in the Soviet Embassy here, they played a key role in the negotiations leading up to the summit and Bessmertnykh came to Washington with the advance party.