More about the research:
In recent years, more archaeological and genetic evidence have been found in both eastern China and western Japan to lend credibility to this argument. Between 1996 and 1999, a team led by Satoshi Yamaguchi, a researcher at Japan's National Science Museum, compared Yayoi remains found in Japan's Yamaguchi and Fukuoka prefectures with those from early Han Dynasty (202 BC-8) in China's coastal Jiangsu province, and found many similarities between the skulls and limbs of Yayoi people and the Jiangsu remains.[8] Two Jiangsu skulls showed spots where the front teeth had been pulled, a practice common in Japan in the Yayoi and preceding Jōmon period. The genetic samples from three of the 36 Jiangsu skeletons also matched part of the DNA base arrangements of samples from the Yayoi remains.
Surprisingly, Japanese also display the highest frequency of haplogroup O3a5, which is a Han Chinese and Sino-Tibetan specific O3 branch.
Xue et. al.
Japanese
Haplogroup O3a5 (O3e) 10/47= 23%
This frequency is about 5% higher than the frequency of O3a5 among Manchus, Koreans and other Northeast Asians.
For North Koreans, the frequency of O3a5 is lower than some Tungusic populations. Overall, the Koreanic haplogroup O3 were the least influenced by Sinitic populations.

Whereas pure haplogroup C3 (M217) was observed at a high frequency among Tungusic (20%) and Koreanic (16%) populations. The frequency of haplogroup C3 among Japanese was only 1%. This means Japanese origins were NOT from Siberia.

Haplogroup D was observed among Japanese (25%) and Tibetans (40%). Was also observed among Han Chinese, Mongolians and Koreans.

DNA sequence SNP study done by Japanese researchers in 2005.

Biggest contributor of DNA of each East Asian people is bolded
Korean DNA sequence is made up of:
40.6% Uniquely Korean
21.9% Chinese
1.6% Ainu
17.4% Okinawan
18.5% Unidentified

Japanese DNA sequence is made up of:
4.8% Uniquely Japanese
24.2% Korean
25.8% Chinese
8.1% Ainu
16.1% Okinawan
21% Unidentified

Chinese DNA sequence is made up of:
60.6% Uniquely Chinese
1.5% Japanese
10.6% Korean
1.5% Ainu
10.6% Okinawan
15.2% Unidentified

The biggest components in Japanese are Chinese, Korean, Okinawan.
It's a fact that Korean peninsula was not as isolated as Japanese islands. But, this meant one people could settle on the peninsula, but Japan became an island of migrants. This is maybe why Koreans have the least Japanese and Chinese components, whereas Japanese and Chinese have higher Japanese and Chinese components to their DNA.