LSU is the only undefeated team in the country, undisputed No. 1, and headed to the BCS National Championship Game.

The Tigers have a dominant defense, a deep and powerful running game, an efficient passing attack, and some of the best special teams in the country.

They don’t have a weakness, and 12 of their 13 victories have come by 13 or more points.

And yet when fall camp began four months ago, this team had a lot of question marks, potential holes in all three areas. LSU has since gone from one of a handful of perceived national-championship contenders in the preseason to the clear-cut best team in the country thanks in large measure to being able to replace a bunch of key players that had moved on from last year’s 11-2 team.

The Tigers had to replace four key players on defense, including All-America cornerback Patrick Peterson, who was also a dynamic kick and punt returner. The others were middle linebacker Kelvin Shepperd, the leading tackler and glue of the defense, and the powerful tackle duo of Drake Nevis and Pep Levingston.

On offense, they had to replace 1,100-yard rusher Stevan Ridley, starting left tackle Joseph Barksdale and wide receiver Terrence Toliver. On special teams, they had to replace All-America place-kicker Josh Jasper and solid punter Derek Helton.

There was definitely talent waiting in the wings, but in every case the less-proven players blossomed so much that there wasn’t a drop-off, and in some cases, there was an upgrade.

“It’s amazing when you can replace those type of guys,” cornerback Morris Claiborne said. “We have a young team and some guys who have seen those guys do it previously. They want to do it better, and they’re just going out and putting the work into it.”

In Peterson’s case it was a group effort. Claiborne, who started alongside Peterson as a sophomore last season, emerged as an All-America-caliber corner, leading the team with six interceptions. Tyrann Mathieu, who inherited Peterson’s jersey No. 7, excelled at nickelback as a freshman last season and excelled at both corner and nickel this season.

Those two also picked up the slack on special teams as Claiborne had a 99-yard kickoff return for a touchdown against West Virginia and Mathieu had punt returns of 92 and 62 yards for touchdowns against Arkansas and Georgia, respectively. Mathieu, who also helped replace Peterson on kickoff coverage, was such a dynamic player on defense and special teams that he was one of five finalists for the Heisman Trophy who was invited to New York for the award presentation Saturday.

“I think that Tyrann Mathieu is a very special player,” Tigers coach Les Miles said. “He plays special teams, he plays defense, and he just seems to make plays.”

Karnell Hatcher moved from safety to middle linebacker to play the position Shepperd manned a year ago, though senior strongside linebacker Ryan Baker took over as the leader of the group and one of the leaders of the defense. Once the Tigers moved into the heart of Southeastern Conference play and started facing more physical offenses, senior Kevin Minter took over in the middle. Minter is fifth on the team with 56 tackles, just two behind Baker. Hatcher has 23.

The middle of the defensive line was shored up by a talented group of youngsters in the wake of Nevis’ and Levingston’s departure. Sophomores Michael Brockers and Bennie Logan excelled as the starters; and sophomores Josh Downs and Ego Ferguson, as well as freshman Anthony “Freak” Johnson, were also key parts of the rotation. Those tackles combined for 18.5 of LSU’s 98 tackles for loss.

The Tigers expected to replace Ridley with a running back committee, and the committee expanded as the season went along. Spencer Ware, who had a breakout game in the Cotton Bowl at the end of last season, was the starter as the season began and pretty much held on to the title, though the workload is pretty evenly shared. Fellow sophomores Michael Ford (team-leading 775 yards) and Alfred Blue (539) added lightning to Ware’s thunder (700), and freshman Kenny Hilliard (320) has shown down the stretch that he offers both. The four have combined for 30 rushing touchdowns.

“I like the best back for a series and for the (particular) plays,” Miles said. “I think a hot hand and a guy that is fresh are the best options.”

Rueben Randle went from being a nice complementary receiver to Toliver last season to being one of the top big-play receivers in the SEC, averaging 18.1 yards per catch and scoring eight receiving touchdowns.

“I worked on being more explosive out of my cuts,” Randle said, “and giving the DB the illusion that you’re running past them and getting in and out of your cuts real fast with the ability to separate and get to the ball.”

On a line featuring several seniors, it was sophomore Chris Faulk who blossomed as the anchor. “I’ve learned a lot (from the older linemen),” Faulk said. “All of our guys really come off the ball. (Offensive line coach Greg Studrawa) put guys at different positions in camp to build chemistry, and he did a great job with that.”

Special teams was a concern because the replacements for Jasper and Helton were untested. Junior Drew Alleman, who hadn’t tried a field goal in a college game, stepped in as the place-kicker and made all but two field goals, scoring all of LSU’s points in a 9-6 victory at No. 2 Alabama on Nov. 5.

Freshman Brad Wing came in and became one of the most versatile weapons as a punter in recent memory. His combination of distance, hang time, and placement helped the Tigers lead the SEC in net punting as they allowed a total of 6 punt-return yards. Wing also has been flawless replacing Helton as the holder.

“We just knew he had that kind of potential,” Miles said. “He’s a very talented punter.”

When Alleman struggled with kickoffs in the early going because of a leg injury suffered in the season opener against Oregon, freshman James Hairston took over the kickoffs and helped LSU improve in that area with 16 touchbacks in the past 10 games after LSU had none in the first three.

“He hits the ball as he is supposed to and continues to improve,” Miles said of Hairston. “He hits the ball deep, long and puts it where we want to put it.”