Adding strings in lists together

Use zip():

>>> lst = ["A","B","A","A","B"]
>>> [x + y for x, y in zip(lst, lst[1:])]
['AB', 'BA', 'AA', 'AB']

You can use map():

s = list(map(str.__add__, lst[:-1], lst[1:]))

A bit better to use operator.concat() (thanks for advice, @MykolaZotko):

import operator

s = list(map(operator.concat, lst[:-1], lst[1:]))

Upd.

I've decided to do some tests on bigger data.

import operator

lst = [...] # list with 10000 random uppercase letters


def test1():
    return list(map(operator.concat, lst[:-1], lst[1:]))


def test2():
    return [x + y for x, y in zip(lst, lst[1:])]


def test3():
    return [v + lst[i + 1] for i, v in enumerate(lst[:-1])]


def test4():
    s = ''.join(lst)
    return [s[i:i + 2] for i in range(len(s) - 1)]


if __name__ == '__main__':
    import timeit
    print(timeit.timeit("test1()", setup="from __main__ import test1, lst", number=10000))
    print(timeit.timeit("test2()", setup="from __main__ import test2, lst", number=10000))
    print(timeit.timeit("test3()", setup="from __main__ import test3, lst", number=10000))
    print(timeit.timeit("test4()", setup="from __main__ import test4, lst", number=10000))

Results:

1. Python 2:

   10.447159509
   11.529946446
   20.962497298000002
   20.515838672
   

2. Python 3:

   10.370675522
   11.429417197
   20.836504865999995
   20.422865353
   

On bigger data map() is a bit (~9%) faster, but there's no significant difference between test1() and test2()

There are several issues in your original code:

sequences = ["A","B","A","A","B"]
lista = sequences
lista.pop(0)
print(lista)

for x in range(sequences):
    mc =sequences[x]+lista[x]

Firstly, the statement lista = sequences does not make a copy of sequences. Instead, lista and sequences become two different names for the same list. What you do using one name also happens to the other. lista.pop(0) is the same as sequences.pop(0). If you want a copy, then import the copy library.

import copy

sequences = ["A","B","A","A","B"]
lista = copy.copy(sequences)
lista.pop(0)
print(lista)

for x in range(sequences):
    mc =sequences[x]+lista[x]

Secondly, your statement range(sequences) is incorrect. The range() function accepts integers as input, not lists. That's what gave you TypeError: 'list' object cannot be interpreted as an integer

# VALID
range(5)
range(3)
range(10)

# INVALID
range(["A","B","A"])
range(["eyes", "nose", "tail"])

sequences is a list. You want range(len(sequences)) notrange(sequences)

In the end, we can modify your original code to work:

import copy

sequences = ["A","B","A","A","B"]
lista = copy.copy(sequences)
lista.pop(0)
print(lista) # prints ["B","A","A","B"]

mc = list()
for x in range(len(lista)):
    mc.append(lista[x] + sequences[x + 1])