A MIPS palindrome - mips

Question

The current code that I have looks as such. It can successfully read if a string is a palindrome when punctuation is not entered.
.data
buffer: .space 80
input: .asciiz "Enter a string: "
output: .asciiz "Your string: "
paly: .asciiz "This is a palindrome "
notp: .asciiz "This is not a palindrome"
.text
main:
li $v0, 4 # system call code for print_str
la $a0, input # address of string to print
syscall # print the input
li $v0, 8 # code for syscall read_string
la $a0, buffer # tell syscall where the buffer is
li $a1, 80 # tell syscall how big the buffer is
syscall
la $a0, buffer # move buffer into a0
li $v0, 4 # print buffer
syscall
la $t1, buffer # begining of the string
la $t2, buffer # end of the string
li $t0, 0
loop:
lb $t3,($t2) # load the byte of the end of the string
beqz $t3,endl # if its equal to 0 then branch out of the loop
addu $t2, $t2,1 # if in loop the increment to next character
jal loop # repeat the loop
upper:
addi $t4,$t4,32
j lowered
lowered:
addi $t0,$t0,1
sb $t4, 0($a0)
addi $a0,$a0,1
j loop
endl:
subu $t2, $t2, 2 # subtracting 2 to move back from \0 and \n
check:
#lb $t4, 0($a0)
#beqz $t4, after
#beq $t4, 10, after
#slti $t2, $t4, 91
#li $t3, 1
#beq $t2, $t3, upper
bge $t1, $t2, palindrome # if both sides are equal then its a palindrome
# call palindrome
lb $t3, ($t1) # load the byte into register t3
lb $t4, ($t2) # load the end byte into register t4
bne $t3, $t4, notpaly # if the two register bytes are not equal its it not a palindrome
addu $t1, $t1, 1 # increment the beginning of the string to next char
subu $t2, $t2, 1 # decrement end of string to next char to compare
jal check # repeat the loop
palindrome:
la $a0, paly # calling paly from data
li $v0, 4 # call for reading string
syscall
jal exit # jump to end
notpaly:
la $a0,notp # calling notp from data
li $v0, 4 # call for reading string
syscall
jal exit # jump to end
after:
li $v0, 4
la $a0, output
syscall
la $a0, buffer
li $v0, 4
syscall
exit:
li $v0 ,10 # call to end program
syscall # call os
Now I know I need to implement code such as to make uppercase lowercase, and to remove punctuation.
With my already stored bits in check: I have some commented code, and this would be to to test if a character is uppercase, then jump to the function and lower it by adding 32. But it does not compile correctly and i am assuming this is because I am not storing the bits correctly.
#lb $t4, 0($a0)
#beqz $t4, after
#beq $t4, 10, after
#slti $t2, $t4, 91
#li $t3, 1
#beq $t2, $t3, upper