Help with stochastic model by use of the Gillespie algorithm

The code just fails for some reason and I can't pinpoint why

num_reactions = 50000;
cIprot = zeros(num_reactions,1);
cIrna = zeros(num_reactions,1);
croprot =zeros(num_reactions,1);
crorna = zeros(num_reactions,1);
cIprot (1) = 0;
cIrna (1)= 0;
croprot(1) = 0;
crorna (1)= 0;
time = zeros(num_reactions,1);
total_time = 1; % total time in seconds
i=1;
while (time(i) < total_time)
for i=1:num_reactions
v1=50*cIrna(i);
v2=1.2*cIprot(i);
v3=50*(1-(((croprot(i))^2)/(100+(croprot(i))^2)));
v4=1.2*cIrna(i);
v5=50.*crorna(i);
v6=0.8.*croprot(i);
v7=50*(1-(((cIprot(i))^2)/(100+(cIprot(i))^2)));
v8=0.8*crorna(i);
V = [v1 v2 v3 v4 v5 v6 v7 v8];
end
Rtot= (v1+v2+v3+v4+v5+v6+v7+v8);
alpha0 = sum(V);
y = rand(); 
y_mod = alpha0*y; 
next_rxn = 0;
for j=1:num_reactions % iterate over reactions
 if  y_mod < sum(V(1:j))
    next_rxn = j; % next reaction is j
    break; % exit loop
 end
end
if (next_rxn == 1)
 cIprot(i+1) = cIprot(i) + 1; 
elseif (next_rxn == 2)
 cIprot(i+1) = cIprot(i) - 1; 
elseif (next_rxn == 3)
 cIrna(i+1) = cIrna(i) + 1;
elseif (next_rxn == 4)
 cIrna(i+1) = cIrna(i) - 1;
elseif (next_rxn == 5)
 croprot(i+1) = cIrna(i) + 1;
elseif (next_rxn == 6)
 croprot(i+1) = cIrna(i) - 1;
elseif (next_rxn == 7)
 crorna(i+1) = crorna(i) + 1;
elseif (next_rxn == 7)
 crorna(i+1) = crorna(i) - 1;
else
 disp('Something went wrong!');
end
delta_t = -log(y) / Rtot;
time(i+1) = time(i) + delta_t;
end
plot (time,cIrna)