Genetic information in DNA is transcribed to RNA and then translated into the amino acid sequence of a Protein.
Transcription
During the process of transcription, the information in the DNA codons of a gene is transcribed into RNA. Suppose that gene X has the DNA base sequence 3’-TACCCTTTAGTAGCCACT-5’.
The resultant RNA base sequence would be 5’-AUGGGAAAUCAUCGGUGA-3’.
Translation
During protein synthesis at the ribosome, the base sequence of the mRNA codons is translated to the amino acid sequence of a protein.
The transcribed mRNA is 5’-AUGGGAAAUCAUCGGUGA-3’. The resultant amino acids would be:
- From the first codon (AUG)
ATT Ile (I), ATC Ile (I), ATA Ile (I), ATG Met (M), ACT Thr (T), ACC Thr (T), ACA Thr (T), ACG Thr (T), AAT Asn (N), AAC Asn (N), AAA Lys (K), AAG Lys (K), AGT Ser (S), AGC Ser (S), AGA Arg (R), AGG Arg (R)
- from the second codon (GGG)
TGT (C), TGC (C),TGG Trp (W) CGT Arg (R), CGC Arg (R), CGA Arg (R), CGG Arg (R) AGT Ser (S), AGC Ser (S), AGA Arg (R), AGG Arg (R) GGT Gly (G), GGC Gly (G), GGA Gly (G), GGG Gly (G)
Note
(I) stands for Isoleucine, (T) Threonine, (N) Asparagine, (K) Lysine, (S) Serine, (R) Arginine, (C) Cysteine, and (G) Glycine.
AUG is the first codon in all mRNAs and it codes for methionine also. This methionine can be cleaved after translation; therefore, methionine may be one of the amino acids or not.
The first codon acts as an initiation codon whereby, translation begins. In most cases, the initiation codon is AUG. the stop codons, which are UAG, UGA, and UAA, signify the end of translation. Translation ends at these codons and the nascent polypeptide is released after release factors bind to the ribosome. Both first and last codons are important in protein synthesis because, without them, there would be no synthesis (Audesirk, Audesirk, and Byers, 2008, p. 120).
Mutations
A mutation is defined as a change in the base sequence of DNA. This may occur as a “mistake” in DNA replication, for example. Suppose that during DNA replication, two mutant DNA sequences are produced as shown below. For the 2 mutated DNA sequences, you will investigate how these changes might affect the sequence of amino acids in a protein.
Here is the original sequence, followed by two mutated sequences, 1 and 2:
- Original sequence 3′- TACCCTTTAGTAGCCACT-5’
- Mutated sequence 1) 3’-TACGCTTTAGTAGCCATT-5′
- Mutated sequence 2) 3’-TAACCTTTACTAGGCACT-5’.
- mRNA for mutated sequence 1: 5”-AUGCGAAAUCAUCGGUAA-3’.
The amino acid sequence would be:
- from first codon AUG
ATT Ile (I), ATC Ile (I), ATA Ile (I), ATG Met (M), ACT Thr (T), ACC Thr (T), ACA Thr (T), ACG Thr (T), AAT Asn (N), AAC Asn (N), AAA Lys (K), AAG Lys (K), AGT Ser (S), AGC Ser (S), AGA Arg (R), AGG Arg (R)
- from the second codon CGA
TCT Ser (S), TCC Ser (S), TCA Ser (S), TCG Ser (S) CCT Pro (P), CCC Pro (P), CCA Pro (P), CCG Pro (P), ACT Thr (T), ACC Thr (T), ACA Thr (T), ACG Thr (T), GCT Ala (A), GCC Ala (A), GCA Ala (A), GCG Ala (A).
mRNA for mutated sequence 2: 5’-AUUGGAAAUGAUCCGUGA-3’
- from the first codon AUU
ATT Ile (I), ATC Ile (I), ATA Ile (I), ATG Met (M), ACT Thr (T), ACC Thr (T), ACA Thr (T), ACG Thr (T), AAT Asn (N), AAC Asn (N) , AAA Lys (K), AAG Lys (K), AGT Ser (S), AGC Ser (S), AGA Arg (R), AGG Arg (R)
- from the second codon GGA
TGT (C), TGC (C),T GG Trp (W) CGT Arg (R), CGC Arg (R), CGA Arg (R), CGG Arg (R) AGT Ser (S), AGC Ser (S), AGA Arg (R), AGG Arg (R) GGT Gly (G), GGC Gly (G), GGA Gly (G), GGG Gly (G)
Proteins occur in different shapes due to characteristic folding during synthesis and these shapes determine the function of the protein. Therefore, if there is a mutation, it implies that there is a change of arrangement of bases; these bases determine the bonds created in proteins during folding and this means that a protein with a changed shape function differently from the initial function before mutation (Audesirk, Audesirk, and Byers, 2008, p. 210)
Inheritance of Traits or Genetic Disorders
Bob and Sally recently married. Upon deciding to plan a family, both Sally and Bob find out that they are both heterozygous for cystic fibrosis, but neither of them has symptoms of the disorder.
Punnet square
Probability of having:
- Healthy child is 1 out 4 → ¼*100= 25%
- Carrier child is 2 out of 4 or 1/2 → 2/4*100= 50%
- Cystic fibrosis child is 1 out 4 → ¼*100= 25%
Cell division, sexual reproduction, and genetic variability
Eukaryotic cells can divide by mitosis or meiosis. In humans, mitosis produces new cells for growth and repair; meiosis produces sex cells (gametes) called sperm and eggs. Although mutations are the ultimate source of genetic variability, both meiosis and sexual reproduction also can contribute to new genetic combinations in offspring.
The process of meiosis occurs after fertilization. During fertilization, the sperms meet the egg to produce a zygote. Both the sperm and the ovum contain genes that may be recessive in the parents but these genes may pair up to produce a dominant phenotype in the offspring but that was recessive in the parents hence the difference (Audesirk, Audesirk, and Byers, 2008, p. 176).
During the prophase 1 stage of meiosis, there is crossing over. In this scenario, adjacent non-sister chromatids break and reattach randomly to the available homologous chromosomes. These random reattachments cause offspring to be genetically different from their parents. It is important to remember that, these recombined chromatids get back into the cycle until the meiosis process is completed. This implies that the final result is the formation of sex cells that have different genetic material from the parents.
References
Audesirk, T., Audesirk, G., and Byers, B. (2008). Biology – Life on earth with physiology (8th Ed.). Upper Saddle River, NJ, Prentice Hall