Nucleic Acids and Protein Synthesis Biology Chapters 12 & 13

DNA DNA is a complex organic molecule made of repeating monomers. Nucleotides are the repeating monomers or subunits that make up DNA. The DNA molecule consists of two long strands, each a chain of nucleotides.

Nucleotides Each nucleotide consists of three parts: Deoxyribose: a five-carbon sugar molecule Phosphate Group Nitrogen Base The sugar and phosphate molecules are the same for every nucleotide, however there are four different nitrogen bases.

Nitrogen Bases The four nitrogen bases are: Adenine, Guanine, Thymine and Cytosine. Purines are organic molecules which have a double ring of carbon and nitrogen atoms and include Adenine and Guanine. Pyrimidines have a single ring of carbon and nitrogen and include Thymine and Cytosine.

Structure of DNA DNA consists of nucleotides bonded to each other to form a long strand. The strands bond together and form the molecule by twisting around each other in a special structure called a double helix.

Double Helix The double helix resembles a twisted ladder. The sides of the ladder are composed of alternating sugar and phosphate molecules. The rungs of the ladder are formed by the bonding pairs of nitrogen bases. The two strands of DNA are held together by hydrogen bonds formed between the purines and pyrimidines. Adenine always bonds with thymine and guanine always bonds with cytosine.

Replication of DNA Replication is the process by which strands of DNA are copied. During replication, the complementary strands unwind and serve as a template for a new complementary strand.

Process of Replication Steps of Replication: Enzyme DNA helicase attaches to a DNA molecule and “unzips” the two strands of DNA by breaking hydrogen bonds. Unpaired strands react with the complementary bases of nucleotides floating in the nucleus. Complementary bases bond with bases in the DNA strands by forming hydrogen bonds. As the hydrogen bonds form, DNA polymerase forms bonds between sugar and phosphate atoms to form the backbone of two new DNA molecules.

Process of Replication

Function of Replication The process of replication occurs at many points on the strand of DNA simultaneously. The function of DNA replication is to produce strands of DNA in a fast and accurate process that contains enzymes that “proof read” the strand. DNA also has enzymes that repair it from chemical and physical damage as well as radiation.

RNA RNA or ribonucleic acid is a polymer consisting of nucleotides. RNA is different from DNA in three ways: Consists of only one strand Has ribose sugar vs. deoxyribose sugar Has the nitrogen base uracil instead of thymine.

Types of RNA RNA exists in three structural forms each with a different role in protein synthesis. Messenger RNA: single, uncoiled strand that transmits information from DNA during protein synthesis and serves as a template for amino acids. Transfer RNA: single strand of RNA folded in hairpin fashion that allows some complementary bases to bond. Ribosomal RNA: RNA in a globular form that is the major constituent of ribosomes.

Images of RNA Types

Transcription Transcription is the process by which RNA is produced from DNA. RNA molecules are transcribed according to the base sequence of the DNA molecule into a complementary strand except uracil is substituted for thymine. All three types of RNA are transcribed in the same manner and then move into the cytoplasm to aid in protein synthesis

Process of Transcription Process of transcription uses the following steps: RNA polymerase binds DNA and causes the strands to separate. An enzyme then causes hydrogen bonds to begin to form between bases on the DNA strand and complementary bases of RNA in the nucleus. RNA polymerase moves down the strand forming sugar-phosphate bonds between the RNA nucleotides. When RNA reaches the termination sequence of DNA bases, it releases the newly made RNA.

Process of Transcription

Protein Synthesis Protein synthesis is the formation of proteins using information coded on DNA and carried out by RNA. Each organism is made up of thousands of proteins. Each protein is made up of one or more polypeptides. Each polypeptide consists of a specific sequence of amino acids. The region of DNA that directs the formation of a polypeptide is called a gene.

Assembly of Amino Acids The function of protein synthesis is to assemble amino acids into a protein. The genetic information need to assemble the protein is transferred from DNA to mRNA through transcription. The mRNA is made up of codons. A codon is a sequence of three base pairs that code for a specific amino acid using tRNA as an intermediate. A few codons do not code for amino acids, but are used as start and stop signals.

Translation The process of assembling proteins from mRNA is called translation. mRNA moves out of the nucleus to a group of ribosomes where the process occurs. The amino acids used for the process are brought to ribosomes by tRNA. Each amino acid is bonded to a specific tRNA that has an anticodon which is complementary to the corresponding mRNA.

Protein Synthesis

Steps of Translation Steps of Translation: Assembly begins when a ribosome attaches to an AUG (start) codon on the mRNA. The codon bonds with the tRNA anticodon which carries the amino acid. The ribosome then moves along the strand of mRNA bonding the appropriate codons and anticodons and adding to the amino acid chain. An enzyme catalyzes the bonds between the amino acids. The chain continues until the ribosome reaches a stop codon and the mRNA is released.

Images of Translation