Unit 2: Cells and Genetic Information

Study Notes

Unit 2: How Do Cells Decode Genetic Information into Functional Proteins?

Cells decode genetic information into functional proteins through a complex system involving control, switches, feedback, and fine-tuning.
Segments of DNA are transcribed into RNA, which is then translated into proteins.
Proteins fold into three-dimensional configurations.
Proteins combine with other proteins, or are modified with sugars or fats, to become functional tools.
Proteins perform various functions, ranging from structural support and motor functions to catalyzing biochemical reactions and monitoring the cell's internal and external environments.

2.1 Information Transfer in Cells Requires Many Proteins and Nucleic Acids

How Is Genetic Information Passed on in Dividing Cells?: Cells create identical copies of their genetic information (DNA) for daughter cells during division. Accuracy of the copies is crucial for the health and inherited traits of the daughter cells. Replication must be precise.
One factor ensuring precise replication is the double-helical structure of DNA: DNA is composed of nucleotides (adenine, thymine, cytosine, guanine). These nucleotides pair up in a specific manner (A with T, and C with G). This complementary base pairing allows each strand to serve as a template for a new complementary strand during cell division.

In most multicellular organisms, every cell carries the same DNA, but this genetic information is used in varying ways by different types of cells.

Cells "do" different things based on the genes they express. For example, nerve cells synthesize neurotransmitters to communicate.
The expression of genes is dictated by what a cell does.

What Are the Initial Steps in Accessing Genetic Information?

Transcription: The first step in decoding genetic information. Enzymes called RNA polymerases build RNA molecules that are complementary to a portion of one strand of the DNA double helix.

RNA molecules differ from DNA molecules:

RNA molecules are single-stranded, while DNA is double-stranded.
RNA uses ribose sugar, while DNA uses deoxyribose sugar.
RNA includes uracil (U) nucleotides instead of thymine (T) nucleotides.

Three General Classes of RNA Molecules:

Messenger RNA (mRNA): Carries coding sequences for protein synthesis.
Ribosomal RNA (rRNA): Forms the core of ribosomes, where protein synthesis occurs.
Transfer RNA (tRNA): Carries amino acids to ribosomes during protein synthesis.

mRNA:

mRNA is highly variable, with thousands of different mRNA molecules present in a cell at any given time; some are abundant, while others are rare.
The lifespan of mRNA varies; structural proteins have longer lifespans, while signaling proteins degrade faster.
The spectrum of mRNA molecules in a cell is called the transcriptome. This transcriptome varies between cell types (e.g., insulin-producing vs. bone cells) and indicates which genes are actively expressed.

What Is the Function of Ribosomes?

Ribosomes are the sites of protein synthesis in cells.
Cells have many ribosomes, related to their protein-synthesizing activity.
Ribosomes are complexes of rRNA molecules and proteins.
In eukaryotes, some ribosomes attach to internal membranes where they synthesize proteins for those membranes or for secretion.

Function of rRNA

rRNA molecules within ribosomes direct the steps of protein synthesis – the linking of amino acids.
rRNA is also called a ribozyme or catalytic RNA.

Eukaryotic and Prokaryotic Ribosomes:

Eukaryotic and prokaryotic ribosomes differ, and these differences are exploited by antibiotics that target prokaryotic ribosomes without affecting eukaryotic ones.

How Does The Whole Process Result in New Proteins?

Transcription of DNA to mRNA is followed by translation, where mRNA is read to create proteins.
mRNA is single-stranded, and its bases are complementary to corresponding DNA portions.
Each amino acid is represented by a three-nucleotide sequence (codon) in mRNA.
tRNA molecules match amino acids to their corresponding codons in mRNA, and the ribosome assembles the amino acids into a protein.

How Do Ribosomes Work?

Ribosomes, composed of large and small subunits, assemble on mRNA strands.
tRNA molecules bring amino acids to the ribosome, matching them to the mRNA codons.
The ribosome links the amino acids together to form a polypeptide chain that folds into a functional protein.
Translation continues until a stop codon is reached, and the ribosome disassembles and releases the protein.

• In prokaryotic cells, transcription and translation are tightly coupled (translation begins before transcription is complete). • In eukaryotic cells, transcription occurs in the nucleus, and translation occurs in the cytoplasm.