Protein Synthesis

Location of DNA in Eukaryotic Cells

DNA in a eukaryotic cell is primarily located in the chromosomes, which are found within the nucleus of the cell. This nuclear DNA is organized into chromatin. Additionally, small amounts of DNA can also be found in the mitochondria (mtDNA) and, in plant cells, in chloroplasts. This is in contrast to prokaryotic cells, where transcription occurs in the cytoplasm, since they lack a defined nucleus to house the chromosomes and their genetic material is typically a single circular chromosome in the nucleoid region.

Transcription Process in Eukaryotic Cells

Definition

Transcription is the fundamental biological process that converts a specific segment of DNA into a complementary strand of messenger RNA (mRNA). This mRNA molecule then carries the genetic information from the DNA in the nucleus to the ribosomes in the cytoplasm, where it serves as a template for protein synthesis.

Steps of Transcription

  1. Initiation:

    • The first step in transcription occurs in the nucleus.

    • In eukaryotes, initiation is a complex process requiring the assembly of transcription factors at a specific DNA sequence called the promoter, typically located upstream of the gene. A common eukaryotic promoter element is the TATA box (5′−TATAAT−3′5'-TATAAT-3') at approximately −25-25 to −35-35 base pairs upstream from the transcription start site.

    • These transcription factors recruit RNA Polymerase II (for protein-coding genes), which then binds to the promoter. Unlike DNA polymerase, RNA polymerase can initiate RNA synthesis de novo without the need for a primer.

    • Once bound, RNA Polymerase unwinds approximately 10-20 base pairs of the DNA double helix, separating the strands to form a transcription bubble and expose the template strand.

  2. Elongation (Building mRNA):

    • RNA polymerase begins to synthesize a single strand of mRNA in the 5' to 3' direction by adding complementary RNA nucleotides to the growing RNA chain, using the template strand (also known as the antisense strand or non-coding strand) of DNA as a guide.

    • The mRNA strand is built following the base pairing rules: adenine (A) in DNA pairs with uracil (U) in mRNA, thymine (T) in DNA pairs with adenine (A) in mRNA, guanine (G) in DNA pairs with cytosine (C) in mRNA, and cytosine (C) in DNA pairs with guanine (G) in mRNA.

    • As RNA polymerase moves along the DNA template, it continuously unwinds the DNA ahead of it and re-winds the DNA behind it, maintaining the transcription bubble.

    • The nascent mRNA strand is released from the DNA template as it is synthesized, preventing the formation of a stable RNA-DNA hybrid.

  3. Termination:

    • Transcription terminates when RNA polymerase encounters specific terminator sequences in the DNA template.

    • In eukaryotes, the primary transcript is cleaved at a specific site downstream of a polyadenylation signal (e.g., 5′−AAUAAA−3′5'-AAUAAA-3' or 5′−UAUAUA−3′5'-UAUAUA-3') within the RNA sequence. This cleavage often occurs around 10−3510-35 nucleotides past the polyadenylation site.

    • After cleavage, the RNA polymerase continues transcribing for several hundred more nucleotides, but the nascent RNA chain is subsequently degraded by enzymes, which eventually