Sequential Expression of Human Genes

To understand the sequential expression of human genes, we need to consider the flow of genetic information from DNA to protein synthesis. This process involves several key components, including messenger RNA (mRNA), DNA sequence, ribosomes, and transfer RNA (tRNA). Among these options, the correct answer is option B: DNA sequence. Let's explore why this is the case.

1. DNA Sequence
The DNA sequence refers to the specific arrangement of nucleotide bases (adenine, thymine, cytosine, and guanine) within a gene. Genes are segments of DNA that contain instructions for building proteins, which are the functional units of cells. The DNA sequence acts as a template for the synthesis of RNA molecules.

2. Transcription: Messenger RNA (mRNA)
Transcription is the process by which DNA is converted into RNA. During transcription, an enzyme called RNA polymerase binds to the DNA at the beginning of a gene and synthesizes a complementary RNA molecule, known as messenger RNA (mRNA). The mRNA carries the genetic information from the DNA to the ribosomes in the cytoplasm.

3. Translation: Ribosome
Translation is the process by which the genetic information carried by mRNA is used to synthesize proteins. Ribosomes are cellular structures composed of proteins and ribosomal RNA (rRNA). They serve as the site of protein synthesis. Ribosomes read the mRNA sequence in groups of three nucleotides, called codons, and match each codon with the corresponding amino acid.

4. Transfer RNA (tRNA)
Transfer RNA (tRNA) molecules play a crucial role in protein synthesis. Each tRNA molecule carries a specific amino acid and has an anticodon that is complementary to the codon on the mRNA. During translation, tRNA molecules bind to the mRNA codons at the ribosome, bringing the corresponding amino acids in the correct sequence. This allows the ribosome to assemble the amino acids into a growing polypeptide chain, forming a protein.

Summary
The sequential expression of human genes involves the following steps:
1. The DNA sequence provides the template for gene expression.
2. Transcription converts the DNA sequence into mRNA.
3. Ribosomes, through translation, read the mRNA sequence and assemble amino acids into a protein.
4. Transfer RNA molecules bring the corresponding amino acids to the ribosomes.

Therefore, while all the given options play a role in gene expression, the DNA sequence is the initial and essential component in the sequential expression of human genes.