Chapter 18: Problem 9
Describe the human genome in terms of genome size, the percentage of the genome that codes for proteins, how much is composed of repetitive sequences, and how many genes it contains. Describe two other features of the human genome.
Short Answer
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Answer: Two additional features of the human genome are the presence of introns and exons, and the existence of pseudogenes. Introns are non-coding DNA sequences within a gene, while exons code for functional proteins or RNA molecules. Pseudogenes are non-functional copies of coding genes that once expressed functional proteins but have lost their function due to mutations.
Step by step solution
01
Size of the human genome
The human genome consists of approximately 3.2 billion base pairs (3.2 Gb) across 23 pairs of chromosomes (#1-22 autosomes and the X and Y sex chromosomes).
02
Protein-coding percentage
Only about 1-2% of the human genome is coding for proteins, which contains around 20,000-25,000 protein-coding genes. The majority of DNA in the non-coding region is responsible for critical control mechanisms and for producing different types of RNA, whose roles may not fully understood at the moment.
03
Repetitive sequences
Approximately 50% of the human genome consists of repetitive sequences, which can be due to transposable elements and other repeated DNA elements. These sequences may play a role in gene regulation, chromatin organization, and genome dynamics.
04
Number of genes
The human genome contains around 20,000-25,000 protein-coding genes but can result in a varying number of proteins due to alternative splicing, which allows a single gene to encode multiple proteins. Additionally, the genome contains non-coding RNA genes (like rRNA, tRNA, and miRNA genes) that play essential roles in gene expression and regulation.
05
Additional feature 1 - Introns and Exons
Human genome contains both introns and exons. Introns are non-coding DNA sequences within a gene, while exons are DNA sequences coding for functional proteins or RNA molecules. During the process of transcription pre-mRNA is produced, and through a process called RNA splicing, introns are removed, and exons are joined to form mature mRNA molecules.
06
Additional feature 2 - Pseudogenes
The human genome also contains a substantial number of pseudogenes, which are non-functional copies of coding genes that once expressed functional proteins. The pseudogenes are either processed or duplicated, have lost their function due to mutations and can't produce functional proteins anymore. However, they can help with understanding gene evolution and can influence the expression of other functional genes.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Genome Size
The human genome is remarkably vast, consisting of approximately 3.2 billion base pairs (abbreviated as 3.2 Gb). These base pairs are organized across 23 pairs of chromosomes that include autosomal chromosomes numbered 1 to 22 and the sex chromosomes, X and Y.
- Autosomal Chromosomes: Consist of 22 pairs that are the same in both males and females.
- Sex Chromosomes: Typically one pair in humans; women have XX and men have XY combinations.
Protein-Coding Genes
Despite the genome's considerable size, only about 1-2% of the human genome is devoted to protein-coding. This fraction includes around 20,000 to 25,000 unique protein-coding genes.
- Protein Coding: Refers to sequences that provide instructions for making proteins, the vital building blocks of organisms.
- Non-Coding DNA: Makes up the majority and includes sequences that help regulate gene expression and produce various RNA molecules.
Repetitive Sequences
Approximately 50% of the human genome consists of repetitive sequences. These are repeated sections of DNA, often found in abundance within genetic material.
- Transposable Elements: These "jumping genes" can move around the genome and lead to mutations or change how genes are expressed.
- Gene Regulation: These sequences can influence how and when genes are turned on or off.
Introns and Exons
Within the human genome, genes are typically composed of introns and exons.
- Introns: These are non-coding regions within a gene that do not code for proteins.
- Exons: These are coding sequences that do get transcribed into mRNA and translated into protein.
Pseudogenes
Pseudogenes are fascinating elements of the human genome. They are essentially relics of once-functional genes that have accumulated mutations, rendering them nonfunctional.
- Processed Pseudogenes: Typically arise from retrotransposition, a copy of mRNA is reverse transcribed and inserted back into the genome without the original gene's regulatory elements.
- Duplicated Pseudogenes: Result from gene duplication events; initially functional but later accumulate disabling mutations.