7.1 The Genome
A cell’s complete complement of DNA is called its genome. Prokaryotes’ genome comprises a single, double-stranded DNA molecule as a loop or circle. The region in the cell containing this genetic material is called a nucleoid.
In eukaryotes, the nuclear DNA exists as thin fibres called chromatin. Once a cell is ready to divide, the chromatin coils around proteins to compact into chromosomes. Chromosomes are encoded with genetic instructions for making RNA and proteins. These instructions are organized into units called genes. Hundreds (or even thousands!) of genes may be on a single chromosome. Genes are segments of DNA that code for particular pieces of RNA, which act as blueprints for building proteins. Humans have an estimated 20 thousand to 22 thousand genes.
Each chromosome consists of two identical structures called sister chromatids. Sister chromatids are joined together at a region called a centromere. Sister chromatids get separated during cell division.
A set number of chromosomes characterizes each species. Human body cells (somatic cells) have two sets of chromosomes in each cell, one set inherited from each parent. Because chromosomes occur in pairs, these cells are called diploid or 2n. There are 23 chromosomes in each set, for a total of 46 chromosomes per diploid cell. Each chromosome in one set is matched by a chromosome of the same type in the other set, so there are 23 pairs of chromosomes per cell. Each pair consists of chromosomes of the same size and shape and contains the same genes. The chromosomes in a pair are known as homologous chromosomes.
Human cells that contain one set of 23 chromosomes are called gametes, or sex cells; these eggs and sperm are designated n, or haploid.
Types of Chromosomes
Human cells contain two types of chromosomes: autosomes and sex chromosomes.
Autosomes
Autosomes are chromosomes containing genes for characteristics unrelated to biological sex. These chromosomes are the same in males and females. The great majority of human genes are located in autosomes. Of the 23 pairs of human chromosomes, 22 pairs are called autosomes (pairs 1-22 in Figure 7.1.2).
Sex Chromosomes
Exercise 7.1.1
Text Description
- The familiar X-shaped chromosome represents:
- How DNA always looks in eukaryotic cells
- How DNA in eukaryotic cells looks once it is replicated and the cell is about to divide
- Female sex chromosomes only
- How DNA appears immediately after cytokinesis
2. Chromosomes In Eukaryotic Cells
- centriole
- centromere
- chromatid
- DNA
3. Humans have 46 chromosomes. (True/False)
4. Autosomes refer to any chromosome that is not a sex chromosome. (True/False)
5. The X chromosome determines biological sex. (True/False)
6. Alleles for the same types of traits are found on homologous pairs of chromosomes. (True/False)
7. Which of the following is considered a homologous chromosome:
- chromosome 22 and the X chromosome
- the two copies of chromosome 22 that make up a pair
- all of the chromosomes in a skin cell and all of the chromosomes in a muscle cell
- chromosomes 21 and 22
Answers:
- How does DNA in eukaryotic cells looks once it is replicated and the cell is about to divide
- centriole
- True
- True
- False
- True
- The two copies of chromosome 22 that make up a pair
Text Description
Place the terms related to DNA in order of structure size from largest to smallest
- Homologous Pair
- Nucleotide
- Nucleus
- Gene/Allele
- Chromosome
Answers:
- Nucleus
- Homologous Pair
- Chromosome
- Gene/Allele
- Nucleotide
“6.1 The Genome” from Biology and the Citizen by Colleen Jones is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
“4.13 Mitosis and Cytokinesis” from Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.
