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By the end of this section, you will be able to:
  • Describe the structure of prokaryotic and eukaryotic genomes
  • Distinguish between chromosomes, genes, and traits
  • Describe the mechanisms of chromosome compaction

The continuity of life from one cell to another has its foundation in the reproduction of cells by way of the cell cycle. The cell cycle    is an orderly sequence of events that describes the stages of a cell’s life from the division of a single parent cell to the production of two new daughter cells. The mechanisms involved in the cell cycle are highly regulated.

Genomic dna

Before discussing the steps a cell must undertake to replicate, a deeper understanding of the structure and function of a cell’s genetic information is necessary. A cell’s DNA, packaged as a double-stranded DNA molecule, is called its genome    . In prokaryotes, the genome is composed of a single, double-stranded DNA molecule in the form of a loop or circle ( [link] ). The region in the cell containing this genetic material is called a nucleoid. Some prokaryotes also have smaller loops of DNA called plasmids that are not essential for normal growth. Bacteria can exchange these plasmids with other bacteria, sometimes receiving beneficial new genes that the recipient can add to their chromosomal DNA. Antibiotic resistance is one trait that often spreads through a bacterial colony through plasmid exchange.

The illustration shows a prokaryotic cell with a single, circular chromosome floating free in the cytoplasm.
Prokaryotes, including bacteria and archaea, have a single, circular chromosome located in a central region called the nucleoid.

In eukaryotes, the genome consists of several double-stranded linear DNA molecules ( [link] ). Each species of eukaryotes has a characteristic number of chromosomes in the nuclei of its cells. Human body cells have 46 chromosomes, while human gametes (sperm or eggs) have 23 chromosomes each. A typical body cell, or somatic cell, contains two matched sets of chromosomes, a configuration known as diploid    . The letter n is used to represent a single set of chromosomes; therefore, a diploid organism is designated 2 n . Human cells that contain one set of chromosomes are called gametes, or sex cells; these are eggs and sperm, and are designated 1n , or haploid    .

The 23 chromosomes from a human female are each dyed a different color so they can be distinguished. During most of the cell cycle, each chromosome is elongated into a thin strand that folds over on itself, like a piece of spaghetti.  The chromosomes fill the entire spherical nucleus, but each one is contained in a different part, resulting in a multi-colored sphere. During mitosis, the chromosomes condense into thick, compact bars, each a different color. These bars can be arranged in numerical order to form a karyotype. There are two copies of each chromosome in the karyotype..
There are 23 pairs of homologous chromosomes in a female human somatic cell. The condensed chromosomes are viewed within the nucleus (top), removed from a cell in mitosis and spread out on a slide (right), and artificially arranged according to length (left); an arrangement like this is called a karyotype. In this image, the chromosomes were exposed to fluorescent stains for differentiation of the different chromosomes. A method of staining called “chromosome painting” employs fluorescent dyes that highlight chromosomes in different colors. (credit: National Human Genome Project/NIH)

Matched pairs of chromosomes in a diploid organism are called homologous (“same knowledge”) chromosomes . Homologous chromosomes are the same length and have specific nucleotide segments called genes in exactly the same location, or locus    . Genes, the functional units of chromosomes, determine specific characteristics by coding for specific proteins. Traits are the variations of those characteristics. For example, hair color is a characteristic with traits that are blonde, brown, or black.

Questions & Answers

how many cells on the human
Amar Reply
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Nyabuoy
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Nyuongatdet
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fernando
who many cell are in the human body
Ayasso Reply
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Prince Reply
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Jonathan Reply
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Randa
what is chemosynthesis
Irene
who many cell are in the human body
Ayasso
there are billion cells in human body
fazeela
what are three stages of mitosis
jerry
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jerry
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jerry
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fazeela
anyone to explain each of the following,, prophase, metaphase, anaphase and telophase
jerry
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Mbah Reply
what is the difference between chlorophyll and photosynthesis
Rahman Reply
Chloe is the green pigment found in green plants while photosynthesis is the process by which plant produce their own food
mary
photosynthesis is the production of food by plant while chlorophyll is the green pigment that is found in chloroplast..
jerry
chrolophyll (green colouring matter in leaves) while photosynthesis (process by which green plants make their own food)
Nakhombi
what are the functions of parts of microscope
Bami Reply
base to provide support
Ian
only base what about the other
Bami
has only one function
Mark
Mirror ... used to reflect light
Irene
outline 2 ideas of Darwinism theory
Fatma Reply
important of protein in plants
Mark Reply
Protein in plants- repair damaged tissues -
Danisha
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Lizzicios Reply
science is a broad field of study.
Prince
enzyme
Mark
science (study of living en non-living things
Nakhombi
what is cell?
Mirembe Reply
what are antibiotics?
Mirembe
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Oscar
A cell is the smallest unit of a living thing or organ..
Gloria
Antibiotics are antibacteria's/medicine that inhibites the growth of or they destroy micro organisms in the body
Gloria
what are genotype
Abibatu Reply
what are the two types of twins
Fraza Reply
what are osteocytes
Fowowe Reply
WHAT ARE NOBEL GASES
Mbasa Reply
Noble gases are the ones that have octet electron arrangements therefore they are very stable and does not need any more electrons.
Danisha
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Izoo
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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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