3.2. Chromosomes

Prokaryotic chromosome

Consists of a single, circular DNA molecule containing all the genes needed for the cell’s basic life processes

Characteristics of prokaryotic chromosome

• Not associated with any proteins (naked DNA)
• Since it’s one chromosome, there is only a single copy of each gene
• Usually one or more plasmids

Plasmid

Small extra DNA molecule

Characteristics of plasmids

• Very small, circular and naked
• Only contain a few genes that may be useful to the cell but are not necessary for its basic life processes (e.g. genes for antibiotic resistance)

Replication of plasmids

• Not always replicated at the same time as the prokaryotic chromosome
• Many copies of plasmids may exist in the same cell
• A plasmid may not be passed to both cells formed by cell division

Conjugation

Process in which copies of plasmids are transferred from one cell to another

Steps of conjugation

1. Pilus of doner cell (F+) attaches to recipient cell (F-)
2. Pilus contracts, drawing cells together to make contact with one another
3. One strand of the plasmid (F) DNA is transferred from donor cell to recipient cell
4. Donor synthesizes complementary strand to restore the plasmid
5. Recipient synthesized complementary strand to become F+ cell with pilus

Plasmids in gene transfer

• Natural gene transfer: Upon death of a prokaryote, a plasmid may be absorbed by a cell of a different species
• Artificial gene transfer: Scientists use plasmids to transfer genes between species

Eukaryotic chromosomes

Consists of a single immensely long linear DNA molecule that is wrapped around histone proteins

Histone proteins

Globular proteins that are wider than the DNA

Arrangement of histone proteins

Adjacent histones are separated by short stretched of DNA molecule that is not wrapped around a protein

Replication of chromosomes

During interphase, DNA is replicated to create two identical strands, called chromatids

Sister chromatids

Pair of chromatids that make up the double structure of a chromosome; formed by DNA replication

Centromere

Narrow region that joins sister chromatids together

When and how are chromosomes visible?

In interphase, they are too narrow to be seen under a microscope. In mitosis/meiosis, they become much shorter and fatter by supercoiling. Hence. they are visible through stains that bind to either the DNA or the proteins

How do chromosomes physically differ from each other?

In length an in the position of the centromere (close to the end or to the center)

Minimum amount of chromosome TYPES in eukaryotes + # in humans

At least 2. In humans, there are 23

Genetic material in chromosomes

• Each chromosome type contains specific genes arranged in a standard sequence along the DNA molecule
• This allows parts of the chromosome to be swapped during meiosis

Homologous chromosomes

Chromosomes that carry the same sequence of genes but not necessarily the same alleles

Homologous chromosomes in eukaryotes

Each chromosome in an eukaryote is homologous to at least one chromosome in a second eukaryote of the same species. This allows breeding within members of a species

Types of nucleus

Haploid and diploid

Haploid nucleus

Nucleus that has a full set of chromosomes (n), that is, one chromosome of each type

Gamete

A haploid cell involved in sexual reproduction: the female egg and the male sperm

• Since n = 23 for humans, human gametes have 23 chromosomes

Diploid nucleus

Nucleus that has two full sets of chromosomes (2n), that is, two of each type

Characteristics of diploid nuclei

• Contain all the DNA necessary for protein synthesis and cell function
• Contains two copies of the same gene

Benefits of having two copies of the same gene

• If a dominant allele is present, recessive mutations can be avoided
• Hybrid vigor

Hybrid vigor

Phenomenon in which organisms with two different alleles have stronger growth and health

Why can’t members of different species breed?

During fertilization, both gametes must have the same number of chromosomes in order for the zygote to be viable. However, each species has a characteristic number of chromosomes

Number of chromosomes + evolution

• Sometimes during evolution, n can change, but these events are very rare
• n is not linked to how advanced a species is in evolutionary terms

Sex determination

In humans, it is determined by one pair of chromosomes (pair 23)

• females (XX)
• males (XY)

Autosome chromosomes

Chromosomes that do not determine the sex of offspring (pairs 1 to 22)

Chromosomes in human gametes

• Each female egg cell carries one of the two X chromosomes
• Half of male sperm cells carries the X chromosome, while the other half carries the Y chromosome

What determines the offspring’s sex?

The chromosome that is carried by the sperm that fertilizes the egg:

• X → female
• Y → male

Physical differences between X and Y chromosomes

• X chromosome is relatively large and has its centromere near the middle
• Y chromosome is much smaller and has its centromere near the end

Genes in X chromosome

• Contain a lot of genetic material
• Many of the genes code for both male and female characteristics. Hence, all humans need at least one

Genes in Y chromosome

• Contain a small number of genes
• Small part of the Y chromosome has the same genetic sequence as a small part of the X chromosome
• Remaining genes are not found in the X chromosome and are not needed for female development

Example of gene in Y chromosome

SRY or TDF gene

SRY or TDF gene

Gene involved in the development of testicles and testosterone production

• Since a fetus with XX lacks the TDF gene, ovaries develop instead of testicles, and female sex hormones are produced instead of testosterone

Karyogram

A micrograph of all chromosomes in homologous pairs, arranged from longest to shortest

How is a karyogram made?

A cell in mitosis is stained and placed on a microscope slide. It is then burst by pressing the cover slip, spreading out the chromosomes

How are chromosomes of a different type but similar size distinguished?

By the position of the centromere and the binding pattern of the chromosome (revealed by the stain)

Karyotype

The number and types of chromosomes that an organism has in its cell nuclei

What are karyotypes used for?

• Deduce sex by the appearance of pair 23 (sex chromosomes)
• Diagnose abnormalities

Down syndrome

When an individual has three copies of chromosome 21 instead of two

• Total of 47 chromosomes
• Often called Trisomy 21

Symptoms of Down Syndrome

Distinctive facial features, hearing loss, heart and vision disorders, and mental and growth retardation

Autoradiography

Technique used to study DNA by labelling it using radioactive isotopes

John Crains

Use autoradiography to measure the length of DNA molecules in E. coli

Describe Crains’ autoradiography

1. Cells were grown for two generations in a culture medium containing tritiated thymidine

   1. Thymidine consists of the base thymine, which is used by E.coli to make nucleotides for DNA replication
   2. Tritiated thymidine contains tritium, a radioactive isotope of hydrogen
   3. Hence, E.coli incorporated these bases into their DNA d replication, making it fully radioactive after a few generations

2. Cell walls were digested by the enzyme lysozyme, releasing the free-flowing DNA

3. DNA was fixed into position onto a dialysis membrane

4. Thin film of photographic emulsion was applied to the surface of the membrane an left in darkness for two months

   1. Some of the tritium atoms in the DNA decayed and emitted high energy electrons, which reacted with the film

5. After two months, the film was developed an examined in a microscope

   1. Visible black dot appeared at each point where a tritium atom decayed, showing the length and shape of DNA

Crains’ discovery

E. coli contains a single, circular chromosome of DNA with a length of 1100μm (very long since cell is 2μm)

Uses of Crains’ method

Used to produce images of eukaryotic chromosomes. A chromosome from a fruit fly revealed that eukaryotic DNA was linear rather than circular