3.2 Chromosomes

Differences between chromosomes of prokaryotes and eukaryotes

# of chromosomes: P - 1 E - 2 or more Shape: P - circular E - linear Histones: P - not present E - present Presence of Plasmids: P - sometimes E - never Organized into pairs: P - no E - yes

How is a karyotype obtained?

1. Cells that are in metaphase give the clearest view of chromosomes of an organism 2. Stains are used to make the chromosomes show up. Some stains give each chromosome type a distinctive banding pattern. 3. Micrograph can be taken of the stained chromosomes 4. The chromosomes are then arranged according to their size and structure digitally. The position of the centromere and the pattern of banding allow chromosomes that are of a different type but similar size to be distinguished.

Karyotypes are studied by looking at karyograms. They can be used in two ways.

1. To deduce whether an individual is male or female. 2. To diagnose Down syndrome and other chromosome abnormalities. This is usually done using fetal cells taken from the uterus during pregnancy. If there are three copies of chromosome 21 in the karyotype instead of two, the child has Down syndrome. This is sometimes called trisomy 21. While individuals vary, some of the component features of the syndrome are hearing loss, heart, and vision disorders. Mental and growth retardation are also common.

Diploid nucleus

A diploid nucleus has two chromosomes of each type. It has two full sets of the chromosomes that are found in its species. Diploid nuclei in humans contain 46 chromosomes for example. Diploid nuclei have two copies of every gene, apart from genes on the sex chromosomes.

Haploid nucleus

A haploid nucleus has one chromosome of each type. It has one full set of the chromosomes that are found in its species. Haploid nuclei in humans contain 23 chromosomes for example.

Karyotype vs. Karyogram

A karyogram is an image of the chromosomes of an organism, arranged in homologous pairs of decreasing length. A karyotype is a property of an organism - it is the number and type of chromosomes that the organism has in its nuclei.

Chromosomes in a karyogram

As most cells are diploid, the chromosomes are usually in homologous pair.s They are arranged by size, starting with the longest pair and ending with the smallest.

Autoradiography

Developed by John Cairns in 1962 to demonstrate that a bacterium chromosome is made of a single circle of DNA Technique in which radiation from a substance is captured on film and used to obtain images of DNA so their lengths can be measured The film is exposed to radioactive particles being given off by the substance itself Described as DNA being able to "take their own pictures"

Chromosomes in Eukaryotes

Eukaryote chromosomes are made of linear DNA molecules associated with histone proteins

Histone How are adjacent histones separated?

Globular in shape and are wider than the DNA Adjacent histones in the chromosome are separated by short stretches of the DNA molecule that are not in contact with histones. (This gives a eukaryotic chromosome the appearance of a string of beads during interphase)

What do homologous chromosomes carry?

Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.

When are chromosomes visible?

Only during cell division

Chromosomes in Prokaryotes

Prokaryotes have one chromosome consisting of circular DNA molecule containing all the genes needed for the basic life processes of the cell (usually found in the nucleoid region). The DNA in bacteria is not associated with proteins, so is sometimes described as naked. Because only one chromosome is present in a prokaryotic cell, there is usually a single copy of each gene

How does an autoradiography work?

STEP 1: Inject radioactive materials (radio markers) into DNA samples so that it will expose the film faster - these are usually molecules that contain the radioactive isotope ^3H STEP 2: The radio markers "follow" the molecules as they complete cellular processes (ex. DNA replication). In his experiment, John Cairns used [3H] thymidine. STEP 3: Capture the radiation on film to determine the properties of molecules.

Sex determination

Sex is determined by sex chromosomes

Homologous

Similar in size and shape

What are plasmids?

Small extra DNA molecules that are commonly found in prokaryotes but are very unusual in eukaryotes They are usually small, circular and naked, containing a few genes that may be useful to the cell, but not those needed for its basic life processes They replicate independently. They are not always replicated at the same time as the chromosome of a prokaryotic cell or at the same rate. Hence there may be multiple copies of plasmids in a cell and a plasmid may not be passed to both cells formed by cell division. Copies of plasmids can be transferred from one ell to another, allowing spread through a population.

The two chromosomes in humans that determine sex 1. X Chromosome

The X chromosome is relatively large and has its centromere near the middle. It has many genes that are essential in both males and females. All the humans must therefore have at least one X chromosome.

The two chromosomes in humans that determine sex 2. Y Chromosome

The Y chromosome is much smaller and has its centromere near the end. The Y chromosome only has a small number of genes. A small part of the Y chromosome has the same sequence of genes as a small part of the X chromosome, but the genes on the remainder of the Y chromosome are not found on the X chromosome and are not needed for female development.

A karyogram shows...

The chromosomes of an organism in homologous pairs of decreasing length.

Chromosome Numbers

The number of chromosomes is a characteristic feature of members of a species. Organisms with a different number of chromosomes are unlikely o be able to interbreed.

Gametes

The sex cells that fuse together during sexual reproduction. Gametes have haploid nuclei, so in humans both egg and sperm cells contain 23 chromosomes.

What is the function of chromosomes?

They carry all genetic information the cell needs to survive

What does a nucleosome consist of? Function?

Two molecules of four different histone proteins DNA wraps around the eight histone proteins twice (attracted to histones because DNA is negatively charged and the histones are positively charged) Held together by another histone protein which also aids further packaging of the DNA molecule Helps to highly condense and supercoil chromosomes

What is DNA in the form of when cell is not dividing?

When the cell is not dividing, DNA is in the form of chromatin and chromosomes are not visible structures