3.2 Chromosomes

Question 1

What are bacterial chromosomes?

Answer 1

Prokaryotes have one chromosome consisting of a circular DNA molecule. The DNA in prokaryotes is not associated with histones so it is sometimes described as naked.
Because only one chromosome is present in a prokaryotic cell, there is usually only a single copy of each gene.
They may have plasmids. BUT EUKARYOTIC CELLS DO NOT HAVE PLASMIDS

Question 2

What are plasmids?

Answer 2

Some prokaryotic cells have plasmids but eukaryotic cells do not.
Plasmids are 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. For example genes for antibiotic resistance are usually located in plasmids.
Plasmids are not always replicated at the same time as the chromosome of a prokaryotic cell and therefore there may sometimes be two plasmids or none at all. They can spread through a population by multiplying however. They can even cross the species barrier. This happens if a plasmid that is released when a prokaryotic cell dies is absorbed by another cell of a different species.

Question 3

What DNA is in the plasmids?

Answer 3

Plasmids are 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. For example genes for antibiotic resistance are usually located in plasmids.

Question 4

What can audio-radiography be used for?

Answer 4

To produce pictures of structures in cells, for example the prokaryotic DNA was discovered this way in E.coli. This was done by the scientist John Cairn.

1) Cells were grown for two generations in a culture medium containing tritium, which is a radioactive isotope of hydrogen. So radioactively labelled DNA was produced by replication in the E.coli cells.
2) The cells were then placed onto a dialysis membrane and their cell walls were digested using the enzyme lysosome. The cells were gently burst to release their DNA onto the surface of the dialysis membrane.
3) A thin film of photographic emulsion was applied to the surface of the membrane and left in darkness for two months. During that time some of the atoms of tritium in the DNA decayed and emitted high energy electrons which react with the film.
4) The film was then developed after 2 months.

The images showed that the chromosome in E.coli is a single circular DNA molecule with a length of 1,100up.

Question 5

What are eukaryotic chromosomes?

Answer 5

Eukaryotic chromosomes are linear DNA molecules associated with histone proteins inside a nucleus.
Histones are globular in shape and are wider than the DNA. There are many histone molecules in a chromosome with the DNA molecule wound around them.

In Eukaryotic cells there are at least two types of chromosome. In humans for example there are 23. These different chromosomes carry different genes. However humans have 46 individual chromosomes so two of each type. The two of each type are called homologous chromosomes, they carry the same genes, for example eye colour and hair colour but they may not contain the same alleles for these characteristics.

Question 6

What are homologous chromosomes?

Answer 6

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

Question 7

Explain the 23 pairs of chromosomes in humans?

Answer 7

In humans for example there are 23. These different chromosomes carry different genes. However humans have 46 individual chromosomes so two of each type. The two of each type are called homologous chromosomes, they carry the same genes, for example eye colour and hair colour but they may not contain the same alleles for these characteristics.

Question 8

What living organism has the smallest genome?

Answer 8

A virus would but it is not living so a prokaryote.

Question 9

What are haploid nuclei?

Answer 9

Haploid nuclei have one chromosome of each pair. A haploid nucleus has one chromosome of each type. Haploid cells of humans for example have 23 chromosomes.
Gametes are the sex cells that fuse together during sexual reproduction. Gametes have haploid nuclei.

Question 10

What are diploid nuclei?

Answer 10

Diploid nuclei have pairs of homologous chromosomes. It has two chromosomes of each type, perhaps with different alleles. Diploid nuclei in humans have 46 chromosomes.

The advantage of this is that diploid nuclei have two copies of every gene, this means that the effects of a harmful recessive mutation can be avoided if a dominant allele is also present.

Question 11

What do chromosome numbers mean?

Answer 11

Chromosome number is a characteristic of a species, organisms with a different number of chromosomes are unlikely to be able to interbreed so all the interbreeding members of a species need to have the same number of chromosomes.

Question 12

How is sex determined?

Answer 12

Sex is determined by sex chromosomes and autosomes are chromosome that do not determine sex.
There are two chromosomes in humans that determine sex:
- The X chromosome, which is relatively large and has its centromere near the middle.
- The Y chromosome is much smaller and has its centromere near the end.
The X chromosome has many genes that are essential in both males and females. All humans must therefore have at least one X chromosome. Some of the genes on the Y chromosome are the same as those on the X but the others cause the foetus to develop as a male. This is called either SRY or TDF. It initiates the development of male features, including testes and testosterone production. So if a foetus has a Y it develops into a male and if it does not then it does not have the TDF and will instead develop ovaries and testes and female sex hormones are produced not testosterone.

Question 13

What are autosomes?

Answer 13

Chromosomes that do not determine sex.

Question 14

What is the X chromosome like?

Answer 14

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

Question 15

What is the Y chromosome like?

Answer 15

• The Y chromosome is much smaller and has its centromere near the end.
  Some of the genes on the Y chromosome are the same as those on the X but the others cause the foetus to develop as a male. This is called either SRY or TDF. It initiates the development of male features, including testes and testosterone production.

Question 16

How are the sex chromosomes passed on?

Answer 16

Females have two X chromosomes. Females pass on one of their two X chromosomes in each egg cell, so all offspring inherit an X chromosome from their mother. The gender of the human is decided by the one sex chromosome carried in the sperm.

Question 17

What are Karyograms?

Answer 17

A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.
The chromosomes of a cell are clearest when they are supercoiled in mitosis. They are very clearest in metaphase, stains have to be used to make the chromosomes show up. If dividing cells are stained and placed on a microscope slide and are then burst open by pressing on the cover slip the chromosomes become spread. A micrograph can be taken. They can now be arranged digitally, according to their size and structure.
They are arranged in homologous pairs by size starting with the longest and finishing with the shortest.

Question 18

What is the difference between a Karyogram and a karyotype?

Answer 18

A karyogram is an image of the chromosomes in decreasing length. A karyotype is the property of an organism - it is the number and type of chromosomes that that organism has in its nuclei. Karyotypes are studied by looking at karyograms.

Question 19

What are karyotypes?

Answer 19

They are the properties of an organism - the number and type of chromosomes that that organism has in its nuclei. They are studied by looking at karyograms.

Question 20

How are karyograms used?

Answer 20

• To determine whether an individual is male or female.
• To diagnose Down syndrome and other chromosomal abnormalities. This is usually done by taking foetal cells from the baby in the uterus.

Question 21

In meiosis where does the halving of the chromosomes take place?

Answer 21

The halving of the chromosomes happens in the first division. This is because two pairs of homologous chromosome pair up, so there are 4 chromosomes in each bundle. Then in the first division these are pulled apart and so where there were two different types of chromosomes there are now only one. Like you had two red ones and two blue ones, and the red ones were pulled to one side and the blue ones to the other, so those chromosomes are now one colour in each. This goes from the diploid number to the haploid number.
Then these chromatids separate during meiosis 2 and so they still have the same number of chromosomes except now there is only one chromosome of that type.

Question 22

What happens before meiosis?

Answer 22

DNA is replicated so that all chromosomes consist of two sister chromatids. So basically what was 2 different alleles of chromosome 1 is now 4 copies of chromosome 1. The identical 1’s are in pairs and the other identical 1’s are in a pair.

1) The homologous chromosomes then pair up, so all the 1’s come together so there are 4. A pair of homologous chromosomes is bivalent (associated in pairs) and the pairing process is sometimes called synapsis.
2) Soon after synapsis a process called crossing over takes place. A junction is created where one chromatid in each of the homologous chromosomes breaks and rejoins with the other chromatid. Crossing over occurs at random positions anywhere along the chromosomes. At least one will occur in each, or several can occur. Because a crossover occurs at precisely the same position on the two chromatids involved, there is a mutual exchange of genes between the chromatids. This means instead of having 2 copies of 2 slightly different copies of a type of chromosome, chromosome 1 for example, you will have 4 different ones.

Question 23

What is synapsis?

Answer 23

The pairing of the copied homologous chromosomes so they are in 4’s. 4 copies of chromosome 1, 2 are identical and the other 2 are also identical to each other.

Question 24

What happens at crossing over? SL

Answer 24

In meiosis when the pairs of homologous chromosomes have come together, a process called crossing over takes place. A junction is created where one chromatid in each of the homologous chromosomes breaks and rejoins with the other chromatid. Crossing over occurs at random positions anywhere along the chromosomes. At least one will occur in each, or several can occur. Because a crossover occurs at precisely the same position on the two chromatids involved, there is a mutual exchange of genes between the chromatids. This means instead of having 2 copies of 2 slightly different copies of a type of chromosome, chromosome 1 for example, you will have 4 different ones.

Question 25

What introduces variety in meiosis?

Answer 25

• The orientation of bivalents is random and therefore randomly ones are pulled to either end of the poles, by microtubules.
• Crossing over
• Half chromosomes from each parent

Question 26

Why is the chromosome number in the cells halved?

Answer 26

Initially the two chromosomes (each in pairs of the same copy) are held together by chiasmata, but these slide to the end of the chromosomes and then the chromosomes can separate. The separation of homologous chromosomes is called disjunction. One chromosome from each bivalent moves to one of the poles and the other chromosome to the other pole.
The separation of pairs of homologous chromosomes to opposite poles of the cells halves the chromosome number. It is therefore the first division of meiosis that is the reduction division. Because one chromosome of each type moves to each pole, both of the two nuclei formed in the first division of meiosis contain one of each type of chromosome so they are both haploid.

Question 27

What are chiasmata?

Answer 27

They are junctions that hold the bivalents together in meiosis.

Question 28

What is disjunction?

Answer 28

The separation of homologous chromosomes.

Question 29

Why does the first division of meiosis produce a haploid cell?

Answer 29

Using just chromosome 1 as an example, chromosome 1 is made up of two slightly different versions of the same type, maybe one has the blue eyed gene and the other has the green eyed gene, but they both are chromosome 1 because they contain the eye gene. This is diploid because they have two different versions of chromosome 1. Then the chromosome is replicated and so you get four. They join up and then two of the type are pulled to each pole. Because one chromosome of each type moves to each pole, both of the two nuclei formed in the first division of meiosis contain one of each type of chromosome so they are both haploid.

Question 30

How do you obtain cells for karyotype analysis?

Answer 30

Karyotype is the the characteristics of an organisms chromosomes taken from a karyogram. You obtain the cells to scan 2 ways:

Amniocentesis - This involves passing a needle through the mother’s abdomen wall using an ultrasound to guide the needle.

Chronic villus sampling - A sampling tool that enters through the vagina is used to obtain cells from the chorion, one of the membranes from which the placenta develops. This can be done earlier than amniocentesis but it is a 2% chance of miscarriage rather than a 1%.

Question 31

What is amniocentesis?

Answer 31

This involves passing a needle through the mother’s abdomen wall using an ultrasound to guide the needle. In order to gain cells to test.

Question 32

What is chronic villus sampling?

Answer 32

A sampling tool that enters through the vagina is used to obtain cells from the chorion, one of the membranes from which the placenta develops. This can be done earlier than amniocentesis but it is a 2% chance of miscarriage rather than a 1%.

Question 33

Outline the stages of meiosis?

Answer 33

Prophase 1:

• Chromosomes supercoil
• They have replicated so there are 2 copies of every chromosome and the homologous chromosomes have paired up by synapsis. So there are now 23 sets of 4, instead of 23 sets of 2.
• Crossing over occurs
• The nuclear membrane dissolves.
• Microtubules start to grow out of microtubule organising centres

Metaphase 1:

• Spindle microtubules attach to the homologous chromosome at the equator of the cell.
• The organisation of paternal and maternal chromosomes either side is random and independent of other homologous pairs.

Anaphase 1:
- The microtubules shorten and separate the homologous chromosomes, pulling them to different poles of the cell. So all of version a of chromosome 1 is at one pole and all of version b of chromosome 1 is at the other pole.

Telophase 1:

• The chromosomes are pulled in tight to each pole and a nuclear membrane forms around them.
• The chromosomes uncoil
• Cytokinesis occurs

THEN NO REPLICATION OCCURS AT ALL BEFORE THE SECOND ROUND OF MEIOSIS.

THE SECOND ROUND IS IDENTICAL TO MITOSIS.

Question 34

What is non-disjunction?

Answer 34

Non-disjunction causes Downsyndrome. It is when the pairs of chromosomes do not separate at anaphase in Meiosis. This means both chromosomes of type 1 for example, go to one pole and none go to the other. The result is that a gamete either has two copies of a chromosome or no chromosome.
An extra copy for 21 codes for Downsyndrome.