Gr 11 Chromosomes

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Prokaryotes have one chromosome consisting of a circular DNA molecule
Some prokaryotes also have plasmids but eukaryotes do not
Eukaryote chromosomes are linear DNA molecules associated with histone proteins
In a eukaryote species there are different chromosomes that carry different genes
Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes
Diploid nuclei have pairs of homologous chromosomes
Haploid nuclei have one chromosome of each pair
The number of chromosomes is a characteristic feature of members of a species
A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length
Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex

Karyotype

A karyotype is an organized profile of a person's chromosomes. Two chromosomes specify sex, XX for female and XY for male. The rest are arranged in pairs, numbered 1 through 22, from largest to smallest. This arrangement helps scientists quickly identify chromosomal alterations that may result in a genetic disorder.

To make a karyotype, scientists take a picture of the chromosome from one cell, cut them out, and arrange them using size, banding pattern, and centromere position as guides.

Click to complete Karyotype activity

In a eukaryote species there are different chromosomes that carry different genes

Eukaryotic chromosomes are linear molecules of DNA that are compacted during cell division (mitosis or meiosis)

Each chromosome has a constriction point called a centromere, which divides the chromosome into two sections (or ‘arms’)

The shorter section is designated the p arm and the longer section is designated the q arm

Eukaryotic species possess multiple chromosomes that may differ in both their size and the position of their centromere

Staining chromosomes with particular dyes (e.g. Giemsa stain) will additionally generate unique banding patterns

Each chromosome will carry specific genes and the position of a particular gene on a chromosome is called the locus

The region in which a locus is positioned can be identified via three points of reference:

The first point of reference is a number (or letter) which denotes the chromosome (e.g. 7q31 refers to chromosome 7)
The second point of reference is a letter (p or q) to denote which arm the locus is positioned on (e.g. 7q31 is on the q arm)
The third point of reference is a number corresponding to the G band location (e.g. 7q31 is at the longitudinal position 31)

HOMOLOGOUS CHROMOSOMES

There are 22 matching pairs of chromosomes. These are called homologous chromosomes. (The word ʻhomologousʼ means similar in structure and composition.) Each pair is given a number. In the original zygote, one of each pair came from the mother, and one from the father. There is also a non-matching pair labelled X and Y. There are, therefore, two sets of 23 chromosomes – one set of 23 from the father and one set of 23 from the mother.

■■ The non-matching X and Y chromosomes are the sex chromosomes, which determine the sex of the individual. All the other chromosomes are called autosomes. It is conventional to position the two sex chromosomes to one side in a karyogram, so that the sex of the organism can be recognised quickly. In humans, females have two X chromosomes, and males have one X and one Y chromosome. The Y chromosome has portions missing and is therefore smaller than the X chromosome.

■■ The pairs of chromosomes can be distinguished not only by size and shape, but because each pair has a distinctive banding pattern when stained with certain stains, as shown in Figure 16.3. Each chromosome has a characteristic set of genes which code for different features. Scientists are gradually identifying which genes are located on which chromosomes and what their precise functions are. For example, we now know that the gene for the genetic disorder cystic fibrosis is located on chromosome 7. The gene for a particular characteristic is always found at the same position, or locus (plural: loci), on a chromosome. Figure 16.4 shows a map of some of the genes on the human female sex chromosome (X), which, if faulty, are involved in genetic diseases.

Each chromosome typically has several hundred to several thousand gene loci, many more than shown in Figure 16.4. The total number of different genes in humans is thought to be about 20 000–25 000. Each member of a homologous pair possesses genes controlling the same characteristics (Figure 16.5). A gene for a given characteristic may exist in different forms (alleles) which are expressed differently. For example, the gene for eye colour has two forms, or alleles, one coding for blue eyes and one for brown eyes. An individual could possess both alleles, one on the maternal chromosome and the other on the paternal chromosome.

Sexually reproducing organisms inherit their genetic sequences from both parents

This means that these organisms will possess two copies of each chromosome (one of maternal origin ; one of paternal origin)
These maternal and paternal chromosome pairs are called homologous chromosomes

Homologous chromosomes are chromosomes that share:

The same structural features (e.g. same size, same banding patterns, same centromere positions)
The same genes at the same loci positions (while the genes are the same, alleles may be different)

Homologous chromosomes must be separated in gametes (via meiosis) prior to reproduction, in order to prevent chromosome numbers continually doubling with each generation

Diploid

Nuclei possessing pairs of homologous chromosomes are diploid (symbolised by 2n)

These nuclei will possess two gene copies (alleles) for each trait
All somatic (body) cells in the organism will be diploid, with new diploid cells created via mitosis
Diploid cells are present in most animals and many plants

Haploid

Nuclei possessing only one set of chromosomes are haploid (symbolised by n)

These nuclei will possess a single gene copy (allele) for each trait
All sex cells (gametes) in the organism will be haploid, and are derived from diploid cells via meiosis
Haploid cells are also present in bacteria (asexual) and fungi (except when reproducing)

Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex

In humans, sex is determined by a pair of chromosomes called the sex chromosomes (or heterosomes)

Females possess two copies of a large X chromosome (XX)
Males possess one copy of an X chromosome and one copy of a much shorter Y chromosome (XY)

The Y chromosome contains the genes for developing male sex characteristics (specifically the SRY gene)

In its absence of a Y chromosome, female sex organs will develop
The sex chromosomes are homologous in females (XX) but are not homologous in males (XY)

Hence the father is always responsible for determining the sex of offspring:

If the male sperm contains an X chromosome, the growing embryo will develop into a girl
If the male sperm contains a Y chromosome, the growing embryo will develop into a boy
In all cases the female egg will contain an X chromosome (as the mother is XX)

The remaining chromosomes in the organism are called autosomes (they do not determine sex)

Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed
Gametes are haploid so contain only one allele of each gene
The two alleles of each gene separate into different haploid daughter nuclei during meiosis
Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles
Dominant alleles mask the effect of recessive alleles but co-dominant alleles have joint effects
Many genetic diseases in humans are due to recessive alleles of autosomal genes, although some genetic diseases are due to dominant or co-dominant alleles
Some genetic diseases are sex linked
The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes
Many genetic diseases have been identified in humans but most are very rare
Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer

X Linked Chromosome Disorders Video

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