Download Genetics – word list

Genetics – word list
DNA
the molecule which contains genes. This will be looked at in more detail.
it is shaped in a double helix (spiral)
Chromosomes X-shaped objects found in the nucleus of a cell. The x is made from long
coils of DNA (so…as DNA contains genes, so do chromosomes)
Gene
is a section of DNA molecule and also part of a chromosome.
Allele
also a gene. When you have two different versions of the same gene, you
have to call them alleles instead of genes
Dominant
the dominant allele is the one that will determine the characteristic that will
appear – even if another allele (recessive) is present
Recessive
this allele does not affect the characteristic that will appear…because of the
dominant one. But, if there are only two recessive alleles, then it will.
Homozygous
when two alleles are the same for the particular gene
Heterozygous
when the two alleles are different for a particular gene
Genotype
a description of the genes that you have
Phenotype
a description of the physical appearance given by that gene
Mitosis
cell division when one cell splits into two cells
Meiosis
another process of cell division which creates gametes. Meiosis only
happens in the ovaries or testes.
Diploid
this is the name given to the cells which have all the chromosomes (i.e. 46)
Haploid
this is the name given to the cells, which have half the chromosomes
(i.e. 23)
Gamete
either a sperm or egg cell. All gametes are haploid.
Zygote
this is the name given when the egg and sperm has fused together
Blastocyst
this is the name given after the zygote has begun to split via mitosis
Cell division
Fertilisation: the fusion of male and female gametes to form a zygote
The zygote eventually forms the new organism by means of cell division. This is called
MITOSIS.
The elements received by the parents are in the nuclei of the gametes. These are formed by
MEIOSIS.
Gametes are sex cells. A haploid has ½ the number of chromosomes. Gametes contain a
nucleus and a cytoplasm.
Both of these are produced by meiosis:
Egg
Sperm
Larger:
food for initial stages of cell
Smaller:
little food reserve
division
target for sperm
non-motile (can’t move by itself)
motile – mitochondria
few are produced
produced in large numbers
Model of inheritance
Male gamete
n
Organism
2n
Meiosis
Fertilisation
Mitosis
Zygote
2n
Organism
2n
Female gamete
n
Assumption: number of male and female gamete contribute equally to zygote.
If the amount in the gamete = n, the amount in the zygote = 2n.
The number of chromosomes in human being cells
Therefore, the number of chromosomes in gamete
There are 2 types of cell division:
1.
Mitosis
2.
Meiosis
=
=
46.
23
Mitosis
-
each daughter cell produced is genetically identical to each other AND parent cell
the chromosome number remains the same
this occurs in animals for growth and repair
this occurs for plants in the tip of the stem and the tip of the root
mitosis only has one division
mitosis produces 2 cells
Meiosis
the chromosome number is halved
each cell produced is genetically different (due to crossing over)
this takes place in gamete formation in humans: testis and ovary
this takes place in plants in the anther and the ovary
meiosis divides twice successively
meiosis produces 4 cells
-
Meiosis
1.
2.
The chromosomes have replicated to produce
homologous pairs of chromosomes (pairs of the
same chromosomes)
3.
The nuclear membrane breaks down. The
homologous pairs of chromosomes are lined up
together at the equator by the spindle fibres.
4.
The homologous pairs of chromosomes are
pulled apart by the spindle fibres. Half the
number of chromosomes are attached to each
spindle fibre.
The homologous pairs of chromosomes are
now separated by new nuclear membranes. The
cytoplasm pinches inwards at the equator.
5.
Each cell has divided again. The result is the
production of four haploid gametes. Each of
these cells are genetically different to each
other.
Mitosis
This is the name given to the form of cell division which occurs in normal body cells.
The result of this division is:
-
2 genetically identical daughter cells
-
the chromosome number is maintained
This type of cell division is used for growth, repair and replacement.
Once the process of cell division has started, it is a continuous process and 4 distinct stages
have been identified by humans: P M A T
Interphase
-
this is often termed the “resting phase” (but it is not really – lots does occur)
-
no chromosomes are visible
-
DNA replication occurs in this phase.
Prophase
-
chromosomes appear; longitudinally split (i.e. double stranded)
-
each strand = a chromatid (i.e. a molecule of DNA)
-
a chromosome is made of two chromatids
-
the 2 chromatids making up a chromosome are genetically identical
centromere
DNA chromatid
-
the centriole:
o a small body just outside the nucleus
o this divides and each half migrates to the opposite
side of the nucleus
o as the centrioles move apart, they give rise to spindle
fibres
o the spindle fibres become attached to the centromere
of the chromosomes when the chromosomes are lined
up at the equator of the cell
Metaphase
-
the double membrane of the nucleus disappears
-
the chromosomes become aligned along the equator of the cell
-
the chromosomes are attached to the spindle fibres by their centromere
centriole
centromere
spindle fibre
Anaphase
-
a short, rapid phase
-
the centromere splits in half and each half repels each other
-
at the same time, the spindle fibres shorten
Telophase
-
this covers the period in which the two groups of chromosomes arrive at the
poles and become surrounded by a nuclear membrane
-
the spindle fibres disappear and the cell cleavage appears
-
the cytoplasm divides into two by means of formation of a cell membrane (or
lamella in plants) between the two sets of chromosomes
-
the telophase may be either a transitory period – short enough only for DNA
replication to occur before the cells divide again – or long : permanent
interphase may follow.
Separation of the chromatids due to…
(a)
(b)
the centromeres split and each half repels each other half
the spindle fibres shorten
Role of spindle fibres
(a) attach the chromosomes / chromatids by the centromere
(b) shorten to separate chromatids (i.e. movement of chromosomes)
Energy for process
from ATP (i.e. respiration)
Is interphase a “resting phase”?
No…
(a)
DNA replication occurs (i.e. each chromatid manufactures a genetically identical
partner – see DNA stages later on)
(b)
a build up of ATP for the process of DNA replication and cell division
Outcome
(a)
(b)
(c)
2 daughter cells
same chromosome number as each other daughter / parent cell
genetically identical to each other and parent cell (i.e. no variation)
Consequence of mitosis
(a)
from fertilised egg:
- repeated cell divisions give rise to a population of cells which later
become tissues and organs
-
all cells have the same genetic complement
(b)
each individual of a species has a characteristic number of chromosomes
e.g. humans: 46 (i.e. 23 pairs)
(c)
Genetic information carried on chromosomes varies
(d)
Some cells lose their genetic material once they are developed
(e.g. red blood cells, xylem cells)
mitosis is used for growth, repair and replacement