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[C.2.3]-Mindmap_of_ChromosomesGenesDNA - Coggle Diagram
[C.2.3]-Mindmap_of_ChromosomesGenesDNA
Our body is made out of
cells
. Inside these cells contain
nucleus
, which contains the
chromosomes
. If we take a closer look, these chromosomes contains
DNA
, which in here, you can find the
genes
.
DNA :
short for deoxyribonucleic acid
inside the DNA molecule contains deoxyribose, a sugar containing five carbon atoms
is the chemical that is the basis of inheritance in nearly all organisms
usually found in the
nucleus
of a cell, in the
chromosomes
can replicate (make an exact copy of ) itself
when a cell divides by mitosis, each new cell receives
exactly the same
type and amount of DNA. The cells formed are
genetically identical
.
is the only chemical that can replicate itself exactly
Chromosomes :
come in pairs
the number of chromosomes in living beings will always be even
Genes :
a small section of DNA that determines a particular feature (characteristic)
can determine features by
instructing cells
to produce particular proteins which then leads to the development of the feature.
can be also described as a section of DNA that codes for a particular protein.
The stages of protein synthesis
For proteins to be made, the genetic code must be copied, then transferred out of the nucleus to the cytoplasm. This is carried out by a different kind of nucleic acid called
ribonucleic acid (RNA)
. There are 2 types of RNA :
Messenger RNA (mRNA), which forms a copy of the DNA code
Transfer RNA (tRNA), which carries amino acids to the ribosomes to make the protein
Protein synthesis takes place in 2 stages,
transcription
and
translation
.
Transcription
:
is the process of taking a single gene of DNA and copying it into a structure called mRNA.
the process is done in the nucleus.
Translation :
is the process of taking this mRNA strand and using it to produce a protein.
the process is done in the ribosomes.
Base-pairing rules in transcription
:
Guanine = Cytosine
Thymine = Adenine
Cytosine = Guanine
Adenine = Uracil (U)
Transcription
: The RNA nucleotides link up to form an mRNA molecule -> Joins together, making the sugar phosphate backbone of the molecule. -> If a section of DNA corresponding to a protein
has been transcribed
, the mRNA molecule leaves the DNA and passes out of the nucleus to the
cytoplasm
. -> the mRNA leaves through pores in the nuclear membrane -> DNA helix
'zips up'
again.
Translation
: The code consisting of sets of 3 bases in the mRNA are called
codons
. Each codon codes for a particular amino acid. The mRNA molecule attaches to a ribosome. There is a particular tRNA molecule for each type of amino acid. The tRNA molecule carries its amino acid to the ribosome, where its specific anticodon pairs up with the 3 bases of the corresponding mRNA codon.
Process of translation
:
the 1st tRNA binds at the mRNA does so at the 'start codon', which always has the base sequence AUG. This codes for the amino acid metionine.
Another tRNA brings along a 2nd amino acid. The anticodon of the 2nd tRNA binds to the next codon on the mRNA.
A bond forms between the methionine and the 2nd amino acid.
The 1st tRNA molecule is released and goes off to collect another amino acid.
More tRNA molecules arrive at the mRNA and add their amino acids to the growing chain, forming a protein.
At the end of the chain a 'stop codon' tells the translation machinery that the protein is complete, and it's released.
The structure of DNA
A molecule of DNA is made from 2 strands of molecules called
nucleotides
. Each nucleotide contains :
a sugar molecule (deoxyribose)
a phosphate group
nitrogen containing group called a
base
There are 4 bases :
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Base-pairing rule (always link with each other) :
Adenine = Thymine
Cytosine = Guanine
Thymine = Adenine
Guanine = Cytosine
When a cell is about to divide, firstly it must make an exact copy of each DNA molecule in the nucleus. This process is known as
replication
. As a result, each formed cell receives the exact same amount and type of DNA.
The process of replication :
The polynucleotide strands of DNA seperate.
Each strand acts as a template for the formation of new strand of DNA.
DNA polymerase assembles nucleotides into 2 new strands according to the base-pairing rule.
2 identical DNA molecules are formed, each containing a strand from the parent DNA and a new complementary strand.
The genetic code
The template strand
, is the only strand of a DNA molecule that actually codes for the manufacture of proteins in a cell. The other strand is called
the non-template strand
.
Most of the proteins manufactured are enzymes, which go on to control processes within the cells. Some are structural proteins, like
keratin
and
myosin
.
Proteins consists of chains of
amino acids
. A sequence of
3
bases in the template strand of DNA codes for
1
amino acid. Because 3 bases are needed to code for 1 amino acid, the DNA code is a
triplet code
. The sequence of triplets that codes for all amino acids inside a protein is a
gene
.
The triplet base that codes for individual amino acids are the same in all organisms.
The DNA code is a
universal code
.
Gene mutations
A
mutation
is a change in the DNA of a cell. Sometimes, mistakes are made and the wrong nucleotide is used when the DNA is replicating. This results a gene mutation. It can alter the sequence of the bases in a gene. This can lead the gene coding for wrong amino acid and protein. There are a few ways in which gene mutations can occur.
For example :
In duplication, the nucleotide is inserted twice instead of once. This makes the whole gene and protein different.
In deletion, a nucleotide is missed out. This leads to the same result as the one in duplication.
In substitution, a different nucleotide is used. This may lead to the new triplet unable to code for a different amino.
In Inversions, the sequence of the bases in a triplet is reversed. This can lead to different amino acid and altered protein structure.
Gene mutations can make everything
go wrong,
like the cell will die and the mutation will be lost. Mutations in the gametes/in the cells that divide to form gametes
can be passed on to the next generation.
This leads to the start of
genetic diseases
.
But some gene mutations can become an advantage to an individual. Like insects can become resistant to insecticides. This is an example of
natural selection
The structure of chromosomes
Not all humans have 46 chromosomes
. Cells with only half the DNA content of other cells, are
haploid cells
.
Pairs of matching chromosomes are called
homologous pairs
. They carry genes for the
same
features, and these genes are arranged at the same positions and sequence along the chromosome. Cells with chromosomes in pairs like this are
diploid
cells.
Because a chromosome contains a particular DNA molecule, it will also contain the genes that make up that DNA molecule.
Nearly
all human cells contain
46 chromosomes
. The cell from the male has
22 pairs of chromosomes
and 2 that do not form a pair,
the X and Y chromosomes.
A body cell from a female has
23 matching pairs
including a pair of
X chromosomes.
When 2 gametes fuse in
fertilization
, the 2 nuclei join to form a single diploid cell (a zygote). This cell has, once again, all its chromosomes in
homologous pairs
and 2 copies of every gene.