Dna

Front 1

What is DNA?

Back 1

DNA or deoxyribonucleic acid contains the instructions in order to make another you, and is your genetic code. It contains traits and features inherited from one generation to the next.

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What are Genes?

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Genes are are genetic instruction that codes for a particular trait, genes are made up of DNA and are organised into larger structures called

chromosomes, which are located with in the

nucleus of the cell.

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What are Chromosomes?

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Your body constantly makes new cells for replacement, growth and repair. It achieves this by a process called mitosis, which is a type of cell division. Prior to cell division the DNA

replicates itself and the long molecule (2-3 metres) bunches up into 46 little packages called

chromosomes.

When the cells are not dividing, the chromosomes are not visible as the coils are unwound and the DNA is spread throughout the nucleus.

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Sex Cells and Meiosis

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Meiosis is another type of cell division and is used in the production of sex cells, or gametes: Ova & Sperm. Meiosis results in the chromosome number being halved, so instead of pairs of chromosomes in each resulting cell, there is only one chromosome from each pair.

The genetic information that you received from the mother was packaged into 23 chromosomes in the nucleus of her egg cell (Ovum), and the

genetic information that is received from the father is packaged into 23 chromosomes in the nucleus of the sperm that fertilised the mothers egg cell. when the gamed fuse together at

fertilisation, the resulting Zygote contained 23 pairs of chromosomes (one from each pair)

- a total of 46 chromosomes.

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Somatic cells

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Cells of your body that are not sex cells are called somatic cells, with the exception of red blood cells, all of your somatic cells

contain

chromosomes in pairs within their nucleus. This double set of

genetic instructions (one set from each

parent) makes up your

genotype. The visual

expression of your genotype as a particular trait or feature is called the phenotype. The

phenotype may also be influenced by your

environment.

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Autosomes

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Chromosomes can be divided into two main types: Autosome and Sex chromosomes.

Autosomes

of the 46 chromosomes in your somatic cells, 44 are present in both males and females and can be matched into 22 pairs on the basis of there relative size, position of centromere and stained banding patterns. These are called autosomes. they are numbered from 1 to 22 on the basis of there size, one being the largest, and 22 being the smallest.

The members of each matching pair of chromosomes are described as being homologous. this that don't match are called non-homologous. For example: two number 21 chromosomes would be described as being homologous, and a number 11 chromosome and a number 21

chromosome would be non-homologous.

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Sex chromosomes

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The two remaining chromosomes are the Sex chromosomes. in humans these differ between males and females. Females posses a pair of X chromosomes (XX) and males have an X and Y chromosome (XY). The sex chromosomes are important in determining an individual's gender.

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chromosome changes

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sometimes a genetic mutation can occur that

results in more or less of a particular type of chromosome. Down syndrome is an example of a trisomy mutation in which there are three number 21 chromosomes instead of two. Turner syndrome is an example of a monosomy mutation that results in only one sex chromosome (XO).

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Karyotypes

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Differences between the chromosome pair size, shape and banding can be used to distinguish them from each other. Scientists use these differences to construct a karyotype. cells about to divide are treated and stained, mounted on slides for viewing, and photographed. These photographs are cut up and rearranged into pictures that show the chromosomes in matching pairs in order of size from largest to smallest. Karyotyping can reveal a variety of chromosomal disorders such as Down syndrome and Turner syndrome.

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Nucleotides

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DNA was made up of repeating units called nucleotides. Each of these nucleotides consisted of a sugar, a phosphate group and a nitrogenous base. In the nucleotides that make up DNA, the sugar is deoxyribose and the nitrogenous base in each nucleotide is one of four different types: adenine (A), thymine (T), guanine (G) or cytosine (C).

The nucleotides are joined together in a chain. The sugar and phosphate parts make up the outside frame and the nitrogenous bases are joined to the sugar parts.

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Base pairing

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The concept of base pairing states that in DNA every adenine (A) binds to a thymine (T), and every cytosine (C) binds to a guanine (G). This is now known as Chargaff’s rule.

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Nitrogenous bases in pairs

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A DNA molecule is made up of two chains of nucleotides. Hydrogen bonds join them at their complementary (or matching) nitrogenous base pairs.

DNA molecules have the appearance of a double helix or spiral ladder. Using the spiral ladder metaphor, DNA could be considered as having a sugar–phosphate backbone or frame, and rungs or steps that are made16up of complementary base pairs of nitrogenous bases joined together by hydrogen bonds.

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Triplets

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The sequence of nucleotides in DNA is often described in terms of the nitrogenous bases that they contain. For example, if the first nucleotide contains guanine, the second contains adenine and the third thymine, then this sequence would be described as GAT. This sequence of three nucleotides in DNA is referred to as atriplet. Although some of these DNA triplets code for a start (e.g. TAC) or stop (e.g. ATT, ATC or ACT) instruction, most code for a particular amino acid. The triplet GAT, for example, codes for the amino acid aspartine.

The sequence of these triplets in DNA contains the genetic information to make your body’s proteins. This includes all of your hormones, enzymes, antibodies and many other proteins that are essential for your survival. If one of these triplets (or its bases) is incorrect or missing, it may result in a protein not being coded for or produced — which could result in death.

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Protein Synthesis

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The instructions for making proteins are coded for in the sequence of nitrogenous bases in DNA. Within the nucleus, these instructions are transcribed into another type of nucleic acid called RNA in a process called transcription. This RNA copy then moves to a ribosome in the cytoplasm where the genetic message is translated into a protein.

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RNA

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Like DNA, RNA is a type of nucleic acid and is made up of nucleotides. Its nucleotides, however, are different from those of DNA. RNA contains the sugar ribose (instead of deoxyribose), and uracil (instead of thymine) is one of its nitrogenous bases. It is also shorter and single-stranded.Another difference is that the triplet code in mRNA is referred to as a codon. The complementary mRNA codon for the start triplet TAC in DNA, for example, would be AUG.

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Transcription

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The first step in making a protein involves the unzipping of the gene’s DNA. When the relevant part of the DNA strand is exposed, a special copy of the sequence is produced in the form of

messenger RNA (mRNA). The process of making this complementary mRNA copy of the DNA message is called transcription.As its name suggests, messenger RNA (mRNA) passes through the pores of the nuclear membrane into the cytoplasm to take its genetic copy of the protein instruction message to ribosomes. These may be free floating in the cytosol or attached to the rough endoplasmic reticulum.

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Translation

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Once the mRNA has reached the ribosome, its message needs to be translated into a protein. The ribosome and another type of molecule called transfer RNA (tRNA) are involved in this process. tRNA already located in the surrounding cytosol collects and transfers the appropriate amino acid to its matching code on the mRNA. These amino acids are joined together by peptide bonds to make a protein.

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Proteins

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Proteins form parts of cells, regulate many cell activities and even help defend against disease.

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Genes switched on or off

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Different genes are responsible for different characteristics, such as the colour of flower petals, the markings on a snail shell, or a person’s blood group or eye colour. Every body cell in an organism has the same set of genes called a genome, but not all genes are active. Some have to be switched on to act and some have to be switched off at different stages in the life of a cell.

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Where are your genes?

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Located on specific chromosomes, humans possess around 20 000–24 000 genes. The position occupied by the gene on the chromosome is called its locus. Genes that are located on the same chromosome are described as being linked.

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Genome maps

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The total set of genes within an individual or cell is referred to as its genome. The study of genomes is calledgenomics. Genome maps describe the order of genes and the spacing between them on each chromosome. The genome size is often described in terms of the total number of base pairs (or bp). The genome size for organisms varies considerably: humans have about three billion base pairs, fruit flies about 160 million and brewer’s yeast around 12 million.