Chromosomes, Genes and DNA

• Chromosomes, genes, and DNA are key components in science

Cells

• Cells are the basic biological units of living organisms.
• The human body consists of trillions of cells, making it highly complex.
• Some organisms, like amoebae, are made of a single cell and are much simpler.
• With the exception of sex cells, all cells within an organism contain identical DNA in their nuclei.

DNA

• Inherited traits are passed down through a genetic code.
• The genetic code is written in deoxyribonucleic acid (DNA), located in the nucleus of cells.
• Genetic code refers to instructions contained in a gene required to make a specific protein.
• Sequences of nucleotide bases (adenine A, thymine T, cytosine C, guanine G in DNA; uracil U replaces thymine in RNA) are read in sets of three bases, called codons.
• Each codon specifies a particular amino acid or a 'start' or 'stop' signal for protein synthesis.
• DNA serves as a genetic blueprint and a unique ID tag, with secrets from the past and future.

Genes

• A gene codes for a particular trait (e.g., ear lobe shape, blood group, eye color).
• Genes help to make a particular polypeptide (protein) by providing instructions to build them.
• Genes comprise specific segments of DNA organized into larger structures called chromosomes.
• Genes are located within the nucleus of the cell

Chromosomes

• A long, double-stranded molecule (2-3 meters) bunches into 46 packages called chromosomes.
• The term chromosomes comes from (chromo = 'colored' + some = 'body'), due to staining for visibility.
• Chromosomes are only visible when a cell is about to divide.
• When cells divide, chromosomes are not visible because the coils are unwound, and the DNA is spread throughout the nucleus.
• Centromere is where sister chromatids attach to the short and long arms of chromosomes.
• Telomere is the cap at the chromosome's end.
• Kinetochore is a region associated with the centromere that is important in cell division.

Autosomes

• Of 46 chromosomes found in somatic cells, 44 are present in both males and females.
• The 44 chromosomes are matched into 22 pairs based on size, centromere position, and banding patterns.
• A pair of chromosomes are described as homologous.
• Chromosomes that don't match are called non-homologous.

Sex Chromosomes

• The two remaining chromosomes are the sex chromosomes.
• Females possess two X chromosomes (XX), males have one X and one Y chromosome (XY).
• Sex chromosomes determine an individual's sex.

Chromosome Patterns - Karyotypes

• Karyotypes are constructed by scientists using differences in chromosome pair size, shape, and banding.

Types of Cells and Division

• Cell division includes Mitosis and Meiosis.
• Mitosis results in somatic cells (body cells).
• Meiosis relates to gametes (sex cells).
• Mitosis produces new body cells for replacement, growth, and repair.
• The cells produced through Mitosis are identical to each other and the original cell, with the same number of chromosomes.
• Somatic or body cells contain chromosomes in pairs within their nucleus.
• Meiosis is the type of cell division used to produce sex cells (or gametes).
• Ova (singular ovum) and sperm are two types of gametes.
• In Meiosis, the chromosome number is halved, and only one chromosome comes from each pair
• Cells with one of every chromosome are referred to as haploid.
• Genetic variation can increase the survival of a species, to which Meiosis contributes.

The Nucleotide

• DNA comprises repeating units called nucleotides which consists of: -a sugar (deoxyribose in DNA) -a phosphate group -a nitrogenous base [adenine (A), thymine (T), guanine (G) or cytosine (C)]

A with T and G with C

• In 1950, Erwin Chargaff contributed significantly to understanding DNA structure through his analysis of nucleotides and ratios.
• In DNA, every adenine (A) binds to a thymine (T) and every cytosine (C) binds to a guanine (G).
• A strand of DNA has equal numbers of A and T, and equal numbers of C and G.

The Universal Genetic Code

• All DNA consists of nucleotides with the same four nitrogenous bases: adenine, guanine, cytosine, and thymine.
• The specific order of these bases makes each organism unique.
• DNA contains instructions for proteins to be created, with different DNA sections (genes) coding particular proteins.

Stepping Down the DNA Ladder

• Nucleotides are monomers of nucleic acids, so DNA molecules are polymers of nucleotides.

Nitrogenous Bases in Pairs

• A DNA molecule has two chains of nucleotides.
• Hydrogen bonds join nucleotides at their complementary (or matching) nitrogenous base pairs.
• A segment of DNA with one strand with the code GATTACA has a complementary strand of CTAATGT

Unlocking DNA Codes

• The DNA code is read three bases at a time.
• DNA triplets can code for a 'start' command (e.g., TAC).
• DNA triplets can code for a 'stop' command (e.g., ATT, ATC, or ACT).

Introducing RNA

• Like DNA, ribonucleic acid (RNA) is a kind of nucleic acid.
• RNA is made up of nucleotides that are different from nucleotides of DNA.
• Instructions for making proteins are coded for in the sequence of nitrogenous bases in DNA.
• DNA cannot leave the nucleus because it is too big to fit through the nuclear pores.
• DNA makes mRNA (messenger RNA) via transcription.
• RNA then moves to a ribosome in the cytoplasm, and the genetic message is translated into a protein via translation.

Transcription

• A special copy of the DNA sequence is produced in the form of messenger RNA (mRNA) when the relevant piece of the DNA strand is exposed.
• This process of this copy is called transcription.

Transcription - The steps

• DNA is unzipped, and two strands are seperarted
• One strand is used as template to build off.
• RNA nucleotides complementary to the DNA are added creating a mRNA strand with RNA polymerase.
• Remember there is no T in RNA and instead replaced with a U.
• Newly synthesised mRNA is released. For example, a TACGACG DNA would be transcribed into AUGCUGC.
• mRNA passes its genetic copy through the nucelar membrane to the ribosomes.
• Introns (non coding sequences) are removed and sections called exons which have the information required to form a protein in translation.

Translation

• Once mRNA reaches the ribosome, its message must be translated into a protein.
• The ribosome is in groups of three, known as a codon.
• Each codon contains instructions to ass a specific amino acid.
• Each tRNA molecule has three bases called an anti-codon that is complementary to the codon on the mRNA.
• Anti-codon ensures that it is delivering the correct amino acid.

Translation - The Steps

• mRNA enters ribosome and is read one codon at a time always starting with AUG.
• A tRNA molecule then brings the codon, that amino acid detemined by the amino acid chart.
• The apporiate amino acid is added to form, protein (or polypeptide), continuing with the next codon being read.
• When a STOP codon is, the process ceases, and the protein is released to continue with its purpose.

Explore cells as the basic biological units of living organisms. Learn about DNA, the genetic code, and nucleotide bases. Understand codons and their role in protein synthesis.